Download Enhancing Land Controlled-Source Electromagnetic Surveys using

Enhancing Land Controlled-Source
Electromagnetic Surveys using Vertical
Electric Field Measurements
A look at the influence of different subsurface parameters for the Schoonebeek
field
Author:
Joeri Brackenhoff
Supervisors:
Andreas Schaller, MSC
Dr. Guy Drijkoningen
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Abstract
Nowadays, marine Controlled Source ElectroMagnetic surveys are commonly used for oil exploration
whereas similar surveys on land are still a rarity. In recent years theoretical studies have shown that
using a horizontal source and vertical receivers on land will result in lower noise levels measured by
the receivers. In order to test this, a survey on land near the village of Schoonebeek in the
Netherlands has been planned. There a reservoir filled with oil is located in the subsurface. An
analytical modeling code has been developed and different parameter combinations have been
tested to study the best setup for this survey.
Using the modeling code it has been shown that the horizontal source and vertical receiver setup
would indeed be feasible, that frequencies below 1 Hz should be used for transmitting and that the
offset of the receivers should be between 3 and 6 km. Due to steam injection in the reservoir the
thickness of the steam layer and therefore the electric conductivity of the reservoir will change.
These changes can be detected by repeated measurements over time. Parameters such as the depth
of the reservoir and changes in the overburden were shown to have little influence.
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Table of Contents
Abstract ................................................................................................................................................... 2
1. Introduction ......................................................................................................................................... 4
2. Theory.................................................................................................................................................. 5
2.1 Electromagnetic Theory ................................................................................................................ 5
2.2 CSEM theory .................................................................................................................................. 6
2.3 Models ........................................................................................................................................... 8
3. Sensitivity study................................................................................................................................. 10
3.1 Survey parameters ...................................................................................................................... 11
3.1.1 Components ......................................................................................................................... 11
3.1.2 Frequency ............................................................................................................................. 12
3.1.3 Receiver offset ...................................................................................................................... 15
3.2 Subsurface parameters ............................................................................................................... 16
3.2.1 Conductivity of the reservoir ................................................................................................ 16
3.2.2 Reservoir thickness ............................................................................................................... 19
3.2.3 Reservoir depth .................................................................................................................... 21
3.2.4 Overburden .......................................................................................................................... 23
Conclusion and discussion ..................................................................................................................... 25
Acknowledgements ............................................................................................................................... 26
Self-Evaluation ....................................................................................................................................... 27
References ............................................................................................................................................. 28
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1. Introduction
In the past couple of decades there has been much development in the field of exploration
geophysics with the aim to gain better understanding of the subsurface. A lot of these methods have
been used as logging tools such as natural radioactivity and resistivity but there are also methods
that do not need a borehole. Examples of these methods are the use of seismic waves and
electromagnetism. The seismic method has been widely utilized on both land and in marine
environments but the electromagnetic method has been sparsely used even though the theory stems
from 1953 (Constable et al, 2010). The electromagnetic method is mostly used in marine
environments because the contrast between the high conductivity of salt seawater and the low
conductivity of hydrocarbon reservoirs give relatively good imaging results (Andreís and MacGregor,
2007). The method has been utilized on land mostly in combination with the seismic method as the
electromagnetic method has very good intrinsic resolution but poor spatial resolution (Knaak and
Snieder, 2013). Recent studies have shown though that there is a high potential for the
electromagnetic method on land particularly for the vertical component Ez (Streich et al. 2010). This
is because most of the electromagnetic field on land which could be perceived as noise is horizontal
and thus has no strength in the vertical field (Hunziker et al, 2011).
Studies into the possibility of using the vertical component have rarely advanced beyond the
theoretical stage. In order to study the feasibility of the vertical component a survey was planned in
Schoonebeek, Drenthe, the Netherlands. Located near the German-Dutch border the subsurface
holds one of the richest oilfields of continental Europe. The survey included receivers oriented in the
x-, y- and z-direction for both the electric and magnetic fields. The magnetic and horizontal x- and ydirection electric field receivers were buried at shallow depths but the vertical z-direction electric
field receiver has to be placed at several different depths to create a dipole which requires a
borehole. This component is much more difficult to place than the other components (Streich et al.
2010).
Before the survey was conducted a feasibility study was done in order to check if there was a
theoretical backing for the survey. The code that was used was written by Dr. Hunziker after it was
validated against analytical solutions and a similar code. This code was then run using a model based
on the subsurface of Schoonebeek derived from well logs. The model was run several times with
different parameters for frequency or the thickness of the reservoir.
In chapter 2 the theory for this thesis is explained. First the basics of Electromagnetic theory are
shown which are then used to explain the CSEM method that was used to conduct the survey in
Schoonebeek. Finally a description of the models used for the feasibility study is done. In chapter 3
the feasibility study itself is shown. The parameters that can be influenced by the survey crew are
first considered which are the receiver setup, frequency and receiver offsets. Secondly the
parameters that could not be influenced are considered namely the conductivity of the reservoir, the
thickness of the reservoir, the depth of the reservoir and the overburden. From the results of this
study a conclusion is drawn what the setup of the survey should be and which parameters need to be
kept in consideration during the survey.
