In recent years, the time-lapse microgravity method has been used for fluid movement monitoring in hydrocarbon reservoirs. The basic principal of this method is to measure different gravity anomaly at surface which representing density changes within the reservoir. This density changes come from different pore fluid in certain time interval which means a fluid movement in the subsurface. Furthermore, we should understand the density changes within the reservoir prior to do the gravity inversion. However, the inversion results sometimes inaccurate and unstable. So, to increase the quality of the inversion we can add the time-lapse borehole gravity log. Time-lapse borehole microgravity measures different gravity anomaly at the wells in certain time interval. This method captures the gravity attraction along the well in certain depth interval with in order to increase the gravity signal in the subsurface. The data obtained from this method looks like log data. This log data can be an additional input for gravity inversion process to improve vertical and lateral sensitivity near the well. The inversion that writer used is damp least square inversion method. In this paper, the writer will show the inversion result between the surface microgravity inversion only versus the combination of surface and borehole microgravity inversion. These scenarios will be applied in synthetic data. The integration of time-lapse surface and borehole microgravity method shows us a better result than the surface microgravity only. It can be seen that the boundary and the thickness of gravity anomaly body are clearer. It means that we can get better vertical and horizontal resolution result. By this simulation, we can conclude that time-lapse borehole microgravity could be an alternative way to improve gravity inversion method and give us more information about fluid movement within the reservoirs.

1. Enhance Gravity Inversion Result Using
Integration of Time-lapse Surface and
Borehole Microgravity
By:
Andika Perbawa(1)(2), Wawan G. A. K(3)
(1)Medco E&P Indonesia
(2) Formerly Geophysical Engineering ITB
(3) Geophysical Engineering ITB
The Bali 2010 International Geosciences Conference and Exposition,
Bali, Indonesia, 19-22 July 2010

2.  Background
 Basic Theory
 Forward Modeling
 Inverse Modeling
 Analysis
 Conclusions
 Recommendations

3.  Time-lapse microgravity survey
 Supporting production management,
 Monitoring fluid movement.
 Acquisition
 At least two gravity measurements (Kadir, et. al., 2003).
 Limitation in vertical resolution.
 Time-lapse borehole microgravity survey
 Enhance the signal sensitivity,
 Sharpen vertical resolution.
 Inversion
 Understanding distribution of density contrast.

4.  Background
 Basic Theory
 Forward Modeling
 Inverse Modeling
 Analysis
 Conclusions
 Recommendations

5. (Plouff, 1976)
   








2
1
2
1
2
1
,, )log()log(arctan,,
i j
iijkiiijki
ijkk
ii
k
k
ijkrqpz yRyxRx
RZ
yx
ZGg rqp

222
kjiijk zyxR 
     kji
ijk 111 
Where:
∆ρ=(+)

6. ∆gmgal
mgal
1st measurement 2nd measurement
=
=
TLSM
TLBM
Density contrast
t1 t2 ∆t=t2-t1
µgal
µgal
∆g

7. ρmatrix (gr/cc) ρoil (gr/cc) ρwater (gr/cc) Φ
2.65 0.85 1 27%
ρb= (1-Φ) ρmatrix + Φ (So*ρoil+Sw*ρwater)
1st Condition (100% oil)
ρb(1) = (100% - 27%)2.65 + 27%(100%*0.85 + 0%*1)
ρb(1) = 2.16 gr/cc
2nd Condition (100% water)
ρb(2) = (100% - 27%)2.65 + 27%(0%*0.85 + 100%*1)
ρb(2) = 2.2 gr/cc
∆ρ=ρb(2)-ρb(1)
∆ρ=2.16-2.2
∆ρ=0.04 gr/cc
(Schön, 1995)

8. ρ1=2
ρ1=2.1
ρ1=2.16
ρ1=2.3
ρ1=2.4
ρ2=2
ρ2=2.1
ρ2=2.2
ρ2=2.3
ρ2=2.4
∆ρ=0
∆ρ=0
∆ρ=0.04
∆ρ=0
∆ρ=0
- =
2nd
condition
1st
condition
Density
contrast
TLBM
response
(+)
(-)
ρ in (gr/cc)

9.  Relationship between data and model parameter:
 Damp least square inversion solution:
  dGGGm
mGd
TT 1


  dGIGGm
TT 12 
 
(Grandis, 2008)
m : density contrast
G : geometry factor matrix
ε : damping factor
I : identity matrix
d : gravity anomaly

