钻井液对裂缝性地层气侵的影响模拟研究

Simulation Study on the Effects of Drilling Fluid on Gas Cut from Fractured Formations

裂缝性地层钻进过程中,容易发生重力置换气侵,裂缝内气体进入井筒后,控制不当易引发井涌、井漏、井喷等复杂情况,气侵发生时裂缝附近的压力波动容易导致井壁失稳。在塔中北坡顺南区块裂缝特征参数分析基础上,使用Gambit建模软件进行前期建模和网格划分,建立了井筒与裂缝的三维模型;运用ANSYS Fluent流体力学软件,建立多相流模型,分析置换过程中裂缝开口处动压对裂缝开口处井壁稳定性的影响;以现场某井的数据为参考,模拟分析钻井液的密度、黏度、排量对重力置换气侵的影响。结果表明,置换气侵发生时,由于压差较大,瞬间流速较大,在0.1s内降低至较低水平;动压的最大值在裂缝的上下两端,最小值在裂缝中上部,这3个位置的裂缝口缝宽容易增加,引起井壁失稳;钻井液的密度、黏度变化对气侵时裂缝口的动压影响较小,变化量小于6%,对气侵速度的稳定值影响也较小;而排量对于气侵时裂缝口的动压影响较大,变化量达到66.9%,降低钻井液排量有利于井壁稳定。

Gas cut caused by gravity displacement is frequently encountered when drilling fractured formations. Well kick, mud loss and well blowout will happen if gas entering the wellbore from fractured formations is not correctly handled. When gas kick is happening, pressure fluctuation around the fractures will result in borehole wall instability. In laboratory studies, based on the analyses of the characteristic parameters of the fractures in Block Shunnan located in the north slope of Tazhong, the early stage modeling and meshing were done using the modelling software Gambit, and 3D models for wellbore and fractures were established. Using ANSYS Fluent hydromechanical software, a multiphase flow model was established with which the effects of the dynamic pressure at the mouth of a fracture on the stability of the borehole wall at the same place during gravity displacement were analyzed. Drilling data of a well, such as the density, viscosity and flow rate of the drilling fluid were used to analyze their effects on gas cut caused by gravity displacement. It was found that when gravity displacement took place, high pressure differential resulted in high instant flow rate of gas, which was then reduced to a low level in just 0.1 sec. The maximum dynamic pressure was found at the upper and lower ends of the fracture, while the minimum dynamic pressure was found at the middle-upper position of the fracture. The widths of the fracture at these three points were easy to increase, causing the wellbore to destabilize. Changes in the density and viscosity of the drilling fluid caused change in the dynamic pressure at the mouth of the fracture by only 6%. The steady rate of gas cut was also less affected by the changes in the density and viscosity of the drilling fluid. On the other hand, flow rate of the drilling fluid during gas cut greatly affected the dynamic pressure at the mouth of the fracture, causing the dynamic pressure to change by 66.9%, indicating that reduced flow rate of drilling fluid during gas cut is beneficial to bringing gas cut under control.