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

Abstract

A B S T R A C TAs 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 microfractures 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. I N T R O D U C T I O NPressure strongly influences the mechanical and transport properties of rocks, such as porosity, velocity, permeability and resistivity. In a fluid-saturated rock, both pore pressure and confining pressure control the rock properties. Seismic and borehole techniques measure these rock properties in order to infer subsurface information. For example, time-lapse seis- * mic is often employed to find bypassed reserves and reservoirpressure distribution. Differences in seismic attributes (a combination of compressional-and shear-wave velocities and density) between monitor and base surveys are used to infer pore pressure and saturation changes caused by depletion and injection.To relate changes in seismic attributes to reservoir conditions, a thorough understanding of pressure and saturation effects on rock properties is essential. Gassmann's theory (1951) is generally accepted as a prediction tool for velocity change

Pore-Pressure-Coefficient Anisotropy Measurements for Intrinsic and Induced Anisotropy in Sandstone

“…4 and 5, respectively. We compared our compressional-wave velocity (Vp) measurements with those reported by Xu et al (2006) for Lyons sands. Although their measurements were taken under hydrostatic state of stress, their initial measured compressional-wave velocity at zero stress (i.e., 4250 m/s) agrees with our values for the vertical sample (i.e., 4283 m/s).…”

The pressure dependence of acoustic velocity and quality factor — New petrophysical models

“…(6) applies well in practice. It can also be seen that in the lower pressure region, the increase in 3324.9 825.2 0.1341 Lyons sample (Xu et al 2006) 3944.4 784.7 0.1069 velocity with increasing pressure is very steep and nonlinear; this is due to the closure of microcracks. In the higher pressure domain, the increase in velocity (with increasing pressure) is moderate as fewer number of cracks are closed.…”

“…It had an average porosity of 16%, permeability of 75 mD, and a bulk density of 2.61 g/cm 3 . The Lyons sample (Xu et al 2006) was a low-porosity, Permian aeolian deposit composed of mostly wellsorted quartz grains (90%) with less than 3% clay content. The Lyons sandstone was composed of rounded grains with a grain size of about 0.2 mm cemented well.…”

“…Quality factor-pressure function on sample marked with V175 (solid line -calculated data produced by the quality factor model, asterisks -measured data). Data obtained from Best (1997) 0.61 Lyons sample (Xu et al 2006) 0.38 Berea sample (Prasad and Manghnani 1997) 4.8 Sample marked with V175 (Best 1997) 6.75…”

“…Data obtained from Pressure dependence of longitudinal wave velocity on Lyons sample (solid line -calculated data produced by the velocity model, asterisks -measured data). Data obtained fromXu et al (2006) …”