Occurrence state and production mechanism of formation water in tight sandstone reservoirs of Sulige area, Ordos Basin

The Sulige gas field in the Ordos Basin is the largest unconventional continuous tight sandstone reservoir in China.There are many water-producing gas wells in the Upper Paleozoic strata in the west and the north of Sulige region,where the reservoir has no unitive gas-water interface.The intrinsic rules of gas-water distribution are complicated in this area.Based on the dynamics of hydrocarbon accumulation in tight sandstone,integrating oil and gas accumulation elements and reservoir present microscopic pore structure and fluid flow mechanism,the occurrence state and production mechanism of formation water in Sulige region have been discussed in this paper.It has been showed that the main reason for complicated gas-water distribution is the difference between hydrocarbon generation expansive force and capillary pressure.The occurrence state of formation water in this area behaves 3 types,i.e.irreducible water,capillary water and free water.The irreducible water is basically located in the high gas generation intensity area in the east of Sulige region,where the testing gas wells are mainly gas-producing ones.The capillary water is normally located in the low gas generation intensity area in the west and the north of Sulige region,where the testing gas wells are mainly gas/water-producing ones.The free water is sporadically located in the area.