KMS Technologies KJT Enterprises Inc. Presentation Thomsen, L., Strack, K. M., Brady, J., Biegert, E. 2003 A Novel Approach to 4D: Full Field Density Monitoring Society of Exploration Geophysicists, Annual Meeting, Dallas, Invited Paper in workshop 4D Time Lapse Rock Properties
A Novel Approach to 4D: Full Field Density Monitoring by Leon Thomsen 1, Kurt Strack 2, Jerry Brady 1, Ed Biegert 3 1 BP 2 KMS Technologies 3 Shell SEG Research Workshop Dallas, Oct. 31, 2003 6/19/2007 1
Outline Motivation Principles Surface Borehole The Future 6/19/2007 2
The key to effective reservoir management: 4D 4D means: Measurements in 3D + elapsed time Not necessarily seismic monitoring More frequent is better Hence, cheaper is better Boreholes offer significant opportunities for monitoring 6/19/2007 3
4D gravity makes sense when 4D seismics has difficulty Surface difficulties: Permafrost, sand, swamp, Infrastructure Subsurface difficulties: Gas cloud Salt bodies 6/19/2007 4
4D gravity makes sense when production makes reservoir density changes High porosity Gas -> brine (oil -> brine) Formation collapse 6/19/2007 5
Outline Motivation Principles Surface Borehole The Future 6/19/2007 6
As a reminder Every particle in the entire earth pulls on a gravity meter; the sum of all this force is F g = mg 980 ; cm / s 2 = Fluid movements make changes in gravity in micro-gals,ie in parts per billion! 980 Gals 6/19/2007 7
As a reminder II The direction of gravity defines the vertical: g In a vertical borehole, we can measure the vertical gradient of gravity: g z, z g z g z z 6/19/2007 8
As a reminder III The gravitational attraction of an infinite slab of thickness z is: g z = 4πGρ z Hence the measured gravity gradient can be expressed in terms of the equivalent density of an infinite slab: gz 4πGρ apparent z 6/19/2007 9
We can do it because Gravity meters are better than when you were a student They are getting even better! We can detect and interpret changes better than absolute quantities. 6/19/2007 10
Conventional (relative) gravity meter measures force Refinements of this basic design have been the state of the art until recently. But they lack the long-term stability needed for 4D gravity. 6/19/2007 11
Absolute gravity meter measures acceleration directly This design appears to offer more promise for 4D gravity, if sufficient accuracy can be achieved. FG-5 Absolute Gravimeter courtesy of Micro-G Solutions Inc. 6/19/2007 12
Relative & absolute gravity meters Absolute Relative 6/19/2007 13
Outline Motivation Principles Surface Borehole The Future 6/19/2007 14
Waterflood Monitoring with 4D Surface Gravity Perform baseline gravity survey prior to injection Water injection creates a positive gravity anomaly Gravity survey repeated yearly 4D gravity signal = (repeat baseline ) 4D gravity difference inverted for density changes Reservoir density change => water movement Constraints on the inversion: Changes confined to reservoir Mass balance 6/19/2007 15
Prudhoe Bay, Alaska After Hare et al., 1999 6/19/2007 16
Prudhoe Bay: Change in bulk density 2.30 Bulk Density (g/cm 3 ) 2.28 2.26 2.24 2.22 2.20 2.18 S g = 0.2 S g = 0.4 Water Gas S g = 0.6 2.16 0.0 0.2 0.4 0.6 0.8 1.0 φ = 0.22 ρ ma = 2.65 g/cm 3 ρ o = 0.762 g/cm 3 ρ w = 0.986 g/cm 3 ρ g = 0.198 g/cm 3 Residual water saturation = 0.32 Fraction of Oil Swept After Brady, 1998 6/19/2007 17
Gravity Inversion Vs Density Model 6/19/2007 18
Modeling gravity is easier than modeling seismic 6/19/2007 19
Absolute Gravity Meter Repeats in 2002 Survey microgals Station Repeats Average Standard Deviation Standard Error 999 7 470.4 3.1 1.2 102 5 9365.5 5.6 2.5 457 3 4847.3 1.7 1.0 106 2 9895.0 4.7 3.3 406 2 5951.2 0.7 0.5 409 2 5968.3 1.7 1.2 431 2 5609.7 6.0 4.3 458 2 4839.6 2.6 1.9 503 2 3364.7 3.0 2.1 509 2 1906.5 6.4 4.5 Average 3.5 2.2 The noise floor for repeat surveys is a few microgals. 6/19/2007 20
Preliminary GPS results Elevation errors lead to gravity errors: 3 µgals/cm 6/19/2007 21
4D Surface Gravity at Prudhoe Bay: Conclusions 1. General water front shape and movement can be monitored with the surface gravity technique 2. Mass-balance techniques can account for more than 95% of the water added. 3. Inversion modeling can determine the average waterflood front and a reasonable location for the leading edge of the waterflood front. 4. Absolute gravity meter has been refined to the point that it is a field-worthy tool that can be used as an integral portion of the surface gravity survey. 6/19/2007 22
Modeling can easily test whether these conclusions apply to another field Waterflood in reservoir, seen from above 4D gravity anomaly at the surface. 10 µgal contour Conclusion: for this field, you can't see the water finger from the surface 6/19/2007 23
