川西新场构造欠平衡钻井欠压值的确定

Definition of Underpressure Value of Underbalanced Drilling in Xinchang Structure,Western Sichuan

川西地区地层岩石致密、硬度大、可钻性差,机械钻速低,欠平衡钻井能有效提高机械钻速。由于该地区地层压力高、泥页岩发育稳定性差,要成功实施欠平衡钻井关键是确定合理的钻井液密度。为此,提出需要获得精确的地层孔隙压力和坍塌压力,确定合理的欠压值,为优化设计提供科学依据。在分析欠压实理论的基础上,运用伊顿法并利用测井资料,结合实测资料,建立了新场构造钻遇地层准确的孔隙压力;通过井壁力学理论分析,结合实钻井资料,确定了井眼扩大率为60%是新场构造允许井眼最大垮塌范围;然后依据井眼扩大率为60%时计算出地层坍塌压力,计算坍塌压力与孔隙压力之差表明,新场地区液体欠平衡钻井欠压值在0.3g/cm3左右。同时,根据建立的压力剖面,新场构造须二段地层坍塌压力、孔隙压力与上部须三段地层坍塌压力一致,目前以须二段为目的层的常规钻井四开次钻井井身结构可以进一步优化、简化,以利于提高钻井速度。

The formations of western Sichuan have poor drillability and low ROP for dense and hard rocks.Underbalanced drilling could effectively enhance ROP.As high formation pressure and unstable shales development,determining rational drilling fluid density is the key to implement unbalanced drilling.Therefore ,accurate formation pore pressure and wellbore collapse compressive stress should be acquired for determining rational underbalanced value,which will provide a scientific basis for optimizing design.Based on the undercompaction theory,accurately pore pressure of the drilled formation may be gained by Eaton method and logging data and the measured data.Through theoretical analysis of wellbore mechanics and real drilling data ,maximal well-bore collapse permissibility range of Xinchang formation is figured out as 60% of wellbore expansion rate.Then the formation caving pressure may be calculated.The difference between the formation caving pressure and pore pressure indicates that underpressure value of liquid-underbalanced drilling in Xinchang area is about 0.3g/cm3.In addition,caving pressure and pore pressure in XujiaheⅡ member of Xinchang structure are the same as that in upper Xujiahe Ⅲ member on the pressure sectional view.So,the well framework at 4th section of conventional drilling aimed to Xujiahe Ⅱ member can be optimized and simplified further so as to enhance drilling speed.