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2. Theory
2.1 Electromagnetic Theory
Electromagnetics, after this abbreviated to EM, is as the name suggests a combination of the science
that combines electricity and magnetism. An EM-field is a combination of both the electric field and
the magnetic field which are closely related to each other according to the Maxwell equations. In the
space time domain these equations are given as:
where a bold symbol denotes that the symbol has vector properties and the (x,t) denotes that it is
space and time dependent. E is the electric field (strength) in V/m, H is the magnetic field in A/m, ρ is
the charge density in C/m3, ε is the electric permittivity in F/m, µ is the magnetic permeability in
H/m, σ is the electric conductivity in S/m and ∂t is the time derivative. Equation 1 is Gauss’s law,
equation 2 is Gauss’s law for magnetism, equation 3 is Faraday’s law and equation 4 is the MaxwellAmpère law. These laws show that if an electric field exists and varies in either time or space a
corresponding magnetic field exists and vice versa. This is an important property of EM fields as it
means that an electric field can create a secondary magnetic field that in its turn can create a
secondary electric field. These secondary fields are dependent on the parameters µ and ε of the
medium in which they are created.
Figure 2.1: Behavior of EM fields in subsurface. (from www.geopartner.pl)
In figure 2.1 the behavior of EM fields in the subsurface is shown with a simple transmitter and
receiver setup. The transmitter emits a field that is either magnetic or electric which is the primary
field. The field then travels through the subsurface until it comes in contact with a conductor with
different parameters for electric conductivity, electric permittivity and/or magnetic permeability.
Because the primary field varies in space and time a secondary field is created according to equations
1 to 4 using the parameters of the conductor. This secondary field then travels back to the receiver.
The way the secondary field is created shows that the field strength of the EM fields is very
dependent on the medium parameters. The behavior of the EM fields in the subsurface is not as
simple as this however. There are three other major factors that contribute to the strength of the EM
fields according to Constable (2010) and Eidesmo et al. (2002). The three main reasons are geometric
spreading, galvanic jumps and inductive attenuation. These are all represented in figure 2.2.
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Figure 2.2: Mechanisms which determine the amplitude of an electric field in the subsurface. (from Constable
(2010)).
Geometric spreading is an important property of EM fields for low frequencies. EM fields are
governed by the wave equation, but for low frequencies the dampening term of this wave equation
become dominant and the EM field starts to behave like a diffusive field (Loseth et al. 2006). A
diffusive field spreads it energy in all directions which means that with increasing distance r from the
source an EM field loses its energy with a factor of 1/r3. Galvanic jumps are strongly related to the
change in medium parameters of the subsurface. The most important property is the electric
conductivity. The electric field in a medium 2 traveling from medium 1 with electric field strength E 1
is calculated by equation 5:
Finally there is inductive attenuation which follows from equations 3 and 4. This is due to the fact
that as the EM field travels through the subsurface it is in fact changing in time and space due to
geometric spreading. These changes cause the EM field to create secondary EM fields while they
travel through the subsurface losing part of their energy to these fields.
2.2 CSEM theory
CSEM stands for Controlled Source ElectroMagnetic and is a test where a long line source is used to
generate an EM field that travels through the subsurface and is then picked up by receivers. The
reason that it is called controlled source EM is that it is possible to produce a field with certain
properties such as its frequency (Constable, 2010). In figure 2.3 the CSEM line method is represented
schematically. One of the most important things to note is the fact that there is not a single point
source for the EM field but instead there is a long line source: a line source strengthens the field,
thus the longer the line, the stronger the field. This line source emits the EM field which is later
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picked up by several receivers. These are usually oriented vertically and buried about 0.15 m in the
subsurface (Streich and Becken, 2010). However using this layout the only fields that can be
measured are the horizontal electric and the three components of the magnetic field. The vertical
electric field is impossible to measure this way, because it is zero at the surface. In order to measure
these fields a borehole has to be drilled in which several receivers have to be placed at different
depths, even though this is said to be impractical (Streich et al, 2010).
The reason why one would do this is because the vertical components are much more sensitive to
changes of the subsurface with depth as follows from the galvanic jumps. However not all of the field
passes through the reservoir. Some of it even travels through the air and is called an air wave. This air
wave has very low attenuation and is therefore very strong. It goes straight down near the receivers
and can because of its high amplitude suppress the lower amplitudes that do contain information
about the reservoir. If the borehole with receivers is completely vertical however the air wave will
not be recorded (Hunziker et al. 2011). It should also be noted that the noise which is the electric
field strength caused by anything else than the source, such as railways, is stronger at shallow depth
and will therefore be lower deeper in the borehole.
Figure 2.3: Schematic representation of CSEM-line source method on land. The lower conductivity of the
reservoir causes the energy of the field to accumulate in its medium.