Outline Motivation Principles Surface Borehole The Future 6/19/2007 24
Borehole gravity measurements Gravimeter: Very deep investigation; need several boreholes to invert for density. Or, subtract independent readings to derive apparent density. Gravity Gradiometer: Direct measurement of gradient g z,z. Fixed frame of reference ρb A B Multi-component Gradiometer: Directional interpretation from g z,x, g z,y, g z,z Higher-order derivatives: Different depths of investigation ρb 1 ρb 2 After Strack et al. 1999 6/19/2007 25
Borehole Gravimeter (BHGM) LaCoste & Romberg zero-length spring balance Capacitor force related to gravitational acceleration Standard deviation: 3 µgal. Spring Beam Capacitor s s Plate Courtesy Edcon Inc. Capacitor s s Plate 6/19/2007 26
Borehole Gravity Used for Reservoir Surveillance Problems with current borehole surveillance tools: All shallow-reading devices have problems with: - Straddles, Scab liners, Gas channels, Gas cones, Perforations Primary Advantage of Borehole Gravity: A very deep-reading tool Gravity meter reaches a few hundred meters Gradiometer: lateral reach ~5X vertical spacing Primary Disadvantage of the BHGM tool: - The tool has never been developed for surveillance - Tool OD is too large - Only works in near-vertical wells - Depth control is difficult to achieve 6/19/2007 27
Borehole Gravity Applications: Large Gas Cone 180 180 80 Large Cone 140 140 60 0.6 feet feet 100 100 60 60 20 20 1000 1000 0 feet 0 feet -1000-1000 0 0 500 500 1400 ft 1400 ft Original Original Gas/Oil Contact Gas/Oil Contact 1000 1500 2000 2500 1000 1500 2000 2500 feet feet Differential Gravity (µgal) 40 0.4 20 0.2 0 S gas -20 10000 10500 11000 11500 12000 12500 Measured Depth (ft) 6/19/2007 28 After Brady, 1999
Borehole Gravity Applications: Small Gas Cone 180 180 140 140 50 40 Small Cone 0.6 feet feet 100 100 60 60 20 20 1000 1000 0 feet 0 feet -1000-1000 0 0 500 500 200 ft 200 ft Original Original Gas/Oil Contact Gas/Oil Contact 1000 1500 2000 2500 1000 1500 2000 2500 feet feet Differential Gravity (µgal) 30 0.4 20 0.2 10 0 S gas -10 10000 10500 11000 11500 12000 12500 Measured Depth (ft) 6/19/2007 29 After Brady, 1999
Borehole Gravity Applications: Salt dome overhang Model Data 6/19/2007 After Black, 1998 30
Borehole Gravity Applications: Salt dome overhang II Model Data 6/19/2007 After Black, 1998 31
Borehole Gravity Applications: Waterflood fingering These wells indicate: The spatial variation at any one time; The temporal variation at a single place. 6/19/2007 32
Cross-section though the finger 30m reservoir at 2500m depth Do these wells see the finger early? Maximum density change ρ=.05 gm/cm 3 6/19/2007 33
The 4D gravity signal can be seen for ~ 200m Borehole gravity, finger model apparent density, gm/cm3 2000m Depth, m 2300 2350 2400 2450 2500 2550 2600 2650 2700 2750 2800 0 1 1000m 100m breakthrough 50m out 100m out 150m out 200m out 500m out 2000m, offset 1000m, offset 0m, offset 6/19/2007 34
The gravity gradient signal falls off much faster Borehole gravity gradient, finger model apparent density, gm/cm3 Depth, m 2300 2350 2400 2450 2500 2550 2600 2650 2700 2750 2800 0 0.5 2000m 1000m 100m breakthrough 50m out 100m out 2000m, offset 1000m, offset 0m, offset 6/19/2007 35
Outline Motivation Principles Surface Borehole The Future 6/19/2007 36
The path to reservoir surveillance through borehole gravity : Take new measurements downhole: Absolute gravity Optical interferometry Long-term stability-> 4D gravity Cheap, permanent instruments in many wells Active reservoir management 6/19/2007 37
Borehole Absolute Gravimeter Evacuated chamber Gimbal mount for strongly deviated wells Fiber-optic link uphole Piezo-launcher Small size: Initially 2.5-3.5 Ultimately 1-11/16 Very inexpensive Sensors < $5000 Laser = $100s Fiber Optics = $10s Note: accuracy does not reduce as the size is reduced. 6/19/2007 38 After Strack et al. 1999
Borehole Absolute Gravity Gradiometer Chamber 1 Separation adjustable Chamber 2 Courtesy EMG Inc. 6/19/2007 39
Multiple Absolute Station System "MASS" Laser Splitter GAS 100'-1000' OIL Sensor heads Multiple Stations provide monitoring network Drift-free instruments allow long term monitoring 6/19/2007 40
This system does not yet exist Now in engineering phase No significant problems loom Manufacturing phase in ~ 1 year We welcome partners in the development and early application of this new means for 4D active reservoir management. 6/19/2007 41
Acknowledgements We thank: BP KMS Technologies Shell and their technology partners for making this development possible. 6/19/2007 42
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