Study On Water-based Anti-sloughing Drilling Fluid Technology For Shale Gas Formation

Because of the development of beddings,micro fractures,nano pores and organic matters such as kerogen in shale gas formation,wellbore instability phenomenon frequently occurs in the drilling of horizontal wells,while the problems such as friction,hole cleaning and so on are serious,which seriously restrict the development of shale gas.At present,oil-based drilling fluid is still the major drilling fluid for shale gas drilling.However,there are many problems of OBM such as high cost,poor environmental protection and so on.Also,lost circulation of OBM often occurs in the fractured formation,and is difficult to deal with,which can cause great economic losses.High performance water-based drilling fluid,which can replace OBM is the key technology for high efficient drilling and development of shale gas,and is also a major engineering and technical problems in the world.Through the comprehensive analysis of the composition characteristics of minerals and organic matters,microstructure characteristics,physicochemical properties,absorption law,wetting characteristics and mechanical properties of Lomgmaxi shale in Sichuan Basin target area,we studied the effects of main physical and chemical factors on wellbore stability and revealed the wellbore instability mechanism of shale gas formation.The wellbore instability of fractured shale formation is closely related to its mineral composition and microstructure characteristics.The surface hydration of illite and other clay minerals,and the development of nano scale beddings and fractures are the main factors of wellbore stability in shale gas formation.Also,the existence of organic matter,such as kerogen,has an important influence on shale stability.Through thermogravimetric analysis,adsorption isotherm measurement,spectroscopic characterization and so on,we studied the characteristics and its internal mechanism of shale hydration,and revealed the adsorption behavior of shale inhibitor on the surface of clay minerals and its microscopic inhibition mechanism.In the early stage of hydration,the hydration ability of illite is far superior to that of montmorillonite,and the strong binding water molecules are arranged on the surface of illite,which reduces the total potential energy of the system and weakens its capacity to control the weak binding water molecules.The adsorption of polyamine,K⁺and NH₄⁺on clay minerals surface is more in line with the Langmuir adsorption model,and is monolayer adsorption.Polyamine is mainly adsorbed on clay minerals surface by cation exchange and hydrogen bond.The electrostatic force and hydrogen bond interaction make its adsorption strength higher than K⁺and NH₄⁺.The adsorption of water molecules on inhibitor-clay minerals surface is more in line with the Freundlich adsorption model.The adsorption of inhibitors can significantly reduce clay surface hydration ability,the effect of polyamine is optimal.In addition,the effect of surface hydrophobicity and the neutralization of the protonated amine groups can further weaken clay hydration ability.Through the surface modification and graft polymerization of nano SiO₂,we prepared a temperature sensitive intelligent nano plugging agent SD-SEAL.Experimental results showed that SD-SEAL was sensitive to temperature,and when the temperature was higher than its LCST value,SD-SEAL played a dual role of physical plugging and chemical inhibition,slowed down pressure transmission and filtrate invasion remarkably.We prepared graphene oxide sheets with high monolayer rate through an improved Hummers method.In addition,we prepared an ultra-low permeable film-forming agent through the in-situ polymerization of GO.Together with SD-SEAL,it can block the pressure transmission of shale,reduce shale permeability by two orders of magnitude,and greatly improve shale stability.Based the analysis of micro-nano scale wellbore instability mechanism of shale and the multivariate cooperation principle for wellbore stability,we put forward the design principles of water-based drilling fluid for shale gas drilling.Based on the self-made nano plugging agent,ultra-low permeable film-forming agent and combined with new polyamine inhibitor,we developed a water-based drilling fluid with excellent comprehensive performances for shale gas formation,which can effectively plug micro-nano scale fractures and pore-throats,inhibit shale hydration.At the same time,it has many other excellent properties,such as pollution resistant,better environmental performance and low cost.