滤饼质量对井壁周围应力和井壁稳定性的影响

无论是过平衡钻井,还是欠平衡钻井过程,都可能导致井周围地层孔隙压力改变,这样就使通过假定地层孔隙压力恒定而得到的井壁周围应力分布和地层的安全钻井液密度范围与实际偏离。滤饼质量直接影响到井壁附近地层的孔隙压力,定量地分析了滤饼质量对井周地层径向应力、切向应力、轴向应力和剪应力的影响。结果表明:一方面随着滤饼质量的提高,井筒内钻井液液柱压力对井壁地层的支撑作用逐渐增强,井壁地层抗剪切垮塌的能力相应提高,井壁稳定性变好;另一方面,地层颗粒间的接触应力逐渐增大,岩石抵抗张性破裂的能力增强,地层的破裂压力提高,井壁稳定性变好。滤饼质量好坏对低渗透地层的影响尤其显著。

Effect of Filtrate Cake Quality on Borehole Stress and Hole Stability

The formation pore pressure is assumed as stable, while it changes in the surrounding formation of the hole during drilling, no matter it is over-balanced drilling or under-balanced drilling, thus deviations are produced on the borehole stress distribution and the safe range of drilling fluid density between the assumption and the real condition. As the quality of filtrate cake influences the change of formation pore pressure near the borehole, thus how the filtrate cake quality affect on the radial stress, axial stress, surrounding stress and shearing stress is analyzed quantitatively. The results show that with the improvement of filtrate cake quality, the supporting effect of drilling fluid column to borehole becomes more remarkable, and the anti-shearing and sloughing control capability also improve, thus the wellbore tends to be more stable. In addition, the contacting stress between formation fines increases also, and the tension fracturing resistance of rock becomes higher, with higher fracturing pressure detected; this also improves the stability of the borehole. The effect of filtrate cake quality on low permeable formation is especially remarkable.