Influence of pore pressure on velocity in low-porosity sandstone: Implications for time-lapse feasibility and pore-pressure study

摘要

As seismic data quality improves, time-lapse seismic data is increasingly being called upon to interpret and predict changes during reservoir development and production. Since pressure change is a major component of reservoir change during production, a thorough understanding of the influence of pore pressure on seismic velocity is critical. Laboratory measurements show that differential pressure (overburden minus fluid pressure) does not adequately determine the actual reservoir conditions. Changes in fluid pressure are found to have an additional effect on the physical properties of rocks. The effective-stress coefficient n is used to quantify the effect of pore pressure compared to confining pressure on rock properties. However, the current practice in time-lapse feasibility studies, reservoir-pressure inversion and pore-pressure prediction is to assume that n = 1. Laboratory measurements, reported in both this and previous research show that n can be significantly less than unity for low-porosity rocks and that it varies with porosity, rock texture and wave type. We report the results of ultrasonic experiments to estimate n for low-porosity sandstones with and without microcracks. Our results show that, for P-waves, n is as low as 0.4 at a differential pressure of 20 MPa (about 3000 psi) for a low-porosity sandstone. Thus, in pore-pressure inversion, an assumption of n = 1 would lead to a 150% underestimation of the pore pressure. Comparison of the effective-stress coefficient for fractured and unfractured samples suggests that the presence of microfrac-tures increases the sensitivity of P-wave velocity to pore pressure, and therefore the effective-stress coefficient. Our results show that the effective-stress coefficient decreases with the differential pressure, with a higher differential pressure resulting in a lower effective-stress coefficient. While the effective-stress coefficient for P-wave velocity can be significantly less than unity, it is close to one for S-waves.