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Effect of connate water on stress sensitivity of tight gas reservoirs

ABSTRACT The current research on stress sensitivity was mainly focused on reservoirs without connate water. But the connate water exists in tight gas reservoirs widely. Thereby research of stress sensitivity with connate water is meaningful for reasonable development of tight gas reservoirs. The micro-pore structure and clay minerals of core samples selected from tight gas reservoirs were analyzed through mercury injection experiment, X-ray diffraction analysis, and scanning electron microscope (SEM) technologies. For tight gas reservoirs, the clay content was high which is an important factor leading to strong stress sensitivity, and the characteristic of pore structure belonged to pore-matrix type. In view of the above, the mechanical capillary model considering connate water was established in this paper, which was used to analyze the influence of connate water on stress sensitivity. Core samples under different connate water saturations were adopted for the experiments of stress sensitivity. Results show that stress sensitivity of tight gas reservoirs with connate water is strong, and increases with decrease of permeability and increase of connate water saturation, which is consistent with the conclusion from the forced capillary model. Relationships between stress-sensitivity coefficients, absolute permeability, and connate water saturation were studied through experiments and a numeric expression used to quantificationally characterize stress sensitivity with connate water was obtained. On this basis, a productivity model considering stress sensitivity with connate water was established. By calculating the productivity of gas well of different connate water saturation, it can be found that productivity damage occurs due to stress sensitivity, and productivity damage increases with the increase of connate water saturation. Therefore, if the pressure drawdown rate is controlled and connate water saturation is decreased, productivity damage caused by stress sensitivity will decrease.