深井安全钻井液密度窗口影响因素

考虑钻井液渗滤造成井壁岩石孔隙压力变化和钻井液与地层岩石温差产生的附加应力和应变,推导了孔隙度与孔隙压力和温差的理论关系,建立了考虑孔隙压力、温差及孔隙度变化的深井安全钻井液密度窗口计算模型。应用模型计算结果表明:①深井钻井井壁岩石与钻井液温差一定时,随着钻井液渗滤作用的增强,井壁岩石孔隙压力增加,导致坍塌压力增大,破裂压力减小,安全钻井液密度窗口变小,不利于安全钻井。②当井壁岩石孔隙压力一定时,若钻井液使井壁岩石降温,则随着温差的增加,坍塌压力减小,破裂压力增加,安全钻井液密度窗口范围变大,有利于安全钻井;若钻井液使井壁岩石升温,则随着温差的增大,坍塌压力增大,破裂压力减小,安全钻井液密度窗口变小,不利于安全钻井。

Analysis on influential factors for safe window of drilling fluid density in deep well

Considering the pore pressure variation of sidewall rock, which is caused by drilling fluid filtering, and additional stress and strain which is caused by temperature difference between drilling fluid and formation rock, the theoretical model of variation of porosity, which has considered variation of pore pressure and temperature difference, is derived, and the calculation model of safe window of drilling fluid density in deep well, which has considered variation of porosity, temperature difference and pore pressure, is finally established. Application of this calculation model shows that: when temperature difference between sidewall rock and drilling fluid in deep well is definite, with increased function of drilling fluid filtering, as increased as pore pressure of sidewall rock, the caving pressure is increased, fracturing pressure is decreased, safe window of drilling fluid density is narrowing, and unfavorable for safety drilling; and when pore pressure of sidewall rock is definite, if drilling fluid lower the temperature of sidewall rock, then as increased as temperature difference, the caving pressure is decreased, fracturing pressure is increased, safe window of drilling fluid density is widen, and favorable for the safe drilling. Moreover, if drilling fluid rises the temperature of sidewall rock, then as increased as temperature difference,the caving pressure is increased, fracturing pressure is decreased, safe window of drilling fluid density is narrowing, and unfavorable for the safety drilling. In this paper, the effects of drilling fluid filtering and temperature difference between drilling fluid and formation rock on safe window of drilling fluid density are theoretically explained, which is a of significance to the present calculation model of safe window of drilling fluid density.