Processing and modelling of time-lapse changes in shear-wave splitting in Vacuum Field, New Mexico

• Main Author: Angerer, Erika
• Published: University of Edinburgh 2001
• Subjects: 553
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.640542

Time-lapse seismic is proving to be a valuable method for monitoring subsurface pressure changes and fluid movements in producing hydrocarbon reservoirs. This case study aims to show the importance of multicomponent seismic data in time-lapse monitoring as shear wave splitting proves to be a highly sensitive diagnostic for monitoring hydrocarbon production. The investigated 4D, 3C land seismic data are from Vacuum Field, New Mexico, USA with a fractured dolomite forming the reservoir of interest. In 1995,<i> </i>over a period of two months, two seismic surveys were acquired before and after a pilot tertiary recovery program of CO₂-injection in a single well. The pore fluid pressure in the reservoir increased significantly from a ‘normal’ pressure of 10.6 MPa to an ‘overpressure’ of 17 MPa. CO₂is a highly compressible fluid at these pressures and therefore decreases the bulk modulus of the reservoir fluid. After the application of a data processing sequence that aims to preserve anisotropy and maximise repeatability, the interval time analysis of the reservoir interval shows a significant 10% change in shear-wave anisotropy and a 3% decrease in the <i>P</i>-interval velocities to the south and east of the CO₂-injection well. The fast shear wave becomes the slowest after the injection. Further, the rms-amplitudes of the shear-wave parallel to the maximum horizontal stress direction decrease in the area of the anisotropy anomaly after injection. In 1997 and 1998 two further surveys were acquired one year apart in Vacuum Field during a major CO₂-injection program. Due to repeatability issues these data are analysed separately. Small pore fluid pressure increases of up to 1.4 MPa in the reservoir led to velocity decreases of generally less than 2% in <i>P </i>and in shear. In order to interpret the observations several 1D models incorporating both saturation and pressure changes are matched to the data in the vicinity of the injection well. Due to the high stiffness of the dolomitic reservoir rock the saturation changes have little effect on the seismic velocities.