As can be seen in figure 2.3 not all of the field lines reach the depth of the reservoir and even if they
do their amplitudes may have become so low that they are below the noise-level. If the field is still
strong enough when it is picked up by the receiver it will however have a high intrinsic resolution
(Andréis and MacGregor, 2007). As discussed before the field changes as it travels through the
subsurface and keeps creating new secondary fields. This is the cause of the high intrinsic resolution
of the received data. The spatial resolution on the other hand is very poor (Knaak and Snieder, 2013;
Andréis and MacGregor, 2007; Constable, 2010). The penetration into the earth is very dependent on
the properties of the subsurface and therefore cannot be determined exactly though it should be
noted that mediums with low conductivity have EM fields traveling through them with higher energy
as can be deduced from equation 5 which is also represented in figure 2.3. Also the diffusive
behavior of the field means that there is a change in energy so that the field strength at the receiver
is very different from the one at the source. The fact that the spatial resolution is so poor is the
reason that the CSEM method is rarely used alone. It is usually combined with the seismic method,
which has a good spatial resolution but a poor intrinsic resolution (Constable, 2010). The seismic
method is used to discover an anomaly that could possibly be a reservoir. By using the CSEM method
it can be determined if this reservoir is filled with oil, water or something else. Because of the high
differences between the conductivities of water and oil the response of the reservoir will differ
greatly depending on the fluid in the reservoir.
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2.3 Models
The modeling that has been done utilized a code called EMmod and was created by Dr. Jürg Hunziker
from TU Delft. The code can model the electric field from 1D-heterogeneous models in 3D. It is based
on propagation in homogeneous layers and reflection and transmission at the boundaries of the
layers. For a detailed description of the code one can refer to Hunziker et al (2014). The code has
been validated using another modeling code and analytical solutions. The models than were run
through EMmod are all one dimensional meaning that their conductivities all vary in only one
direction which in this case is depth.
Figure 2.4. Different one dimensional model ran through the codes all of which contain a reservoir which can be removed
from the model. (a): Full-space model with one constant value for the electric conductivity in all directions. (b): Half-space
model with a surface above which the electric conductivity is constant and below which there is a constant conductivity. (c)
Model based on the subsurface of Schoonebeek created by Andreas Schaller. (d) updated model based on Schoonebeek.
In figure 2.4 the four models that have been used for this thesis are represented. As mentioned
before all the models are heterogeneous in one dimension so they are plane-layered meaning that
the subsurface is simplified to be flat layers. Figure 2.4a shows the full-space model which is the
simplest model that exists. It has only one value for electric conductivity and no boundaries in any
direction. The half-space model that is shown in figure 2.4b is very much like the full-space model
with one difference. Unlike the full-space model the half-space model does have a boundary
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condition namely a surface between the conductive subsurface and the non-conductive air above it.
The last two models are the most complicated of all. In figure 2.4c a model is shown based on the
subsurface of Schoonebeek. It was derived from well logs by Andreas Schaller, MSc. This model has
several jumps in conductivity most noticeably at a depth of 685 m. Here the reservoir filled with oil is
located. This model was based on very old well logs and as such was updated after a new log was
made which reached to a depth of about 125 meters. The updated model is represented in figure
2.4d. The most notable changes are that a gradient of increasing conductivity was discovered in the
first 80 meters of depth. After this the conductivity remains more or less constant up to a depth of
125 meters. After this the rest of the model is represented with the data from figure 2.4c. The model
shown if figure 2.4d will be used in this study unless stated otherwise.
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3. Sensitivity study
Different model parameter have been evaluated in order to determine which properties of the
reservoir influence the response measured at the receivers the most. The Schoonebeek model used
in this study is based on logging data and can therefore be assumed to be well defined. However,
variations of certain parameters are still possible. The thickness of the reservoir for example has
been estimated to be 15 meters. This could also be 20 meters or maybe only 10 meters and it does
not have to be continuous over the entire area. By changing the parameters one at a time and see
what this does to the reservoir response all of these data can be combined to see what the best
combination of parameters would be. Not all of these parameters can be influenced but a few can.
The ones that can be influenced by the crew of the survey is the frequency of the measurement, the
offset or position of the measuring equipment, the components which are measured and whether
the frequency or time domain is used to interpret the data. The parameters that cannot be affected
by the crew are the conductivity, reservoir depth and reservoir thickness. First the different sort of
components will be modeled with the most interest in the vertical E31 component, because that
component is the prime point of investigation for this study. In this notation the first number in the
subscript denotes the orientation of the receiver and the second number denotes the orientation of
the source. In this notation a 1 stands for an x-orientation, 2 for a y-orientation both of which are
horizontal and a 3 stands for a vertical z-orientation. After this the other parameters are considered.
Using these results a combination of the ideal parameters is created. The first set of parameters that
are considered are the once that can be influenced during the survey starting with the components.
The way that parameters are considered is by checking if the response of the model with a reservoir
is the significantly different than if the same model was ran with the reservoir removed. An example
for this is given in figure 3.1
Figure 3.1: Electric field strength in V/m of Schoonebeek V2 model for component E 31 at depth of 100 m at 0.5 Hz frequency
(a) with reservoir (b) without reservoir. (c) ratio between (a) and (b). (d) ratio between (a) and (b) with electric field strength
below noise level removed. The area of the figures is 10 by 10 km.
