As an important industrial material, bentonite has been widely applied in water-based drilling fluids to create mud cakes to protect boreholes. However, the common mud cake is porous, and it is difficult to reduce the filtration of a drilling fluid at high temperature. Therefore, this paper endowed bentonite with a thermo response via the insertion of N-isopropylacrylamide (NIPAM) monomers. The interaction between NIPAM monomers and bentonite was investigated via Fourier infrared spectroscopy (FTIR), isothermal adsorption, and X-ray diffraction (XRD) at various temperatures. The results demonstrate that chemical adsorption is involved in the adsorption process of NIPAM monomers on bentonite, and the adsorption of NIPAM monomers accords with the D–R model. With increasing temperature, more adsorption water was squeezed out of the composite when the temperature of the composite exceeded 70 °C. Based on the composite of NIPAM and bentonite, a mud cake was prepared using low-viscosity polyanionic cellulose (Lv-PAC) and initiator potassium peroxydisulfate (KPS). The change in the plugging of the mud cake was investigated via environmental scanning electron microscopy (ESEM), contact angle testing, filtration experiments, and linear expansion of the shale at various temperatures. In the plugging of the mud cake, a self-recovery behavior was observed with increasing temperature, and resistance was observed at 110 °C. The rheology of the drilling fluid was stable in the alterative temperature zone (70–110 °C). Based on the high resistance of the basic drilling fluid, a high-density drilling fluid (ρ = 2.0 g/cm3) was prepared with weighting materials with the objective of drilling high-temperature formations. By using a high-density drilling fluid, the hydration expansion of shale was reduced by half at 110 °C in comparison with common bentonite drilling fluid. In addition, the rheology of the high-density drilling fluid tended to be stable, and a self-recovery behavior was observed.
Shale gas has become the major source of natural gas. However, shale is rich in clay and easily collapsed by water invasion. This not only causes collapse of the reservoir but also causes the loss of natural gas and can even cause local earthquakes and affect the safety of human beings. This paper describes an investigation of the relationship between hydration and collapse. Shale samples were obtained from a series of wells drilled in the lower Silurian Longmaxi Formation at a depth of 3500 m. The different hydrated shales were simulated to analyze the hydration−collapse relationship. Magnetic resonance analysis and mechanical analysis were combined to analyze the collapse of the hydrated shales. The collapse progression was found to follow an S-shaped growth curve that can be divided into three parts, namely, the potential period, the exciting period, and the mature period. The hydration state and degree of damage were determined from the magnetic resonance of water molecules. This paper proposes a mechanism for shale hydration collapse based on basal and numerical data that can be used to predict shale collapse as a function of hydration.
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