| The Junggar Basin is rich in oil and gas resources,and it has been an important exploration and development block in the western region of SINOPEC.During the early stage of oil and gas development,wellbore instability in Qingshuihe and Toutunhe formation has been becoming the most serious problems,which significantly restricts the exploration and development of the Junggar Basin.In order to solve these problems,the technical measures of anti-sloughing drilling fluid of Junggar Basin,preparation of anti-sloughing agent based on oil sludge and novel micro-nano plugging agent,high inhibitive and tight plugging anti-sloughing water-based drilling fluid were studied respectively,which provides reliable drilling fluid technology for the exploration and development of the Junggar Basin.The mineral compositions,micro-structure characteristic and physical-chemical properties of brittle shales were investigated respectively due to brittle shale instability problems of the Junggar Basin.Wellbore instability of brittle shales is closely related to its multi-scale pore structure characteristics,and micro-fractures and nano-scale fissures are the main controlling factors.What's more,drilling fluid density less than or close to the formation collapse pressure is also one of the important causes of wellbore instability in brittle shales.According to the multivariate cooperation principle of wellbore stability,wellbore strengthening principles of multi-scale tight plugging were put forward based on the multi-scale pore structure characteristics,and enhancing sealing performance of drilling fluids to plug nano-scale fissures and micro-fractures is key point to solve wellbore instability of brittle shales in Junggar Basin.What's more,improving shale hydration inhibitive ability and providing the effective stress support of reasonable drilling fluid density are beneficial to wellbore stability of brittle shales in the the Junggar Basin.The novel preparation method of anti-sloughing agent for drilling fluids was investigated using oilfield oil sludge as raw materials,and this simple and economically feasible method could makes full use of the effective components of oil sludge and does not produce secondary pollutants.High performance anti-sloughing agent(SDOS)was also developed,which extends the resource reuse of oil sludge to drilling fluid technology for the first time and explores a new path to the resource reuse of oil sludge.Experimental results indicated that anti-sloughing agent(SDOS)with adjustable softening point(50?120?)could stably disperse in the water-based drilling fluids,and form thin and tough filter cake,thus improving the anti-sloughing performance of drilling fluids.Furthermore,the action mechanism of anti-sloughing agent(SDOS)was investigated in the physical plugging and chemical adsorption aspects by use of ZETA potential,contact angle measurement,filter cake compressibility and penetration tests.The results indicated that anti-sloughing agent(SDOS)could be deformable to internally seal pore or micro-fractures to reduce fluid pressure penetration,and the chemical adsorption of SDOS could also render the shale more hydrophobic to further improve the anti-sloughing properties.The micro-nano plugging agent(WS-1)was successfully synthesized via emulsion polymerization using modified nano silica in order to meet demand of multi-scale tight plugging of brittle shales.It possesses uni-modal distribution from 80 nm to 200 nm and exhibits good dispersion stability and compatibility in the water-based drilling fluid.The synergistic effect on shale stability of micro-nano plugging agent(WS-1)and chemical shale stabilizer(DLP-1)was investigated quantitatively through the pressure transmission and membrane efficiency tests,core fracturing-plugging tests and contact angle measurement,and shale strengthening micro-mechanism based on the physical/chemical tight sealing effect was also discussed in detail.Experimental results indicated that the micro-nano plugging agent(WS-1)work synergistically with chemical shale stabilizer(DLP-1)to impart shale stability due to superior properties of decreasing shale permeability and increasing shale membrane efficiency.According to wellbore strengthening principles of multi-scale tight plugging,high inhibitive and tight plugging anti-sloughing water-based drilling fluids for development and exploration well drilling respectively were established and optimized based on anti-sloughing agent(SDOS)and micro-nano plugging agent(WS-1).It was concluded from the field trial test results that the optimized drilling fluids exhibit stable performance,and no wellbore instability exists during the drilling process of brittle shales with the average enlargement rate of less than 10%,thus meeting the wellbore stability requirement of brittle shales in Junggar Basin. |
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