The Effect of Overburden Pressure on Relative Permeability

Abstract

Abstract In routine core analysis, porosity and permeability, both relative and absolute, are measured permeability, both relative and absolute, are measured on rock samples which are not under net overburden (confining) pressure. Using these data to predict reservoir performance or estimate reserves can lead to serious errors since all reservoirs are under net overburden pressure. Data collected from constant rate, dynamic displacement experiments were utilized to study the effect of net overburden pressure on porosity and absolute and relative permeabilities. These experiments were conducted on small, consolidated rock samples under net overburden pressures up to 41.37 MPa (6000 psi) and room temperature. The pore pressure was maintained atmospheric. pressure was maintained atmospheric. Examination of the data shows a decrease in porosity and permeability with increase in overburden porosity and permeability with increase in overburden pressure. A correlation between porosity and pressure. A correlation between porosity and overburden pressure and also between permeability and overburden pressure has been developed using linear regression analysis. Both correlations are found to be logarithmic. The irreducible water saturation and residual oil saturation increase with increased overburden pressure levels. While the relative permeability to oil decreases with increasing overburden permeability to oil decreases with increasing overburden pressure, a corresponding negligible decrease in pressure, a corresponding negligible decrease in water relative permeability is observed. Introduction In routine core analysis, porosity, absolute and relative permeabilities are measured on rock samples which are not under net overburden pressure. However, under actual reservoir conditions, the rocks experience a net overburden pressure which is equal to the weight per unit area of the vertical overburden column from the reservoir depth to the surface of the earth less the pressure of the liquid in the pores of the rock. Sometimes only the initial net overburden pressure is considered while subsequent increase in this pressure caused by reduction in pore pressure upon reservoir depletion is ignored. If rock pressure upon reservoir depletion is ignored. If rock properties measured at zero net overburden pressure properties measured at zero net overburden pressure differ from those measured at initial or intermediate conditions, a systematic error will be introduced into reservoir engineering calculations such as well productivity, reserves, and simulation. productivity, reserves, and simulation. Fatt and Davis were first to observe a decrease in absolute permeability of sandstone upon increasing overburden pressure. McLatchie et al. attributed the permeability reduction to compressibility of the rock. Dobrynin studied the effect of net overburden pressure and found results similar to those of Fatt and Davis. Gray et al. found that maximum permeability reduction occurred under uniform pressure. Wilhelmi et al., on the other hand, developed a method for simultaneous measurement of pore and elastic properties of rock under nonuniform stress. The only study on consolidated limestone has been presented by Marek who observed a drastic decrease in absolute permeability accompanied by a little decrease in porosity under overburden pressure. The decrease in permeability and porosity was attributed to pore collapse. Several investigators have studied the effect of various factors on relative permeability such as temperature, interfacial tension, fluid composition, and rock surfaces. However, there has been no attempt to investigate the effect of overburden pressure on relative permeability. Thus, there is a need to establish quantitative relationships between absolute permeability and porosity with overburden pressure and more importantly, to ascertain the effect of overburden pressure on relative permeability. permeability. P. 335