| As for the Low permeability tight sandstone gas reservoir in western Sichuan governed by Southwest Branch Company of CNPC, Staged fracturing technique in horizontal well is an essential technique in the process of prospecting evaluation and building up production. Besides, the large displacement and fluid volume fracturing technique is adopted in the exploitation of Xu5 Gas Reservoir in western Sichuan. however, a great amount of water is needed to prepare the fracturing fluid, when the above two techniques work, which also results in the problem of finding and using water. But, as the exploitation develops further, the produced water in gas reservoir increases year by year. A great amount of produced water need to be disposed, which bring the branch company heavy stress in cost and protecting environment. Therefore, taking above two factors into consideration, we research to use formation water to make fracturing fluid so that waste water can be recycled. It will be a novel way to dispose formation water, to alleviate the stress of formation water processing and fracturing water finding & using, and to implement environmental-friendly exploitation of oil and gas field.Through doing research about the produced water of gas field in western Sichuan, this paper analyzes the current situation of the production of formation water and water- processing in western Sichuan. What’s more, we have analyzed a lot of samples and combined the research achievements of predecessors. Now, we figure out the features and the salinity of formation water and its ionic type and content. The content of formation water is different in different depth. Based on that fact, we divide the water into middle-shallow formation water and the Xujiahe one. According to the compatibility of formation water and reservoir stratum, we decide to prepare guar gum fracturing fluid with middle-shallow formation; drag-reducing fracturing fluid, Xujiahe formation water.To prevent the guar gum fracturing fluid prepared by middle-shallow formation water from being sticky, we developed a special guar gum solubilizer. Concerning to the weakly cross-link of base fluid, we optimized the treatment scheme of sewage treatment plant. All that we have done contribute to make guar gum fracturing fluid formula with middle-shallow formation water. The viscosity of that fracturing fluid keeps around 100mPa·s, if the making time is reduced by 60 min in the case of 45℃ and 170s-1. That kind of fracturing fluid can meet the requirements of operation on the rheological property of fracturing fluid.The pilot test on the guar gum fracturing fluid prepared by middle-shallow fraction water has been implemented successfully, which increases the production by 2.8×104m3/d. However, in constructing site, a lot of problems come out. For instance, the pumping pressure of formation water fracturing fluid gel is increased suddenly in the highly sands-carrying section. The gel become thinner and brittler. The time of cross-linking is prolonged. All these problems will increase the risk of working. So, we decide to make formation water fracturing fluid. Firstly, analyze the factors that influence the tolerance of high-heat and shearing of fracturing fluid. it may be the Aluminum sulfate flocculants put in the treatment of formation water that affects the rheological property of fracturing fluid. In the end, the research confirm the influence of ionic type and concentration of formation water on the making process of formation water guar gum fracturing fluid.The performance of middle-shallow formation water guar gum fracturing fluid is mainly influenced by Ca2+, Mg2+,Cl-, and its influential degree ranks as Ca2+>Mg2+>Cl-. The critical influential concentration of Ca2+ is less than or equal to 1200mg/L; Mg2+, 1500mg/L; Cl-, 10×104mg/L. In accordance with the influence of ionic type and concentration of formation water on the making process of formation water guar gum fracturing fluid, we consider to decrease the concentration of above-mentioned ions into the below of critical concentration, and drop the the concentration of ions, Ca2+, Mg2+, to below critical concentration through diluting formation water with fresh water.Thus, we developed a formation water guar gum fracturing fluid system especially for Xujiahe formation water by dilution. Under this system, The viscosity of that fracturing fluid keeps around 150mPa·s, if the making time is reduced by 120 min in the case of 75 ℃ and 170s-1. When the tension of the surface of gel-breaking fluid is 26.86mN/m, and its anti-swelling rate is 84.2%, the damage rate to the core is just 17.73%. After the indoor research, we make out the order of influencing factors( for drag-reducing fluid)that shows as following: Cl->>Ca2+>Mg2+. the critical influential concentration of Ca2+ is less than or equal to 1×104mg/L; Mg2+, 4000mg/L; Cl-, 6×104mg/L; The total salinity, 15×104mg/L. Having optimized the salt-tolerant drag-reducing fluid, chosen relevant additives and studied the evaluation of comprehensive performance, we develop a formula of formation water drag-reducing fluid for Xujiahe formation water. Its drag-reducing rate is greater than 45%, which meets the requirement of volume fracturing operation of Xu5 Gas Reservoir.We have done several Xujiahe formation water guar gum fracturing fluid tests respectively on the wells of MP23-13, JS33-7, ZX33 and SF309-1. Owing to the inadequate of stirring intensity, making fluid on the spot is confronted with many problems, such as guar gum being sticky slowly, base fluid being not sticky, and bacteria resulting in hydration of guar gum. But the test that we did have solved them completely. If the daily production of gas accumulates to 16.33×104m3/d, it demonstrates that the production is increased obviously, and Xujiahe formation water guar gum fracturing fluid debugged by dilution adapt to stratum appropriately, which also reduce the harm to reservoir stratum. We did a successful pilot test about drag-reducing fluid in the well of X502. This test realizes that The performance of fluid is equivalent to that of drag-reducing fluid prepared by fresh water. At the end of 2014, the emission of formation water is reduced by 1835m3 with formation water fracturing fluid technique, which also prove that this technique enjoys great environment-friendly benefits. What’s more, it is conducive to reduce the dosage of additives to cut the cost down. |
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