Time‐lapse seismic changes in a CO₂injection process in a fractured reservoir

Abstract

The time delay changes of stacked time-lapse shear-wave data show a significant 10% change in anisotropy before and after a CO 2 injection process in a fractured dolomite reservoir in Vacuum Field, New Mexico. The time-lapse Pwave data do not show comparable variations in interval travel times. A combined model based on the anisotropic poro-elasticity theory, APE, (Zatsepin and Crampin, 1997) and on Kuster-Tokszos modelling matches the stacked data. Both the saturation change and the measured pore-fluid pressure increase of 1000 psi are considered. The model explains these observations as a pressure effect with the dynamic opening and closing of near-vertical low aspect ratio pores to which shear-wave splitting is more sensitive than P-wave velocities.

Indication of high pore-fluid pressures in a seismically-active fault zone

“…The polarisations of the seismic stress‐aligned shear‐wave splitting observed in almost all rocks are generally parallel to the direction of maximum horizontal stress (Crampin et al 1980; Crampin 1994, 1999). However, in the presence of sufficiently high pore‐fluid pressures, the polarisations become orthogonal to the maximum horizontal stress (Zatsepin & Crampin 1997) in what are known as 90°‐flips (Angerer et al 2000, 2002).…”

Shear-wave splitting in a critical crust

“…Such 90j flips have previously been observed in vertical seismic profiles in a critically pressurised reservoir in the Caucasus Oil Field (Crampin et al, 1996;Slater, 1997) and in reflection surveys (Angerer et al, 2000(Angerer et al, , 2002 of a critically pressurised CO 2 injection. These 90j flips have also been observed above small earthquakes immediately above the San Andreas Fault in California by Liu et al (1997), who recognised the significance of 90j flips, and also by Peacock et al (1988) and Crampin et al (1990Crampin et al ( , 1991, although at that time, the significance of the 90j flips had not been established.…”

“…This has important implications for hydrocarbon reservoir characterisation, hydrocarbon recovery, and stress-forecasting earthquakes (Crampin, 2000b;Crampin and Chastin, 2001). Substantially improved recovery and calculation (even prediction) of the response of the reservoir to some recovery operations may now be possible by being able to understand and model the changes (Angerer et al, 2000(Angerer et al, , 2002.…”

“…This is a result of the changes in the threedimensional distribution of crack aspect ratios as pore pressure approaches values when the rock would hydraulically fracture. Angerer et al (2000Angerer et al ( , 2002 called such phenomena 90j flips. It is well known that high pore-fluid pressures are needed to relieve frictional stress on lithostatically clamped faults before slippage and earthquakes can occur, and we interpret the changes in shear-wave polarisations in Fig.…”

“…Nevertheless, since the underlying assumption of APE is that the fluid-saturated cracks in in situ rock form a CCC system verging on fracture criticality and failure , even the approximate match of APE to a large range of phenomena is confirmation, albeit indirect, that the cracks in the crust are a CCC system. Crampin and Chastin (2001) at 9IWSA reported the successful modelling (in effect, prediction with hindsight) of the response of the reservoir to two CO 2 injections by Angerer et al (2000Angerer et al ( , 2002. One of the pressures was high enough to cause 90j flips in shearwave polarisations in the injected reservoir, where the faster split shear wave flipped from approximately parallel to approximately orthogonal to the direction of maximum horizontal stress.…”