Now a day, the application of 4D surface microgravity technology for identification density changes of fluid reservoir is rapidly develop. But, it still has a resolution limitation in vertical density variation. Alternatively, borehole gravity meter technique can resolve this problem. In this research, to get effectiveness in measuring borehole gravity response, a forward modeling code program has been created for some complex synthetic models. A complex model means that the reservoir has some faults, anticline, syncline, and wedge out shape. And then, a characteristic analysis of gravity anomaly response has been done relates to amplitude, wavelength, boreholes space and model geometry. The result of synthetic modeling supported by amplitude attribute analysis shows that the shape of complex synthetic model and its depth can be identified clearly. This paper shows a simulation of fluid movement in reservoir with different models at different times. With this simulation, we can see that 4D borehole microgravity is useful to monitoring the fluid movement in reservoir.

1. Synthetic Modeling of 4D Borehole Microgravity for Fluid Movement Monitoring in Complex
Structure Models
Andika Perbawa1
, Indah Hermansyah Putri1
, Wawan Gunawan A. Kadir1
, Susanti Alawiyah1
Institute of Technology Bandung
Abstract
Now a day, the application of 4D surface
microgravity technology for identification density changes
of fluid reservoir is rapidly develop. But, it still has a
resolution limitation in vertical density variation.
Alternatively, borehole gravity meter technique can resolve
this problem.
In this research, to get effectiveness in measuring
borehole gravity response, a forward modeling code
program has been created for some complex synthetic
models. A complex model means that the reservoir has
some faults, anticline, syncline, and wedge out shape. And
then, a characteristic analysis of gravity anomaly response
has been done relates to amplitude, wavelength, boreholes
space and model geometry.
The result of synthetic modeling supported by
amplitude attribute analysis shows that the shape of
complex synthetic model and its depth can be identified
clearly. This paper shows a simulation of fluid movement
in reservoir with different models at different times. With
this simulation, we can see that 4D borehole microgravity
is useful to monitoring the fluid movement in reservoir.
Keywords: 4D Borehole Microgravity, Monitoring,
Complex Model, Amplitude Attributes Analysis
Introduction
If we want to monitor fluid movement in reservoir, in this
case is a movement of oil or water injection, 4D
Microgravity is commonly used. But, this method has a
limitation with vertical resolution. Sometimes, response of
gravity anomaly is too small so our instrument can’t read
that changes. To solve this problem we must to measure the
gravity anomaly closer with the source so it will be increase
our signal and determine the boundary of our target. This
way called borehole gravity measurements.
To do this fluid movement simulation that represented by
density changes in reservoir, writer created a code program
using Matlab language called BHGM AP2009 (Borehole
Gravity Measurements Andika Perbawa 2009). Not like
usual gravity measurement that the acquisition at surface,
BHGM measure the gravity anomaly on borehole or well.
So, the instrument will closer to the source body density.
With this way, we can increase the signal of anomaly
gravity and small response problem will be solved. The
interface program that writer created shown on Figure 1.
The body of anomaly density use a prism or cube approach
(Plouff, 1976) which follow below formula (Equation 1):
2
1
2
1
2
1
,,,,,, )log()log(arctan
i j
iijkiiijki
ijkk
ii
k
k
kjionmonm yRyxRx
Rz
yx
ZGg …(1)
Where gm,n,o is gravity anomaly caused of a cube body in
m,n,o coordinate, G is gravity constant, ∆ρm,n,o is density
Figure 1. Interface of BHGM AP2009 program (Perbawa,
2009).