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In figure 3.1a the logarithmic electric field strength is given of the Schoonebeek model with reservoir
at a depth of 100 meters and a frequency of 0.5 Hz. In figure 3.1b the same result is given only this
time the reservoir was removed from the model and replaced by extrapolating the layer below it
upwards. In this way the subsurface without a reservoir is mimicked. The result is for the component
E31. While the two parts of the figure appear to be very similar it should be noted that the electric
field strength for the result with the reservoir is slightly higher. Because the difference is not very
clear the ratio between the response with and without reservoir are taken and represented in figure
3.1c. It can be concluded from this figure that there is a strong difference between the two results in
a large part of the figure. In the center this difference is low but at higher offsets of about 6 km the
ratio is more than two. However, not all of these data are useful. A large part of the response has an
electric field strength that is too weak. The measurement is done on land and there are a lot of
sources that produce electric fields that do not contain any data of the response of the reservoir. The
strength of these fields is called the noise level and all the field strengths that are below this noise
level are considered to be too weak and to be dominated by the noise. Therefore all of the points
were the field strength is below the noise level are clipped from the figure. The result of this is
displayed in figure 3.1d. On land the noise level has been defined as 10-14 V/m (Rita Streich.
Pers.comm). In figure 3.1d a large part of the figure has been clipped but still a large area with a
strong ratio remains. Similar results like the one found from figure 3.1a and 3.1b will be presented in
the format of figure 3.1d.
3.1 Survey parameters
3.1.1 Components
The reason for this entire study is into the feasibility of using the vertical component with a
horizontal line source. In order to check if this component indeed has this amount of promise it is
first compared with the other possible component configurations. The resulting ratios are plotted in
figure 3.2
Figure 3.2 Ratios between Schoonebeek V2 model with and without reservoir at a depth of 100 m at frequency 0.5 Hz using
a point source for component (a) 11 (b) 12 (c) 13 (d) 21 (e) 22 (f) 23 (g) 31 (h) 32 (i) 33. The area of the figures is 10 by 10
km.
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As a reminder the first number of the component denotes the orientation of the receiver and the
second number denotes the orientation of the source. The first thing that is immediately clear is that
the size of the area of the field that is above the noise level is much smaller when there is no zoriented source in figures 3.2c, 3.2f and 3.2i or a z-oriented receiver like in figures 3.2g and 3.2h
present. This is because the z-oriented field is much weaker than the x- or y-oriented ones. Thus
having a z-oriented source would be a bad idea because almost no part of the field is above the noise
level and the area that is has a very low ratio. When there is a horizontal source the field has a
general higher strength. It should also be noted that figure 3.2a and e are identical except for a
rotation of 90 degrees. This is simply because the source and the receiver orientation between these
figures have also been rotated 90 degrees. A similar relation also exists between figure 3.2b and
3.2d. Because of the source is positioned perpendicular to the receiver there is a line in the (x,y)
plane that has zero amplitude which can be seen in figure 3.2d. While the entire fields in figure 3.2b
and 3.2d are above the noise level, the ratios in these figures are very low and almost 1. This would
mean that detecting the reservoir would be very hard. If the source and receiver have the same
horizontal orientation this is quite different. There is a very small part of the field that is below noise
level but the ratios in places are very high. Still these results are not very good for a survey. The areas
with high ratios are very small and thus forming dipoles for the measurement will be hard. Also these
areas have very specific location which could vary in reality and thus pinpointing them would be very
difficult. This setup has promise but when studying figure 3.2g and 3.2h it becomes clear that the
latter ones are the better choice. There is a large part of the area below the noise level, however this
area is far smaller than in the case of a z-oriented source. Most important however is that the ratio
of the area that is above the noise level is very strong. The two figures are also rotated versions from
each other because the only difference between the figures is that the source is rotated 90 degrees.
Using these results it has been shown that the E31 component shows the best result of all of the
components. Therefore this orientation for the source and receiver will be used for the rest of this
study.
3.1.2 Frequency
The second parameter that will be discussed is the frequency. This parameter is probably the most
easy to change as it can be done at the source without moving the equipment. For this test 3
frequencies were used namely 0.1, 1.0 and 10 Hz. The frequencies were measured at the depths of 5,
50 and 100 meters. The results are summarized in figure 3.3 where every row is a certain frequency
and every column is a certain depth. The results are ordered such that the lowest frequency is on the
top and the lowest depth in the leftmost column. In this test instead of a point source a dipole is
constructed by adding up the data over a length of 1 km in order to model the 1 km line source that
will be employed during the survey in Schoonebeek. The remainder of this thesis will employ this
method.
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Figure 3.3: Overview of ratios between the Schoonebeek V2 model with and without the reservoir for a frequency of (a)(d)(g)
0.1 Hz, (b)(e)(h) 1.0 Hz and (c)(f)(i) 10.0 Hz at a depth of (a)(b)(c) 5 m, (d)(e)(f) 50 m and (g)(h)(i) 100 m. The area of the
figures is 10 by 10 km.
