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1.Gas field development and water production statusJianbei block is located in front of Altun Mountain in Qaidam Basin, on the Jianbei Dongping uplift, with Nanyishan and Xiaoliangshan in the west, Dongping gas field in the East and Dafengshan structure in the south. The reservoir of the block is bedrock, and the lithology is granodiorite in intermediate acid intrusive rocks. The mineral composition is mainly feldspar, quartz, dark minerals hornblende and biotite, and the feldspar is mainly albite. The main reservoir spaces are matrix pores (including dissolution pores and residual pores) and fractures, with an average porosity of 3.3% and an average permeability of 0.05md; Micropores are well developed, and the pores are connected to a certain extent; The fractures are dominated by medium high angle and high guide fractures, with network fractures in some parts. The porosity of fractures is 0.002-0.01%[1-5]. 2.Gas field development and water production statusBy April 2020, 17 gas wells have been put into production; Among them, there are 8 normal wells, 8 intermediate production wells and 1 non deliverable well; The daily gas production is 187000 m3, the daily water production is 491.1 m3, the water gas ratio is 26.3 m3 / 10000 m3, the cumulative gas production over the years is 343 million m3, the cumulative water production is 97700 m3, the gas recovery rate is 0.64%, and the recovery rate is 1.63%. After four years of production, 100 high-yield wells (> 50000 m3) have been converted to low yield wells (0 ~ 10000 m3); At present, there are 8 gas wells with single well production of 0-10000 m3, accounting for 47% of the total production wells. The daily gas production of the block is mainly low. Compared with the development indexes over the years, the gas field development shows the “three fast” development characteristics of rapid production decline, rapid formation pressure drop and rapid water invasion. According to the results of logging interpretation, Jianbei gas field belongs to the block bottom water dry gas reservoir controlled by structure. The original bedrock reservoir has a unified gas water interface with an altitude of - 2019m. 3.Single well water source analysis3.1Water source type of gas wellThe sources of water from gas wells are mainly divided into three types: condensate water, external artificial invasion water and formation water. The formation water includes: primary movable water, secondary movable water, interlayer water, edge water and bottom water. Condensate water is water breakthrough in gas well caused by formation water precipitation from gas when formation temperature and pressure decrease with development. The produced water is called condensate water, which is characterized by low salinity and low production, and has little impact on gas well production. During drilling or gas well operation, some working fluids such as mud filtrate and fracturing fluid will invade into the formation near the gas well. The water yield is generally large at the initial stage of production, and then the gas well continues to produce, and the water yield gradually decreases and finally disappears. The primary movable water in the formation is the sub movable water that will gather in the primary layer at the low part of the reservoir structure. Generally, the reserves of movable water are small. With the continuous production, the water production of the gas well decreases significantly. The secondary movable water in the formation is that the formation pressure decreases gradually, some irreducible water in the reservoir rock gradually forms the secondary movable water in the formation, and with the gas being produced, the water yield of the gas well is characterized by the gradual increase of water quantity, but the ratio of produced water to gas has been kept at a low value. Interlayer water is generally located in the muddy interlayer of the gas layer in the reservoir, and often exists in the form of bound water. Its water production characteristics are that the water free period of production is relatively short or even no water free period. After water breakthrough, the water production of gas wells increases greatly, and the fluctuation range is large, and the water production of gas wells will gradually decrease in the later stage of exploitation. Edge water and bottom water often occur in the middle and late stage of gas well production, and the location of water outlet generally has a certain regional, because the energy of edge water and bottom water is relatively sufficient. When water control and control measures are not taken, the water production of gas well will generally continue to rise. 3.2Comprehensive identification method of water source3.2.1Discrimination method of production water-gas ratioGenerallyThe water yield of gas wells is relatively small and stable at the initial stage of production, and the produced water vapor is relatively small and has an upper limit value. At this stage, the water produced by gas wells mainly comes from condensate water in the formation; With the continuous production of gas wells, the water production rate and water-gas ratio of gas wells increase sharply, and the water production rate increases, which indicates that the edge and bottom water of gas wells may have invaded into the gas reservoir at this time. In order to accurately determine the water source of gas wells, comprehensive analysis should be combined with other determination methods. 3.2.2Discrimination method of salinity of produced waterThe salinity of formation water and condensate water in reservoir is generally different, and the chloride ion content in edge and bottom water is generally much higher than that in condensate water. Therefore, the type and source of water produced by gas wells can be determined according to the detection of water samples produced by gas wells, and the source of water invasion in gas wells can be predicted in advance. The actual geological conditions of field gas reservoir and gas well production are complex, so this method generally needs to combine other methods and gas well working fluid backflow rate to comprehensively determine the gas well water source. 