针对页岩气开发钻井工程中亟待解决的泥页岩钻井井壁安全密度窗口计算难题,通过对Heidug-Wong的广义Biot有效应力原理进行修正,使用Weibull统计模型描述水化应变相关的强度损伤,考虑钻井液封堵层,结合流动-扩散耦合模型建立了钻井液抑制-封堵-渗透作用下的泥页岩安全密度窗口计算方法。以此为基础,分析了钻井液封堵性、抑制性等关键参数对安全密度窗口的影响：提高钻井液封堵性,能够阻缓泥页岩井壁孔隙压力传递,减少溶质扩散;钻井液的抑制性,尤其是对强度损伤的抑制,对水化显著的泥页岩井壁坍塌压力具有决定性的作用,钻井液封堵-抑制性能的提高可以降低坍塌压力、提高破裂压力,从而拓宽安全密度窗口,延长井壁的坍塌周期。若泥页岩内存在渗透流动,钻井液渗透作用能够拓宽安全密度窗口,且钻井液封堵性越强,其影响幅度越大。利用安全密度窗口计算方法可以有效分析坍塌压力、破裂压力与钻井液防塌性能的关系,可用于安全密度窗口的准确计算及钻井液性能的优化。图11表3参29
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 for shale formation under drilling fluid sealing-inhibition-penetration effect has been established combined with a flow-diffusion coupling model. The effects of drilling fluid sealing and inhibiting parameters on safe drilling fluid window were analyzed. The study shows that enhancing drilling fluid sealing performance can slow the pore pressure transmission and reduce solute diffusion; the inhibiting property of drilling fluid, especially inhibition to strength damage, is crucial for the wellbore collapse pressure of shale section with strong 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 cycle 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 fluid density 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.
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