The first thing that can be concluded almost immediately from the figure is that lower frequencies
have a much larger area that is above the noise level than higher frequencies. The depth a field
penetrates increases as the frequency decreases. Therefore at lower frequencies a larger part of the
electric field will penetrate the reservoir and therefore the response will be easier to see. The other
important property that can be distinguished from the figure is that the area below the noise level
decreases with increasing depth. The field that travels through the subsurface loses part of its
amplitude as was explained earlier on in this thesis. The response of the reservoir is traveling upward
through the subsurface thus at a higher depth it will be stronger.
The goal of this sensitivity study is to determine which frequency and depth are feasible for use
during the survey. This does not mean that the survey will be limited to a single frequency for a single
depth. Rather it is desirable to have as many options as possible so that the data can be compared at
different frequencies and depths for more credibility. Looking at figure 3.3 it seems that the
frequency of 10 Hz is not very useful. The area where the electric field strength is above the noise
level is very small although the ratio is very strong. The response at these frequencies would be very
difficult to measure regardless of depth. On the other hand frequencies lower than 1.0 Hz seem to
yield a high ratio with areas of field strength above the noise level that are large enough to place
receivers in.
Up until this point only three frequencies have been considered which gives a general idea of how
the frequency influences the field strength of electric field. Now to see in detail how great this
influence is more frequencies are considered namely two hundred steps from 10-3 to 104 Hz on a
logarithmic scale. For this loop the Schoonebeek V2 model is used and a single point is considered in
this model with and without the reservoir. This point is located at (4000,0,-100) m. This point was
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chosen because it is outside of the central part with the low ratio and the area it is in has a good ratio
for varying frequencies. The result is represented in figure 3.4.
Figure 3.4: Frequency dependence of E-field strength in V/m for Schoonebeek V2 model. The location of the point that is
considered is at x=4000 m, y=0 m and z=-100m.
First the part of the plot that is above the noise level is considered. The field strength of the response
of the model with the reservoir is higher than the one without the reservoir. This is to be expected as
the electric conductivity of the reservoir is much lower than the rest of the reservoir. The red plot
crosses the noise level somewhere around 1 Hz. The field strengths at higher frequencies are below
the noise level. This result agrees with the deduction made from figure 3.3.
Below the noise level there is a strange event. At higher frequencies the field strength of the
response converges, which is not strange as at higher frequencies the field does not penetrate deep
enough in the subsurface to encounter the reservoir. The strange occurrence is the fact that there is
a sudden change in the trend of the line which was decreasing until it suddenly started increasing
again. This increase is very strange and because the field strength increases until it again is above the
noise level this is an event that was worthwhile to investigate. This was done by investigating if there
might be a change in behavior. The frequencies are still considered to be low so there is no possibility
that this even might be due to a change in the EM-field behavior from diffusive to wave. Another
possibility was that there might be a response from the air-ground interface because of the strong
difference in electric conductivity. In order to check if this was the case, the response of the simpler
full-space and half-space from figure 2.4a and 2.4b were used. The results are represented in figure
3.5.
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Figure 3.5: Frequency dependence of E-field strength in V/m for (a) half-space model (b) full-space model. (c) is the air
response for the half-space and (d) is a combination of all previous figures The location of the point that is considered is at
x=4000 m, y=0 m and z=-100m.
The response of the half-space model has the same behavior as the response of the Schoonebeek V2
model from figure 3.4. The response of the full-space model however shows that there is no bend in
amplitude occurring as it does in the half-space and Schoonebeek V2 model. This leads to the
conclusion that there is a high chance that part of the electric field bounces of the interface between
the ground and the earth, because the air is not electric conductive. By subtracting the half-space
model response with the full-space model response the response of the interface between the air
and ground is obtained.
The frequency dependence shows a general trend that for higher frequencies the strength of the
electric field decreases. In order to be sure that the field strength is above the noise level the
frequency level should not exceed 1 Hz. Theoretically speaking every frequency below 1 Hz could be
used however as the frequency decreases the depth to which it penetrates also increases which
means that the response from layers and other elements in the subsurface will influence the
response of the electric field. Therefore it is unwise to measure at extremely low frequencies. The
best range of frequency to use seems to be somewhere between 0.1 and 1 Hz.
3.1.3 Receiver offset
Now that a range for the frequency has been found the offset for the receivers is determined. This is
important as from previous figures it can be seen that if the receiver is too close to the source the
ratio is too low and if the receiver is too far from the source the electric field strength is below the
noise level. In order to check the range in which a response has the highest chance to be recorded
figure 3.3 is again taken into account. The response with the smallest area that is above noise level
for the determined frequency range is at a frequency of 1.0 Hz and a receiver depth of 5 meters. The
response with the largest area above the noise level is at a frequency of 0.1 Hz and a receiver depth
of 100 meters. These results are again displayed next to each other in figure 3.6.
