THE RELATIONSHIP OF PERMEABILITY TO CONFINING PRESSURE IN LOW PERMEABILITY ROCK
1981; Linguagem: Inglês
10.2523/9870-ms
AutoresJames W. Jennings, H.B. Carroll, C.J. Raible,
Tópico(s)Hydraulic Fracturing and Reservoir Analysis
ResumoLaboratory permeability measurements of low permeability (less than 1 md) reservoir rock is significantly affected by test confining pressure. Knowledge of the confining pressure is required to predict permeability at reservoir conditions. A permeability at reservoir conditions. A model is proposed which relates electrical conductivity, permeability, and pore dimensions to confining pressure. The model assumes that rock pores are interconnected by thin cracks and microfractures which can be modeled by rectangular slits. For this model, core permeability is related to confining pressure by a third order polynomial, and electrical conductivity Is polynomial, and electrical conductivity Is related to confining pressure by a first order polynomial, Electrical conductivity and permeability versus confining pressure measurements were made on test cores having laboratory Klinkenberg permeabilities from 20 to 200 microdarcys. The experimental measurements were found to be consistent with the slit model theory.Introduction. Thomas and Ward have shown that the measured permeability of many western gas sandstone cores is significantly decreased by increased test confining pressure, whereas effective porosity is only slightly affected. Jones and Owens developed an empirical method for relating permeability to confining pressure which is valid for a variety of low-permeability western gas sandstone core. The results are consistent enough to suggest that a rock matrix model can be developed to relate pore geometry to laboratory permeability. Their studies suggest that low-permeability reservoir rock consists of a matrix material containing larger pores which contribute mostly to the rock porosity. The larger-pores are interconnected by smaller pore throats that restrict permeability and are easily deformed by changes in confining pressure.Wyllie and Gardner developed a capillary model in which pores and pore throats are assumed to be short sections of capillary tubes. Stacking sections of the capillary tubes together in a random manner forms a model which can relate porosity, electrical conductivity, and permeability. porosity, electrical conductivity, and permeability. Archie's equation is often inferred from the Wyllie-Gardner model. It is possible to extend the capillary model to include the effects of confining pressure. The extension is made by assuming that each capillary tube is a thick-walled cylinder to which the confining pressure is applied. As the confining pressure is increased, the inside diameter of the capillary is reduced which reduces the permeability and electrical conductivity. This model could be useful if the pore throats were nearly round capillary tubes. It is more likely that pore throats in low-permeability sands can be described better as slits, cracks or micro-fractures. For the same cross-section area, a slit will be a much weaker structure than a capillary tube and will undergo a larger change in area due to an applied external stress. Our paper proposes that a slit model can paper proposes that a slit model can describe the effect of confining pressure on permeability and electrical conductivity for permeability and electrical conductivity for low permeability reservoir rock.Slit Model Theory. The following assumptions have been made for the model:Flow paths through the core are independent and consist of a series of pores connected by rectangular slits.The slits are uniform in size in each flow path.Permeability and electrical conductivity are limited primarily by slit dimensions and fluid properties.Viscous flow is assumed; slit entrance and surface roughness effects are neglected.p. 391
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