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(arctani j

iijkiiijki

ijkk

iik

k

kjionmonm yRyxRxRz

yxZGg …(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).

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).

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).

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).