Stress arching effect on stress sensitivity of permeability and gas well production in Sulige gas field

摘要

Permeability is a fundamentally important parameter of hydrocarbon reservoir because it significantly impacts well productivity. In laboratories, it is measured under isotropic load which is equal to overburden pressure and it is often assumed to remain unchanged during production. In really however, stress arch forms above the reservoir and the overburden pressure drops during production. Consequently, there is a clear need to illustrate the stress arching effect on the stress sensitivity of permeability and well dynamic performance. Stress arching ratio, which is defined as the change in overburden pressure divided by the change of pore pressure from initial reservoir condition, is calculated for different reservoir shapes in Sulige gas field. Relationship between overburden pressure and effective stress, considering stress arching effect in Sulige gas field has been established. Laboratory experiments following reservoir depletion path have been conducted under the stress arching ratio of 0.12 and 0.28 that the reservoir may follow. The results show that the stress sensitivity of permeability greatly depends on stress arching ratio. Permeability measured under nonzero stress arching ratio is larger than that obtained from the conventional experiments with the stress arching ratio of 0 under the same condition. At the pressure drop of 25 MPa, the measured permeabilities increase by 23% and 50% for the stress arching ratios of 0.12 and 0.28, respectively, compared with the values obtained by the conventional experiments when the initial permeability of the core is 0.098 md. In contrast, the permeability shows less dependence on stress arching ratio when the initial permeability of the core is large, such as 7.387 md. Furthermore, the effect of stress arching ratio is introduced into the calculation of well productivity based on the experiment results. The gas well productivities increase by 5.03%, 12.46% and 72.48% for stress arching ratios of 0.12, 0.28 and 1, respectively, when the reservoir initial permeability is 0.098 md. The impact of stress arching ratio on dynamic performance of gas well is generally incorporated via look-up tables of transmissibility multiplier in Eclipse, which is approximate to the ratio of the decreased permeability to the initial permeability for different stress arching ratio. The production decline rate and production decline type are closely related with stress arching ratio when the initial permeability is 0.098 md.