The effective stress coefficient, introduced by Biot, is used for predicting effective stress or pore pressure in the subsurface. It is not a constant value. It is different for different types of sediment and it is stress dependent. We used a model, based on contact between the grains to describe the reason for change in effective stress coefficient under stress. Our model suggests that change in effective stress coefficient will be higher at uniaxial stress condition than at hydrostatic condition. We derived equations from the original definition of Biot to estimate effective stress coefficient from one dimensional rock mechanical deformation. We further investigated the effect of boundary condition on the stress dependency of effective stress coefficient and discussed its application in reservoir study. As stress field in the reservoirs are most unlikely to be hydrostatic, effective stress determined under uniaxial strain condition will be more relevant in reservoir studies.
Some of the chalk reservoirs in the North Sea compact during oil and gas production [1, 2]. This is primarily due to the increase of effective stress, as pressure in the pore fluid decreases with hydrocarbon production. Compaction in the reservoir induces surface subsidence. Therefore, increase in effective stress could affect productivity, well bore stability and safety as well as stability of the oil platforms. Prediction of actual effective stress during the production is therefore necessary for production planning, well bore design, placement of platform etc. In addition it is an important parameter to be used for monitoring purpose when mechanical and chemical processes are involved in reservoirs: e.g. geological sequestration of CO2 and Enhanced Oil Recovery (EOR) processes. We derived a relation from Biot’s [3] original equations on effective stress, for the effective stress coefficient, n, under uniaxial strain conditions.