3.2.3Water-based judgment methodThe salinity and density of condensate water in the formation of Jianbei gas field are relatively low. Because the acidic components in the gas dissolve into the condensate water, the PH value of the condensate water is relatively acidic, and the water type is mainly Na2SO4. Engineering fluids (mainly drilling fluids and acid fluids) contain a large amount of other additives, with relatively high salinity and density. The PH value of drilling fluids is usually strongly alkaline, while that of acid fluids is usually strongly acidic. The salinity and density of formation water in gas reservoir are mainly between condensate water and engineering fluid, which is obviously higher than that of condensate water, and the water type is mainly CaCl2 type. 3.2.4Method for judging production performance of gas reservoir.① The waterless gas production period of gas wells in gas reservoirs is short, and there is even no waterless gas production period. The water produced in gas well production immediately and the water yield and water-gas ratio are stable, which indicates that the water produced in gas well production is mainly condensate water in the formation. ② With the continuous production of gas wells, the formation pressure gradually decreases, and the water production of gas wells gradually begins to increase, but the increment is relatively small, and the gas production has been relatively stable, indicating that due to the decrease of formation pressure, a pressure drop funnel is formed around the gas wells, and bound water near the wellbore gradually begins to flow, and the water produced by gas wells mainly comes from condensate water and bound water.③ After a period of gas well production, the water yield of gas well rises sharply, and the gas production begins to drop sharply, which indicates that the edge and bottom water in the formation intrudes into the gas reservoir and has broken through to the bottom of the well because of the pressure drop in the gas reservoir, resulting in an increase in water yield and a significant decline in gas well production. The main sources of produced water in gas wells are edge and bottom water. 3.2.5Method for judging the falling speed of oil pressure.When edge and bottom water invasion occurs in a gas well, the water yield of the gas well is large, the wellbore pressure loss increases, and even the bottom hole effusion is formed in severe cases, and the wellhead pressure decreases obviously. From the production performance of the gas well, the wellhead pressure decreases greatly and rapidly. 3.3Determination method of water production source in gas wellAccording to the theoretical research results of water source types of gas wells, the judgment method of water source of water and gas wells in Jianbei area is established. 3.4Causes of water production in gas wellIf the reservoir of a gas field is gas-bearing and poor in physical properties, water will be easily produced in production. In addition, with the increase of production pressure difference, the water-gas ratio of gas Wells increases, which are two important reasons for water production in the gas field. Formation water is one of the key factors affecting the development of gas fields. Once water is found in a gas well, the relative permeability of the gas phase near the wellbore will be reduced, resulting in a rapid decline in the production of the gas well. In some low-producing gas Wells, it will even crush the gas well. The source of produced water can be judged comprehensively by condensate water volume calculation combined with the characteristics of outlet water of gas well, or by salinity analysis. When exploiting a gas field, we should consider the timing of production to rationalize development. 4.Analysis of water source of typical single wellBased on the above methods, the water source of typical gas wells in Jianbei gas field is comprehensively determined, which provides theoretical basis for reasonable water control of gas wells. According to the test results of salinity and chloride content of produced water in J3 well, the chloride ion and total salinity change little from well opening to September 2019, and the chloride ion content is about 2.0% × 104mg / L, total salinity 3.8 × In this stage, condensate water is mainly used. Since then, with the growth of production time, the production water gas ratio has gradually increased, and the salinity and chloride ion content of the produced water have increased to a certain extent. Therefore, comprehensive analysis shows that since October 2019, the water source of well jian-3 is the primary movable water and condensate water in the formation, and there is no bottom water intrusion. The salinity of formation water and condensate water is generally different, and the content of chloride ion in edge water and bottom water is generally much higher than that in condensate water. Therefore, the type and source of produced water can be determined according to the detection of produced water samples of gas Wells, and the occurrence source of water invasion of gas Wells can be predicted in advance. The actual geological conditions of gas reservoir and the production situation of gas well are complicated. This method generally needs to be combined with other methods and the flowback rate of gas well working fluid to comprehensively determine the water source of gas well. 5Conclusion5.1 The sources of water from gas wells are mainly divided into three types: condensate water, external artificial invasion water and formation water. The formation water includes: primary movable water, secondary movable water, interlayer water, edge water and bottom water. 5.2 The comprehensive discriminant method of water production source mainly includes: water-gas ratio discriminant method, produced water salinity discriminant method, water quality discriminant method, gas reservoir production performance discriminant method, oil pressure decline rate discriminant method. 5.3 Based on the judgment method of water source of gas well, combined with the actual production situation of Jianbei gas field, a set of judgment method of water source of gas field is formed, which provides theoretical basis for reasonable water control of gas well. 5.4 The intensity of water invasion has an influence on the characteristic curve of water invasion, but the influence does not change the basic shape of the characteristic curve. 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