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Figure 3.6: Overview of ratios between the Schoonebeek V2 model with and without the reservoir for (a) 0.1 Hz at 100 m
depth and (b) 1.0 Hz at 5 m depth. The area of the figures is 10 by 10 km. The white line shows the border of the offset
where receivers should be placed
From figure 3.6a it is very clear that for low frequencies any receiver situated more than 3 km away
from the source will be above noise level and thus contain a response of the reservoir. For higher
frequencies this is not the case. From figure 3.6b it can be concluded that a minimal offset of 3 km is
also good for high ratios but there also is a maximum to this area located at about 6 km. Combining
these results it seems that the best offsets to place the receiver are between 3 and 6 km although if
lower frequencies are used for transmitting the receivers could also be placed farther away.
3.2 Subsurface parameters
Up until this point only parameters that can be influenced before or during the survey have been
taken into account and conclusions have been made from the study of these parameters. Now the
parameters that cannot be influenced are considered. As noted before the Schoonebeek V2 model
and other parameters have been derived from old well logs so the reality of the subsurface of
Schoonebeek could differ from these models. In order to check if small changes in certain parameters
will have a strong influence on the response of the reservoir the model will be changed slightly
depending on each parameter.
3.2.1 Conductivity of the reservoir
The subsurface of Schoonebeek contains a large reserve of oil and a lot of different techniques have
been employed in order to recover these reserves. A method that has been utilized for some time
now is the injection of steam in order to increase the recovery rate. The hot steam causes the oil in
the reservoir to heat up and increases the flow rate of the oil. This leads to the oil flowing easier
through the porous subsurface towards the recovery well (www.nam.nl). In doing this the reservoir
becomes partly filled with steam and hot water both of which are much more electrically conductive
than oil. It is unknown if the steam injections in Schoonebeek have altered the conductivity of the
reservoir but if this is indeed the case the influence needs to be considered. A similar approach as
was used by the study of the frequency is taken for the conductivity of the reservoir. First a few grids
with very different electric conductivities of the reservoir are considered. The results are shown in
figure 3.7
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Figure 3.7: Overview of ratios at a frequency of 0.5 Hz for a conductivity of the reservoir of (a)(d)(g) 0.005 S/m, (b)(e)(h) 0.05
S/m and (c)(f)(i) 0.5 S/m at a depth of (a)(b)(c) 5 m, (d)(e)(f) 50 m and (g)(h)(i) 100 m. The grid has an area of about 10 by 10
km.
As expected the area of the ratio that is above noise level increases with decreasing conductivity and
increasing depth of the receivers. As can be easily seen in the picture if the conductivity changes one
magnitude the area which is noise does not change very much but the ratio does. The change in the
ratio between a reservoir conductivity of 0.005 and 0.05 S/m is very drastic. And for the change to
0.5 S/m for the conductivity of the reservoir the ratio is very close to 1. This is not surprising as this
value of the reservoir is very similar to the surrounding layers and it is in the same magnitude.
Just like with the frequencies the grids in figure 3.7 give only a very general overview for one single
parameter. To see how the amplitude of the electric field changes according to the shift in the
conductivity again a single point is taken in this grid. This point is located at (4000,0,-100) m. The
conductivity is at the logarithmic x-axis ranging from 0.0001 S/m to 10.0 S/m.
17
Figure 3.8: Conductivity of the reservoir dependence of E-field strength in V/m for Schoonebeek V2 model at a frequency of
0.5 Hz. The location of the point that is considered is at x=4000 m, y=0 m and z=-100m.
Just as was seen in figure 3.7 it can be concluded from figure 3.8 that if the conductivity of the
reservoir increases the amplitude of the electric field decreases. The gradient at first is very strong
but it flattens out when the amplitude is about the same strength as when the model had no
reservoir. This is probably due to the fact that the layer at these conductivities starts to approach the
same values for conductivity as the surrounding layers. It can be seen that if the conductivity of the
reservoir keeps increasing the gradient reappears. The behavior is thus very much like one would
expect it to be except for one thing. At very low conductivities of the reservoir the gradient is not
down going but rather up going. This once again might be due to the complicated nature of the
Schoonebeek V2 model. Therefore the full-space and half-space models are considered once again in
order to check if this change again might be because of the air ground interface. The results are
displayed in figure 3.9
18
Figure 3.9: Conductivity of the reservoir dependence of E-field strength in V/m for half-space and full-space model at a
frequency of 0.5 Hz. The location of the point that is considered is at x=4000 m, y=0 m and z=-100m.
Figure 3.9 shows an interesting result. Just like the response of the Schoonebeek V2 model at first
the amplitude of the electric field increases with increasing conductivity, but then there is a sudden
change and the amplitude decreases with increasing conductivity. This happens for both full-space
and half-space so it has to be something that happens because of the reservoir, because the only
change is the conductivity of the reservoir. An electric field is usually strengthened by a layer with a
lower conductivity as was shown earlier in this thesis. Therefore it is strange that there is an increase
in the first part of the plot. The conductivity however is extremely low in this part. An explanation for
what might be going on is that the reservoir at this level of conductivity is so resistive that part of the
electric field is effectively blocked and that only the signal that penetrates the reservoir very briefly
manages to travel back to the receivers at the subsurface.
