延长气田石千峰组与石盒子组井壁失稳机理的研讨

延长气田钻进石千峰组与石盒子组过程中经常发生严重井塌。通过对该气田地应力、坍塌地层岩石力学性能、矿物组分、理化性能的测定,地层3个压力的计算,钻井液抑制性、封堵性、浸泡时间对岩石强度及坍塌压力的影响实验研究,以及对测井资料和已钻井资料的综合研究分析,揭示了该气田钻井过程井壁失稳的机理,为解决该气田井壁失稳技术难题提供理论依据。分析得出井壁失温的潜在因素有:地层属于强硬脆性地层,具有在远低于峰值应力的状态下进入扩容状态的特征,压力波动易导致井周微裂缝扩展、交汇,形成不稳定的高渗带;该气田不存在强构造应力,但存在扭转,地层裂缝发育;地层属于晚成岩期,泥岩泥质含量高,弱分散,中强膨胀,需加强钻井液的抑制水化膨胀作用。井壁失稳的主要原因为:钻井液密度低于地层坍塌压力的当量密度,空井时间过长时会造成坍塌压力增高,进一步加剧井塌;现场所使用的钻井液的封堵性不良,在井壁与微裂缝相交的界面上不能形成有效的封堵。

Study on Borehole Collapse Drilling Shiqianfeng and Shihezi Formations in Yanchang Gasfield

Borehole collapse had always been encountered when drilling the Shiqianfeng and Shihezi formations in Yanchang gas field. Laboratory study has been conducted to determine the mechanism governing the borehole wall instability. By measuring the geo-stress, mechanical characteristics of the rocks in formations where borehole collapse was encountered, mineral composition, and physic-chemical properties of the formations, it was found that the Shiqianfeng and Shihezi formations are composed of hard and brittle rocks, is capable of being dilatant at stresses that are far below the peak stress. Fluctuation of pressure forced the micro fractures near the wellbore to extend and converge, forming an instable high permeability zone. Study found that no high tectonic stress exists in Yanchang gas field, but formation reversion and fracture are quite developed. The formations drilled are in late diagenesis, and the mudstones have high clay content, making them moderate to strong swelling. Drilling fluid used in drilling these formations should have good inhibitive capacity. Low mud weight, less than the density equivalent of collapse pressure of formation, caused the formation to collapse. Long period of time for the borehole to be "empty" (with no drill tools in the hole) led to increasing collapse pressure of the formations, aggravating borehole collapse. Poor sealing and plugging ability of the drilling fluid, which cannot effectively seal and plug micro fractures on the borehole wall, was another cause of borehole collapse.