Occurrence state and production mechanism of bound water in tight gas reservoirs in western Sichuan depression

The micro pore structure of tight sandstone reservoir is complex, and bound water exists generally. Its distribution, occurrence and flow mechanism have great influence on gas bearing property and micro seepage. By selecting rock samples of different types of gas reservoirs in western Sichuan depression, the experimental analysis of semi permeable partition method is carried out to reveal the internal relationship between micro pore structure and occurrence state and production mechanism of bound water. The results show that: ①Different types of reservoirs have different distribution characteristics of irreducible water saturation, and the micro pore structure is the internal factor affecting the irreducible water saturation. When the pore radius is large, but the flaky or necked throat is developed, the pore throat separation is poor and there are many micro pores and throats. The original formation water is easy to be trapped in the places where the pore throat size changes rapidly or in the micro pore throat, resulting in high irreducible water saturation. While, as for the shrinkage pores, the tube bundle throats are relatively developed, the pore throat separation is good, the micro pore throat is less or the pore throat volume ratio is small, and the irreducible water saturation is relatively low. ②Under the action of large pressure gradient or the induction of movable water, part of high value bound water can be transformed into critical water, and finally become movable water to participate in the flow and produce with natural gas. It is concluded that the production pressure difference should be controlled in the initial stage of development. Once the original bound water begins to participate in the flow, the water production and wellbore effusion in the middle and later stages of development will be intensified.