The conductivity is a parameter that cannot be easily influenced by human interaction. However it is
apparent that there has to be a significant change in the conductivity of the reservoir in order to
influence the amplitude of the electric field. The change of one magnitude of conductivity has a very
noticeable impact. Steam injection into the reservoir layer for example could create a magnitude
change which could influence the measured results greatly. If this is the case extra caution should be
taken with interpreting the result of the reservoir.
3.2.2 Reservoir thickness
The change in conductivity that could occur due to steam injection is not the only thing that steam
injection could influence. Another very possible occurrence is that due to the difference between the
steam and oil the steam could form a layer of itself above the oil. This could very well influence the
thickness of the reservoir and thus the response. The estimated value of the thickness of the
19
reservoir is 15 meters at a depth of between 685 meters and 700 meters. In order to make a
comparison the thickness of the reservoir is changed, but the depth at which the reservoir top is
located remains the same and the depth of the bottom of the reservoir is changed. For the grid plots
three thicknesses are considered: a thickness of 3, 5 and 15 meters. One value is three times lower
than the Schoonebeek V2 model value and another one is five times smaller. The results are
collected in figure 3.10. Each column contains the response at certain depths and each row does the
same for the thickness of the reservoir.
Figure 3.10: Overview of ratios at a frequency of 0.5 Hz for a thickness of the reservoir of (a)(d)(g) 3 m, (b)(e)(h) 5 m and
(c)(f)(i) 15 m at a depth of (a)(b)(c) 5 m, (d)(e)(f) 50 m and (g)(h)(i) 100 m. The grid has an area of about 10 by 10 km.
The first thing that can be concluded from figure 3.10 is that the amplitude of the electric field
increases as the thickness of the reservoir increases. This is not strange as a thicker reservoir means
that the electric field travels through a layer of lower conductivity for a longer time which
strengthens the amplitude. It is also noteworthy that if the reservoir is only 5 meters thick there is
still a reasonably large area where the amplitude is above the noise level. And within this area the
ratio is still high so the measurements would still pick up on the reservoir response. This is important
because if the reservoir is much thinner than expected there would still be a chance that its response
can be picked up by the measuring equipment.
The general overview represented in figure 3.10 is now examined in greater detail by once again
taking a single point located at (4000,0,-100) m. The parameter that changes is on the x axis and in
contrast to the conductivity and frequency is not logarithmic but linear from 1 to 50 meters
thickness. This is done because the deviation in the reservoir will likely only be a few meters and not
an entire magnitude. An oil reservoir of a hundred meters thickness or more is very unrealistic and
therefore the scale is kept at a low level. The result of the point analysis are collected in figure 3.11
20
Figure 3.11: Thickness of reservoir dependence of electric field amplitude with reservoir (solid line) and without a reservoir
(dashed line) at the location (4000,0,-100) at a frequency of 0.5 Hz. The noise level is marked in blue.
The plot behaves mostly as expected. The amplitude of the electric field is higher when there is a
reservoir present than when there is no reservoir present. The amplitude increases with increasing
thickness of the reservoir. It should be noted that a few meters of change in the thickness of the
reservoir has a strong impact on the strength of the electric field.
After looking at the result for varying thicknesses it is clear that a deviation of a few meters thickness
would have an impact. There would still be a ratio that can be detected because the field strength is
quite high. This result should be taken into account for the survey in Schoonebeek. Combined with
the result for the conductivity of the reservoir the amount of steam injection should be studied in
order to estimate the impact on the measurements made during said survey.
3.2.3 Reservoir depth
The reservoir is estimated to be located at a depth of 685 m to 700 m in the Schoonebeek V2 model.
While it is very unlikely that the reservoir is located at a very different depth than estimated from the
reservoir it should still be checked in order to see if small changes in depth have the same kind of
influence as they did for the reservoir thickness. Once again a few grids are considered to see if there
is a change in the response of the model.
21
Figure 3.12: Overview of ratios at a frequency of 0.5 Hz for a depth of the reservoir of (a)(d)(g) 655 m, (b)(e)(h) 710 m and
(c)(f)(i) 780 m at a depth of (a)(b)(c) 5 m, (d)(e)(f) 50 m and (g)(h)(i) 100 m. The grid has an area of about 10 by 10 km.
Unlike previous parameters that were considered the results in figure 3.12 do not seem to vary much
with increasing depth of the reservoir. Each column appears to have almost the same figures with
only very small changes in the ratio. It seems that the depth of the reservoir has little influence over
the strength of the electric field strength. In order to double check this again a single point is taken at
the location (4000,0,-100) m. Like with the reservoir thickness the scale again is linear and not
logarithmic because there will not be a magnitude change in depth. The result is presented in figure
3.13.
22
Figure 3.13: Depth of reservoir dependence of electric field amplitude with reservoir (solid line) and without a reservoir
(dashed line) at the location (4000,0,-100) at a frequency of 0.5 Hz. The noise level is marked in blue.
