Studies On Complex Gas-Water Relationship For The Tight Sandstone Gas Reservoir Of Shan 2 Member In Qingjian Area, Zizhou Gas Field

To implement a development program named Zizhou gas field rolling extension, the Qingjian area which is located in the south of Zizhou has become to a main construction of industrial zone. In 2012, the well drilling result shows that the area Shan 23 submember sandbody developed, and has a good continuity. But some gas wells product water when we have the gas testing, as well as in the subsequent production. There is no doubt that the water production has become a bottleneck for the development of Shan 2 member gas resevior in Qingjian area. Under the stress of the need of constructing production and stable production for Zizhou gas field, it is particularly important to have a research on complex gas-water relationship of Shan 2 member tight sandstone gas reservoir in Qingjian area.On the basis of predecessors’ studies on the geological background in this area, I think the region is a delta front sedimentary environment through core observation and logging data analysis. I divide the delta front into four types of sedimentary microfacies. They are underwater distributary channel, subsea interdistributary bay, bay marshes and river mouth bars. Sandstone in this area has three types:quartz sandstone, lithic quartz sandstone and lithic sandstone. The quartz content of Shan 2 member gradually decrease from bottom to top, but the lithic content increase gradually. Zizhou gas field’s sedimentary source is from Yulin and Mizhi. Studies have showed that the Qingjian area also have the feature of two source mixed sedimentary.By logging parameters, lithology in this area can be divided into sandstone, mudstone and coal petrography. Then, I divide sandstone into quartz sandstone, lithic quartz sandstone and lithic sandstone. Due to lithic quartz sandstone is a transition type of quartz sandstone and lithic sandstone, it’s hard to distinguish on the logging parameters curve.According to the gas testing results and the dynamic production data, I select gas layers, water layers, gas-water layers and dry layers as the samples, I roughly determined lines of logging parameters for them by the cross-plot method, and I distinguish the gas layers and water layers with P1/2 normal frequency distribution method, P1/2 which is less than 0.45 is water layer, and which is more than 0.45 is gas layer. Selecting another 44 new samples to verify result, comprehensive accuracy is 88.6%. However, in order to improve the recognition effect of gas and water layer, firstly we should get rid of dry bed by cross-plot method. Through stepwise discriminant analysis to establish the discriminant function of gas, water and dry layers, testing the discriminant function with the known samples, the comprehensive accuracy is 87.14%. Combined with these three methods, we can identify reservoir fluid and dry layer better.By single well logging interpretation, I summarized three kinds of water body types, namely the side (bottom) containing water, the lenticular water, Tight sandstone gas residual formation water, and analyze its causes. Aiming at the three main control factors of gas water distribution, which are sand body distribution, reservoir physical property and structure, Thinking of reservoir physical properties in the study area had a greater influence on the differences of gas-water distribution, reservoir physical property is good, good connectivity sand body usually for gas layers, water layers, or gas-water layers, showing the contact relationship of gas on the top and water on the bottom. The sand body of good reservoir properties and connectivity is usually gas layer, or gas and water layer which is gas on the top, water on the bottom. Sand body distribution effects the distribution of gas-water. It shows that the area in the river center which sand body is thicker and connectivity is good is usually the gas layer, the area on river side and bend is easily to form water body. To tell from the regional scale, the study area is isolated sand body, according to the distribution of sand body position, there are both water in higher or lower location of the sand body in tectonics. So the gas-water distribution which is controlled by regional tectonic is relatively weak, but the local micro-structure provides the isolated sand body which has good property and connectivity with gas-water differentiation conditions. Therefore, the high position of local micro-structure is gas, the low position is water.