Mechanisms and Inhibition of Borehole Instability Encountered in Drilling the Shiqianfeng Formation – Shihezi Formation in the East of Yan’an Gas Field
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摘要: 为了研究延安气田东部区域钻井过程中石千峰组-石盒子组井壁失稳的内在原因,对该地层岩样的矿物组分、理化特征、力学特性进行深入分析,并针对井壁失稳机理提出了抑制液相自吸的稳定井壁对策。研究可知:延安气田东部区域石千峰组-石盒子组黏土矿物含量分布在15.44%~47.52%,属弱膨胀中等分散性岩性,岩心中微裂缝、微孔隙发育,是导致液相侵入、井壁坍塌的主要因素;石千峰组-石盒子组岩样在蒸馏水中滚动回收率低于67.2%、线性膨胀率低于8.14%,石盒子组岩样分散相更强,润湿性均表现为亲水性,且石盒子组岩心的亲水性更强;石千峰组岩样经钻井液浸泡后三轴抗压强度由186.04 MPa降低至98.13 MPa;石盒子组岩样浸泡后三轴抗压强度由90.09 MPa降低至49.21 MPa,表明钻井液沿微孔隙、微裂缝侵入后使岩石强度降低;0.3%自吸水抑制剂ZXS-1可使石盒子组岩样水相和油相在岩心内的饱和度由72.6%和86.6%降低至4.7%和33.5%,并在岩石表面形成一层致密的分子吸附层,将岩心表面的润湿性由亲水性转变为疏水性,通过抑制岩样吸水,达到封堵微裂缝、阻止液相侵入的稳定井壁效果。Abstract: To find out the inherent causes related to the borehole wall collapse in drilling the Shiqianfeng formation – Shihezi formation in the east of the Yan’an gas field, samples were taken from the rig-sites and were studied for their mineral composition, physical-chemical features and mechanical characteristics. Based on the study a measure dealing with the borehole wall collapse, which is to inhibit the self-imbibition of liquid into the formation rocks, is presented. From the study it was found that the Shiqianfeng formation – Shihezi formation in the east of the Yan’an gas field contain 15.44% – 47.52% clays, and are a formation with weak expandability and moderate dispersibility. The rock samples are developed with micro-fractures and micro-fissures which are the main causes for the invasion of liquids and hence the collapse of borehole walls. The percent cuttings recovery of the samples tested on hot roller tester in distilled water is less than 67.2%, and the linear percent expansion of the cores made of the samples in distilled water is less than 8.14%. The Shihezi formation shows stronger dispersibility and stronger hydrophilicity than those of the Shiqianfeng formation. After soaking in a drilling fluid, the triaxial compressive strength of the Shiqianfeng sample is reduced from 186.04 MPa to 98.13 MPa, and the triaxial compressive strength of the Shihezi sample is reduced from 90.09 MPa to 49.21 MPa, indicating that the invasion of the drilling fluid into the rocks along the micro-fractures and micro-fissures causes the strengths of the formations to reduce. Using 0.3% self-imbibition inhibitor ZXS-1, the degrees of saturation of water phase and oil phase in the rock samples can be reduced from 72.6% and 86.6% to 4.7% and 33.5%, respectively, and the ZXS-1 additive can form a dense layer of adsorption around the surfaces of the rocks, thereby turning the wettability of the rock surfaces from hydrophilicity to hydrophobicity. By inhibiting the water imbibition into the rocks, the micro-fractures and micro-fissures are plugged and the liquid can no longer invade into the rocks, and the borehole walls are thus stabilized.
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表 1 石千峰组-石盒子组岩样回收率、膨胀率测试结果
井号 层位 液体类型 m热滚后/g 滚动回
收率/%线性膨
胀率/%2 h 16 h Y2004井 石千峰组 蒸馏水 33.6 67.2 5.12 6.46 现场钻井液 42.4 84.8 4.78 5.62 石盒子组 蒸馏水 27.8 55.6 6.85 7.56 现场钻井液 39.6 79.2 5.86 6.98 Y2005井 石千峰组 蒸馏水 32.2 64.4 5.47 6.18 现场钻井液 41.9 83.8 4.89 5.76 石盒子组 蒸馏水 25.4 50.8 7.32 8.14 现场钻井液 38.6 77.2 5.72 6.74 表 2 石千峰组-石盒子组岩样滚动分散后的电动电位
井号 岩心层位 液体类型 电动电位/mV Y2004井 石千峰组 蒸馏水 −30.6 现场钻井液 −25.4 石盒子组 蒸馏水 −34.8 现场钻井液 −26.8 Y2005井 石千峰组 蒸馏水 −31.4 现场钻井液 −26.2 石盒子组 蒸馏水 −36.2 现场钻井液 −25.4 表 3 岩样硬度实验结果
岩心层位 实验条件 实验载荷/kN 硬度/MPa 石千峰组 浸泡前 5.1421 752.3269 浸泡后 4.3198 632.0184 石盒子组 浸泡前 4.2338 619.4360 浸泡后 3.6520 534.3144 -
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