基于井壁稳定分析的井眼轨迹优化方法

井壁稳定问题是钻井工程关注的焦点之一,但目前基于井壁稳定分析优化井眼轨迹方面开展的工作还很少。为此,基于斜井井壁应力分布模型和井壁失稳破坏模式,分别采用Mohr-Coulomb准则、Drucker Prager准则和拉张破坏准则导出了维持井壁稳定的钻井液安全窗口上下限计算模型,利用数值计算方法建立了安全密度窗口、安全和危险井眼轨迹的求解方法,计算和分析了6种典型地应力状态下的井壁稳定性,并分析了相应地应力状态下安全和危险的井眼轨迹。结果表明:①Mohr-Coulomb准则计算结果高于Drucker-Prager准则,主要由于Mohr-Coulomb准则忽略了中间主应力的影响,低估了地层岩石的强度;②在σ_v≥σ_Hσ_h情况下沿最小水平地应力钻进稳定性最好,σ_Hσ_vσ_h情况下钻进稳定性最好的方向为偏离最大水平地应力30°~45°的方向上,σ_(11)σ_h≥σ_v情况下沿最大水平地应力钻进稳定性最好,σ_H=σ_vσ_h情况下直井稳定性最好;③数值计算方法得出的安全井眼轨迹与AZ模型吻合较好,验证了该方法的正确性和准确性。数值计算方法可以直观给出不同轨迹下的安全密度窗口,对于窄安全密度窗口地层,可采用该方法优化井眼轨迹改善井眼稳定性。

Well path optimization based on wellbore stability analysis

Wellbore stability is a focus in drilling engineering, but only a few researches have been conducted on the well path optimization based on wellbore stability analysis. Therefore, based on the wellbore stress distribution model and the wellbore instability mode of deviated wells, the Mohr-Coulomb criterion, the Drucker-Prager criterion and tensional failure criteria were used respectively to derive the computation model of safety window bound of drilling fluid to maintain the wellbore stability, and the numerical method was used to build the solution method of safe density window and safe and hazardous well paths. The wellbore stability was calculated and analyzed under six typical in-situ stresses, and the safe and hazardous well paths under corresponding crustal stresses were also analyzed. The study results show that the calculation results by the Mohr-Coulomb criterion are higher than those by the Drucker-Prager criterion, mainly resulting from that in the Mohr-Coulomb criterion the effect of intermediate principal stress is neglected and the strength of formation rocks is underestimated. When σ_v≥σ_H＞σ_h, the stability is the best while drilling along the direction with the minimum horizontal crustal stress; when σ_H＞σ_v＞σ_h, the best drilling stability is shown in the direction of 30°-45° deviating from the maximum horizontal crustal stress; when σ_H＞σ_h≥σ_v, the stability is the best while drilling along the direction with the maximum horizontal crustal stress; and when σ_H=σ_v＞σ_h, the stability of a vertical well is the best. Moreover, the safe well paths obtained by the numerical method tally better with those by the A & Z model, verifying the correctness and accuracy of this method. Because the numerical method can intuitively present the safe density window under different well paths, it can be used to optimize the well path and improve the wellbore stability of narrow safe density window formations.