Varying law of water saturation in the depletion-drive development of sandstone gas reservoirs

The varying law of on-way water saturation in the depletion-drive development (the formation pressure: 20 MPa depleting to abandonment pressure) of water-bearing gas reservoirs was studied through four groups of cores whose physical properties represent the rocks in Sichuan Xujiahe gas reservoirs, and cores permeability are 1.630×10−3 μm2, 0.580×10−3 μm2, 0.175×10−3 μm2, 0.063×10−3 μm2, respectively. Combined with the rock characteristics of microscopic pore structure and capillary pressure, the interaction mechanism between different permeability sandstone and water was analyzed and verified on two wells. The conclusions are as follows: (1) The water trapping effect in the sandstone reservoir is dependent on the sandstone reservoir permeability (the critical value is 0.175×10−3− 0.580×10−3 μm2). (2) The formation with permeability greater than 0.580×10−3 μm2 is characterized with big size pore and throat, low capillary pressure and weak trapping effect. So a portion of pore water would be displaced by gas and became movable water. (3) The tight formation with permeability less than 0.175×10−3 μm2 is characterized with small size pore and throat, high capillary pressure and strong trapping effect. When the rock core is saturated by moisture gas, the pore water would not be displaced by gas and would be trapped in the sandstone, which results in the water saturation increases rather than decreases. Drawdown pressure should be kept within a proper range to extend the life cycle of wells.