2. contrast of cube body in m,n,o coordinate, x,y,z is border of
cube body in x-axis, y-axis, and z-axis and Ri,j,k is length
between measurement station with body center coordinate.
Data and Method
On Figure 2, we can see characterictic of BHGM log on
time-lapse or 4D data. When the borehole pierce the body,
our response and gravity anomaly amplitude will be
increase. But, if our borehole not pierce the body so, our
response will be weak. With this borehole gravity
characteristic, we can determine geometry of our body
more accurately than if we only use surface gravity data.
Top of body represented by maximum or minimum
amplitude depend on density contrast of our body.
Initially, our reservoir contain full with oil. In certain time
interval, in one place, there is increasing of density in
reservoir because of water injection. And the other place,
there is decreasing because of production activity. Our
target is density changes because of the injection and
production activity called density anomaly. We deploy
some BHGM above that target. And then start to measure
the anomaly gravity in 4 times. The synthetic model and
design survey showed in Figure 3.
On Figure 4, there are 3 production wells and 3 injection
wells. Production wells contribute to decreasing density
contrast on body while injection wells contribute to
increasing density contrast. And then, we create a
simulation fluid movement on a body (represent a
reservoir). That simulation will be shown on Figure 5, 6
and 7.
Figure 2. (a) Example body, (b) design survey and, (c)13
borehole gravity meters that used. Below picture is
characteristic of BHGM log.
Figure 3. (a) Synthetic model of complex structure, (b)
Model map showed on x and y axis, (c) Survey design of
BHGM using 20 wells (60 meter space).
Figure 4. Geometry of body and location of production well
(bold black circle) and injection well (man’s sign).

3. Result and Discussion
Figure below shown gravity anomaly using borehole
gravity meter. Figure 5 is simulation of fluid movement in
first time-lapse. Figure 6 is simulation of fluid movement in
second time-lapse. And Figure 7 is simulation of fluid
movement in third time-lapse. All of time-lapse segments
relative to first survey.
Figure 5. The first simulation of fluid movement. (a) the origin gravity anomaly using
borehole gravity meter, (b) gravity response on surface, (c) gravity anomaly using
amplitude attribute (power).
Figure 6. The second simulation of fluid movement. (a) the origin gravity anomaly
using borehole gravity meter, (b) gravity response on surface, (c) gravity anomaly
using amplitude attribute (power).

4. On Figure 5, 6, and 7 has shown a fluid movement with
yellow arrow as its direction. Decreasing density of body
cause of production showed as blue-red color (upper to
lower) while increasing density of body cause of injection
showed as red-blue color. Those color represent a border or
body (top and base) while center of body represented by
white color (between those color).
If we see surface gravity anomaly, maximum amplitude for
that body is around 1 – 2.5 μGal. It is because of size of
body is small enough or its density contrast is too small. If
we only counting to surface gravity measurement, we
cannot see fluid movement in our reservoir. But, if we
come closer to our reservoir so the gravity anomaly
response will be increase and we can easily interpret where
our injection water located. However if we combine
borehole gravity measurement with surface gravity
measurement the result will be better. In other hand, these
methods help us to understanding about sealing or linking
fault. Two of that fault on model is linking fault because
water injection from other room can move into another
room.
Amplitude attribute help us to increase signal of our target.
This mechanism is multiplication amplitude with amplitude
so the result will be absolute. Big response will be increase
while small response will almost disappear. With this way,
we can see the center of our density contrast and shown
where the fluid move (Figure 6(c) and 7(c))
Conclusions
1.Borehole gravity measurements help us to increase
signal of gravity anomaly while surface gravity
measurement hard to read it.
2.Borehole gravity can show top and base of body
clearly.
3.Borehole gravity can be used to monitor fluid
movement on complex structure reservoir.
4.Borehole gravity helps us to understanding about
sealing and linking fault.
5.Amplitude attribute help us to determine the
direction of fluid movement.
References
Perbawa, A., 2009. Pemodelan Sintetik Metode
Gayaberat Mikro Selang Waktu Lubang Bor.
Bachelor Thesis, Institut Teknologi Bandung.
Plouff, D., 1976. Gravity and magnetic fields of polygonal
prisms and application to magnetic terrain
correction: Geophysics, 41, 727-741.
Acknowledgements
Institute of Technology Bandung
HIMA-TG “TERRA” ITB.
Figure 7. The third simulation of fluid movement. (a) the origin gravity anomaly using
borehole gravity meter, (b) gravity response on surface, (c) gravity anomaly using
amplitude attribute (power).