IMPROVING THE WELLBORE STABILITY OF SHALE FORMATION WITH WATER ACTIVITY REGULATION

Wellbore instability in reactive shale formation is in fact the most significant technical problem in oil and gas drilling engineering. A high performance and environmental friendly water based drilling fluid capable of improving shale stability is always appealing. Numerous methods have been proposed to improve the shale hydration inhibitive properties of water-based drilling fluid. Among these methods, water activity adjust based on semi-permeable membrane theory is an effective approach. In this study, the effect of water activity on the shale hydration was investigated with shale cuttings hot-rolling test and shale core immersion test. Then the influence of water activity of drilling fluid on the interlayer spacing of clay particles was probed. According to the results of shale dispersion experiment, in terms of inhibiting shale dispersibility, the inhibitory abilities of most active regulators first improved and then tended to be stable, with decreasing activity; among them, potassium formate had the best inhibitory effect and an optimum activity of approximately 0.92. According to the results of collapse experiment, borehole stability correlated to the formation water content, i.e., the lower the water content, the more serious the capillary self-absorption, and the stronger the action of core collapse; when water content was high, under the condition of activity equilibrium, there was no osmotic hydration phenomenon, and the core could remain the same. According to the XRD results, different salts had different effects on clay layer interval, and among them, potassium formate exhibits the best performance. Under the action of potassium formate, the clay layer interval declined to 14.18 angstrom. Potassium formate and potassium chloride exhibited synergistic effect, and after compound use, the clay layer interval dropped sharply to 13.60 angstrom.