Study on the Distribution Characteristics of Formation Water in Tight Sandstone Gas Reservoirs of the Shan 23 Member in the Zizhou Gas Field of Ordos Basin

To effectively formulate a scheme for the development of gas reservoirs, the distribution of formation water in the Shan 23 member of the Zizhou gas field of the Ordos basin in China was studied in depth, making full use of data covering formation water, logging and production. The study concluded that the types of formation water of the Shan 23 member in the Zizhou gas field are edge (bottom) water, lenticular water and formation water residue. The edge (bottom) water in the Shan 23 member is mainly distributed in three regions with low structures in the west, south and southeast of the area, respectively, to the well areas of Y47-Y29-Y43, Y64-Y40 and Y69. This layer is generally interpreted as a water layer by well logging and produces a large amount of water discharge in the processes of gas testing and production. The lenticular water in the Shan 23 member is mainly scattered in the middle and southern parts of the area and is generally interpreted as a water layer by well logging, mainly in small water bodies. The typical production characteristics of gas wells that produce formation water residue in a gas reservoir are as follows: With less water production, the gas saturation is high, and there is no obvious information about the water layer in the logging curves. However, during production, there is trace formation water, and as production continues, this part of the water is taken out. The edge (bottom) water is distributed in the lower structure of the area and is mainly distributed in the southwest part of the area. It is clearly controlled by the structure, especially the low-amplitude structure; thus, structure is more important for the control of edge (bottom) water. Structural characteristics have some influence on the lenticular water and the formation water residue in gas reservoirs. The position of the lower structure is the main area that enriches water. In a relatively independent region containing gas, the position of the lower micro structure is also a common distribution area of water. In addition, a larger water body often forms at the pinchout and the bend of a sand body.