As can be seen in figure 3.13 the depth of the reservoir even when it is several tens of meters of has
very little influence. As noted earlier the chance that the reservoir is actually at a very different depth
than estimated in the model is very low. Now it also has been shown that this has very little impact
on the strength of the electric field. This parameter thus does not require extra caution.
3.2.4 Overburden
The reason that two models based on Schoonebeek exist is because the second model was updated
after a log was made of the subsurface of Schoonebeek. While the differences are not extreme there
is still a definitive difference in the first part of the overburden on top of the reservoir. This could
influence the result of the response drastically and as such the updated model was used. The first
model is now briefly considered as it has been shown that the original model might have had some
slight errors. This is also good to check in order to see how much the surroundings influence the
electric field strength resulting from the reservoir.
23
Figure 3.14: Overview of ratios between the Schoonebeek model with and without the reservoir for a frequency of (a)(d)(g)
0.1 Hz, (b)(e)(h) 1.0 Hz and (c)(f)(i) 10.0 Hz at a depth of (a)(b)(c) 5 m, (d)(e)(f) 50 m and (g)(h)(i) 100 m. The area of the
figures is 10 by 10 km.
When comparing the results from figure 3.3 and figure 3.14 it is evident that there is no apparent
difference between the two of them. This is not strange as the Schoonebeek model and the
Schoonebeek V2 model are very much alike. This result seems to indicate that the original model was
in reality reasonably accurate. Slight changes in the model as was shown with reservoir depth do not
seem to matter very much.
24
Conclusion and discussion
The results shown in Chapter 3 of this thesis have provided a good indication for the parameters that
should be taken into account during the survey in Schoonebeek. The setup with a horizontal line
source and a vertical receiver shows much promise for good measurements in the field. The
frequency used for transmitting the signal seems to be the best somewhere between 0.1 and 1 Hz.
The receivers should be placed between 3 and 6 km away from the source in order to receive good
data for every transmitting frequency. It has also been shown that the effects of the steam injection
in the Schoonebeek reservoir are important to keep in mind. The electric conductivity of the
reservoir and the thickness of the reservoir have very strong impacts on the response of the electric
field. The reservoir depth and slight changes in the overburden do not have the same amount of
impact.
All of the results in this thesis are based on simplified 1D-heterogeneous models. For increased
certainty that the layer approximation of the subsurface is valid more realistic and complicated 3Dheterogeneous models based on the Schoonebeek subsurface should be evaluated. The rest of the
Schoonebeek model should also be updated in order to be more accurate to the truth. Data obtained
from the survey in Schoonebeek should also be compared with the results found in this thesis in
order to check if the modeled results were reasonably accurate with reality.
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Acknowledgements
The research I did for this thesis were made possible by many people from whom I have received
support and encouragement without which I could not have finished my work. My sincere gratitude
goes out to these people in particular:
Andreas Schaller, MSc at TU Delft, for his supervision and availability whenever I needed his help.
Without him this thesis would not have been possible.
Dr. Guy Drijkoningen, associate professor at TU Delft, for his supervision and advice which have
helped me with my research and of course for finding this project for me to work on.
Dr. Jürg Hunziker from TU Delft, for allowing me to use his code and assistance on working with his
code.
Dr. Rita Streich from Shell (GSNL), for sharing her knowledge of CSEM-measurements with me.
26
Self-Evaluation
I believe that my BSc project went very well. This is mainly thanks to the fact that I have had very
good supervision. From the first day it was very clear to me what I was supposed to do and how I
should do it. Andreas Schaller always had time free for me and Dr. Drijkoningen was prepared to
make time free in his busy schedule to meet me. The main difficulty that I had during the work was
understanding the theory for Electromagnetism. The subject is very deep and complicated and
although I believe my understanding has greatly increased I still have a lot to learn about the subject.
Another problem I had was working with the code that I employed during the project. It was the first
time I used a code like this and it took me a while to figure it out. Thankfully Dr. Hunziker, its creator,
was very helpful with this. I also found several bugs in the code which halted my progress at times
but which have improved the modeling code and also helped me to know what to look for when
testing models. The environment I was working in was very stimulating. I met a lot of PhD students
who worked very hard and made me aspire to put more effort in myself. It was a good experience to
see what being a PhD is like and what should be taken into account when working on an important
project. The fact that some of my results were used for the actual survey was very encouraging and
made me feel appreciated. I also believe that the fact that I combined my minor and my BSc project
increased the quality of my BSc project significantly. Considering how much I understood about the
subjects and the working of the modeling code I used after my first six weeks the report then would
have a lot less depth. Six weeks for a BSc thesis is in my opinion too short. All of the experience from
my minor was very useful during my BSc project. I also am very happy to have done my BSc thesis for
the section of Applied Geophysics because I wanted to experience if this particular field was
something I wanted to do for my master. After several months I believe the answer to this is yes. The
one thing I would have liked to do was working with the data obtained during the survey in
Schoonebeek. The fieldwork was delayed however so this was no longer possible.
All in all I think working on this project has not only increased my knowledge on EM but also on
functioning within a project, what comes after the bachelor and which master I want to take. The
time I spent on my thesis has been much more effective and I believe the result has shown itself.
27
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