Effective stress and the elastic response of reservoir sandstones and shales

Abstract

Effective stress is a key concept in determining reservoir pore pressures from seismic velocity data. It is usually defined as the difference, ?P between confining (overburden) pressure, Pc and pore pressure, Pp. However, a more general definition is s´=Pc - nPp where n is an effective stress coefficient. Seismic velocity is observed both in the field and in laboratory experiments to vary non-linearly with effective stress. Furthermore, a form of hysteresis in velocity is observed when reservoir rocks are subjected to differing pore pressure histories. The resulting hysteresis can be quantified with the effective stress coefficient. This coefficient is frequently derived from laboratory velocity-effective stress data at ultrasonic frequencies. This paper describes some recent laboratory based, rock physics experiments involving pore pressure history induced velocity hysteresis and the implications for reservoir 4D monitoring.