10.  Background
 Basic Theory
 Forward Modeling
 Inverse Modeling
 Analysis
 Conclusions
 Recommendations

11. Δρ
(gr/cc)
Borehole
location
Positive
anomaly
(red)
Negative
anomaly
(blue)
4 Layers
Layer-1
Layer-2
Layer-3
Layer-4

12. (a)3D view (b)map view
(c)Cross section
Body anomaly location
(black rectangle)

13. Separate into two bodies
∆ρ(-)
∆ρ(+)

14.  Background
 Basic Theory
 Forward Modeling
 Inverse Modeling
 Analysis
 Conclusions
 Recommendations

15. Actual Model TLSM Inversion
TLBM Inversion Joint Inversion
Δρ
(gr/cc)
Actual model Δρ
(gr/cc)
TLSM Inversion
Δρ
(gr/cc)
TLBM Inversion Δρ
(gr/cc)
Joint Inversion

16. Actual model Surface inversion
Borehole inversion Joint inversion

17. Actual model Surface inversion
Borehole inversion Joint inversion

18. Actual model Surface inversion
Borehole inversion Joint inversion

19. Actual model Surface inversion
Borehole inversion Joint inversion

20.  Background
 Basic Theory
 Forward Modeling
 Inverse Modeling
 Analysis
 Conclusions
 Recommendations

21. Δρ
(gr/cc)
Actual model
Joint inversion
Borehole inversion
Surface inversion

22. -0.04
-0.03
-0.02
-0.01
0
0.01
0.02
0.03
0.04
0 100 200 300 400
Density Profile at Layer 1
-0.03
-0.02
-0.01
0
0.01
0.02
0.03
0.04
0 100 200 300 400
Layer 2
-0.03
-0.02
-0.01
0
0.01
0.02
0.03
0.04
0 100 200 300 400
Layer 3
-0.03
-0.02
-0.01
0
0.01
0.02
0.03
0.04
0 100 200 300 400
Layer 4
Layer 4 Origin
Layer 4 from surface
inversion
Layer 4 from All
Borehole inversion
Layer 4 from Join
Inversion
Initial model
TLSM
TLBM
Joint
(∆ρ) (∆ρ)
(∆ρ) (∆ρ)

23. Δρ
(gr/cc)
Actual model
Joint inversion
Borehole inversion
Surface inversion

24. -0.03
-0.02
-0.01
0
0.01
0.02
0.03
0.04
0 100 200 300 400
Density Profile at Layer 1
-0.03
-0.02
-0.01
0
0.01
0.02
0.03
0.04
0 100 200 300 400
Layer 2
-0.03
-0.02
-0.01
0
0.01
0.02
0.03
0.04
0 100 200 300 400
Layer 3
-0.03
-0.02
-0.01
0
0.01
0.02
0.03
0.04
0 100 200 300 400
Layer 4
Layer 4 Origin
layer 4 from surface
inversion
Layer 4 from all
borehole inversion
Layer 4 from join
inversion
Initial model
TLSM
TLBM
Joint
(∆ρ) (∆ρ)
(∆ρ) (∆ρ)

25. RMS error (gr/cc)
surface inversion borehole inversion joint inversion
0.010 0.006 0.005

26.  Background
 Basic Theory
 Forward Modeling
 Inverse Modeling
 Analysis
 Conclusions
 Recommendations

27.  Time-lapse Borehole Microgravity
 Forward modeling : good vertical resolution.
 Inverse modeling : increase sensitivity to detect
density contrast.
 Joint inversion shows the best result to
determine density contrast distribution.

28.  Background
 Basic Theory
 Forward Modeling
 Inverse Modeling
 Analysis
 Conclusions
 Recommendations

29.  Apply joint inversion for real data.
 Try other inversion methods to optimize the
result,
 Try other models (more complex),
 Analyze how much optimum boreholes that we
need and the distance between them,

30. Thank You

31. Back Slide

32. Detail inversion result of TLSM

33. Inversion using 1 borehole
Δρ
(gr/cc)

34. Inversion using 1 borehole
±75 meter
radius

35. Error distribution of iteration
TLSM Inversion Error
TLBM Inversion Error
Joint Inversion Error

36.  Resolution : 1-20 µgal
 Gas-oil  ± 2 µgal
 Gas-water  ± 5 µgal
 Oil-water  0.7-3 µgal
 Accessable casing up to 5½ inch.
 14 degree from vertical.
(Nabighian, et. al., 2005)
Instrument
Instrument of
boreholegravitymeter.
(Goodell, R. R., 1964).