Effect of pore pressure on the velocity of compressional waves in low-porosity rocks

Abstract

The velocity Vp of compressional waves has been measured in rock samples of low porosity to confining pressures Pc of 2 kb for a number of different constant pore pressures Pp. An effective pressure defined by Pe = Pc − nPp, n ≤ 1, is found to be the determining factor in the behavior of Vp rather than an effective pressure defined simply by the differential pressure ΔP = Pc − Pp. As pore pressure increases at constant effective pressure, the value of n increases and approaches 1, but as effective pressure increases at constant pore pressure, the value of n decreases. These observations are consistent with Biot's theory of the propagation of elastic waves in a fluid‐saturated porous solid.

A simple derivation of the effective stress coefficient for seismic velocities in porous rocks

“…The dependence of seismic velocity on pressure has been confirmed for a variety of rocks by laboratory measurements of elastic wave velocities in samples with varying pressure in pore fluids (see, e.g., Wyllie et al, 1958;Todd and Simmons, 1972;Eberhart-Phillips et al, 1989;Prasad and Manghnani, 1997). In general, for a rock subjected to a given confining stress σ c , higher pore pressures P correspond to lower compressional and shear velocities.…”

Seismic detection of transient changes beneath Black Rapids Glacier, Alaska, U.S.A.: II. Basal morphology and processes

“…For exampl e, in an experiment on saturated limestone, Todd and Simmons (1972) found that Vp dropped instanta neo usly as confining press ure was dropped. After several hours in a water bath, Vp rose to the exp ected speed for a full y saturated sample at the new pressure.…”

“…This process is vvell docu mented in soil mechanics (e.g. Todd and Simmons, 1972;Fredlund and R ahardj o, 1993), but is complex a nd hi ghl y d ep endent on grain-size distributi on and strain hi story. Ther efore th ere is no si mple way to caleu late the point on Fig ure 6 at which exoluti on begins; Fred lund and R ahardj o (1993) present data which suggests this point is about 2 ~IPa, which is the same order as loca l overburden pressure.…”

Influence of Variable Pressure on Acoustic and Volumetric Properties of Terrigenous Reservoir Rocks (On the Example of Semyrenkivska Area Samples)

“…4 представлено графіки залежностей ( ) Очевидно, що для різних груп зразків характерний різний характер залежностей між петрофізичними па-раметрами в різних баричних умовах -швидкість може зростати і спадати плавно чи стрибкоподібно, при чому стрибки при прямому циклі вимірювань зазвичай (як і в дослідженнях інших авторів [2,10,11,15,17,19,22,23]) зустрічаються за тисків 0,1 ÷ 15 МПа і відповідають закриттю пустот різних типів, в основному, тріщин та різкій зміні розмірів інших пустот.…”