Determination of Biot's Effective Stress Parameter for Permeability of Nikanassin Sandstone

Abstract In Alberta and British Columbia, a huge amount of tight gas is trapped in relatively impermeable rock formations. Physical fracturing of these formations could enhance the overall formation permeability and, thus improve tight gas extraction. One of the outstanding issues in rock fracturing is to determine the magnitude of applied effective stress. The general effective stress law is defined as: σ eff = σc – α σp, where σc and σp are total confining and fluid pore pressures, respectively. The Biot's constant α is not only a particular material property, but also markedly sensitive to the magnitude of applied confining and pore pressure. The main objective of this study is to experimentally determine stress dependency on the Biot's constant which controls fracturing mechanics in the tight gas formation and gas production rate from the formation at low and high effective stresses, respectively. A series of permeability measurements were conducted on Nikanassin Sandstone core samples from the Lick Creek region in British Columbia under various combinations of confining and pore pressures. In addition, permeability values were measured both along and across bedding planes to investigate any anisotropy in Biot's constant. Introduction Effective stress is the real item which actually controls the mechanical and hydraulic properties of porous rock and soil materials. Terzarghi(1,2) first brought the effective stress principle, which is defined as σeff = σc – σp, into soil mechanics, where σc and σpare the total confining and fluid pore pressures, respectively. The effective stress principle, though basically very simple, is of fundamental significance in rock and soil mechanics. However, in rocks, especially, the fluid-related or petroleum-related rocks, Terzarghi's effective stress principle may not be always valid. Therefore, the Biot's constant a other than 1.0 was suggested to modify the effective stress principle, and the effective stress principle finally is given by σeff = σc – σp(3). The value of Biot's constant a for permeability has been found to be 0.9 for joints with polished surfaces and 0.56 for joints made from tension fracture (4), and 0.6-0.7 for intact Chelmsford granite (5). Keaney et al.(6) estimated that the average value of Biot's constant α for permeability of Tennessee sandstones is 0.75. Berryman(7) found that for a rock whose mineral phase consists of a single mineral, the value of Biot's constant α should not exceed unity. However, Zoback and Byerlee(8,9) found that Biot's constant α of some clay-rich sandstones can be as high as 3–4. Walls and Nur (10) found that α varies from 1.2 for clean sandstone to 7.1 for sandstone containing 20% clay. It turns out that the Biot's constant α is not only a mechanical property depending on many factors such as rock type, porosity, pore geometry, rock constituents and their geometrical arrangement, but also markedly sensitive to the magnitude of applied confining and pore pressure. In Alberta and British Columbia, a huge amount of tight gas is trapped in relatively impermeable rock formations.

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