Calculation of safe drilling mud density window for shale formation by considering chemo-poro-mechanical coupling effect

It is difficult to define safe drilling mud density window for shale sections. To solve this problem, the general Biot effective stress principle developed by Heidug and Wong was modified. The Weibull statistical model was used to characterize the hydration strain-related strength damage. Considering drilling fluid sealing barrier on shale, a calculation method of safe drilling mud density has been established for shale formation under drilling fluid sealing-inhibition-reverse osmosis effect, combined with a flow-diffusion coupling model. The influence of drilling fluid sealing and inhibiting parameters on safe drilling mud density window was analyzed. The study shows that enhancing drilling fluid sealing performance can reduce the pore pressure transmission and solute diffusion; the inhibiting performance of drilling fluid, especially inhibition to strength damage, is crucial for the wellbore collapse pressure of shale section with significant hydration property. The improvement of drilling fluid sealing and inhibition performance can lower collapse pressure and enhance fracturing pressure, and thus making the safe drilling fluid density window wider and the collapse period of wellbore longer. If there is osmosis flow in shale, induced osmosis flow can make the gap between collapse pressure and fracturing pressure wider, and the stronger the sealing ability of drilling fluid, the wider the gap will be. The safe drilling mud density window calculation method can analyze the relationships between collapse pressure, fracturing pressure and drilling fluid anti collapse performance, and can be used to optimize drilling fluid performance.