Simulation Study On Borehole Stability Mechanism

Borehole instability of shallow formations in deep-water drilling is one of the mostcomplex technical problems in the process of deep-water well drilling. Compared withland-based and shallow water drilling, the geological conditions of shallow formations indeepwater are particularly complex，the shallow formations of deepwater are geological loose,uncompacted, poor cementation, easy swelling and dispersing, making serious hydration andborehole collapse.This paper based on research and analysis of the basic physical properties of shallowsediments in deepwater of South China Sea, and explored to use of molecular modelingsoftware Material Studio（MS）to build Na-montmorillonite crystal model, and then thedynamic simulation study was made on its hydration mechanism and hydration process. Thesimulation results showed that the Na-montmorillonite layer space is gradually increased withthe increasing of the adsorbed water molecules on crystal surface，the first, second, third watermolecules saturated layers were completely formed when the water molecules numberreached48,72,96. The cation hydration parameters were calculated by the Visual Studiosoftware programming, and then took the study of elastic and mechanical properties trend ofcrystal and wellbore stability effects and mechanism under the conditions of seven kinds ofsaturation concentration inorganic salts. The simulation results showed that potassiumchloride, sodium chloride and calcium chloride had better impacts compared with the otherinorganic salts to improve the Na-montmorillonite crystal elastic modulus and stabilizewellbore. Through the research in the elastic mechanical properties of Na-montmorillonitecrystal affected by above three different concentrations additions, function expression andcurve were fit between Young’s modulus and the inorganic salt concentration, and then gavethe optimum dosage range of three inorganic salts when the stability of Na-montmorillonitecrystal was in ideal condition which had a good guide for the research and development of deepwater shallow drilling fluid.