Study On The Loss Mechanism Of Changning Shale Gas Well And The Technology Of Preventing Leakage

The formation of Changning block is well-developed with micro-fractures and easy to leak.The on-site drilling fluid has poor leak-proof and plugging effects,resulting in large losses,which is one of the main problems in the drilling process.Therefore,it is necessary to carry out the analysis of the formation loss mechanism in the Changning block,develop new materials for plugging,optimize the leakage-proof and plugging performance of the on-site drilling fluid,strengthen the pressure-bearing capacity of the on-site plugging slurry,and reduce the probability of leakage and the loss caused by it.Improve drilling timeliness and economic benefits.Aiming at the leakage problem in the Changning block,by analyzing the type of leakage,leakage velocity and distribution characteristics,the main reasons for the formation leakage in the Changning block are clarified: the Longmaxi formation in the block has low strata strength,poor cementation,stratum fractures,and drilling construction process at the same time It is difficult to control the density of the drilling fluid in the middle,and it is easy to cause fractured leakage of the formation.The characteristics of the leakage channel have been clarified by using conventional logging curves and imaging logging data such as acoustic time difference and resistivity,revealing the fractured leakage and upper strata of the Longmaxi Formation.The mechanism of malignant leakage;in order to solve the insufficiency of the on-site oil-based drilling fluid and plugging slurry to plug the pressure-bearing capacity,in view of the formation leakage mechanism of the Changning block,the leakage prevention and plugging while drilling,the pressure-bearing plugging and the gel bridging are proposed three technical countermeasures for plugging.Using single factor analysis and multi-factor orthogonal test method to optimize the formula,a gel material with high temperature resistance ability is prepared;three kinds of reinforcement materials,rigid particles,expanded particles,and fibers,are selected to improve rheological fluid loss and Water swelling,etc.,the best process conditions for obtaining high temperature resistant gel materials are: 12% acrylamide + 3% diethyldiallyl ammonium chloride + 0.04% N,N’-methylenebisacrylamide+ 4% acrylic acid with 70% neutralization degree + 3% bentonite + 3% composite plugging agent + 3% plugging fiber + 0.05%ammonium persulfate,the reaction temperature is 50°C.The apparent viscosity and waterabsorbing swelling properties of the gel gradually increase with the increase in temperature,reaching the maximum at 120°C,the apparent viscosity is greater than 150 m Pa·s,the water absorption coefficient is 43,and the API fluid loss is 0 m L;when the temperature reaches 140°C,The apparent viscosity and water absorption coefficient are 95.5 m Pa·s and 28,respectively,which proves that the gel has strong temperature resistance.In order to solve the problem that the oil-based drilling fluid in the Changning block cannot effectively plug the micro-cracks,high-efficiency leakage-while-drilling materials have been selected to improve the loss and loss of the sand bed by the drilling fluid without affecting the basic performance of the drilling fluid.The plugging performance of micro-cracks increases the pressure-bearing capacity of 400 μm micro-cracks from 0 MPa to more than 5 MPa;for the shortcomings of insufficient pressure-bearing capacity of plugging large cracks with on-site plugging slurry,rigid materials,elastic materials,and fibers are selected.The material improves the pressure-bearing capacity of 1/2 mm wedge-shaped cracks;the optimized plugging slurry is combined with the high-temperature resistant gel material to form a bridge-gel plugging slurry to seal the pressure of the 3 mm large cracks the capacity is up to 5 MPa,an increase of nearly 80%;the pressure-bearing capacity of the three bridging-gel systems to seal large cracks of 3mm under high temperature conditions is compared and analyzed.The experimental results show that the high-temperature resistant gel material developed in this paper is combined with the plugging slurry.After configuration,it still maintains a strong plugging and pressurebearing capacity at 140°C,and can withstand pressure up to 6 MPa at 100-130°C.