地质工程一体化漏失机理与预防措施

塔里木库车山前古近系复合盐层严重漏失给安全钻井带来了巨大挑战。针对库车山前复合盐层重点漏失层段，分析漏失段地质特征、地质力学特征，结合钻井施工参数，判断漏失类型，分析易漏岩性组合分布与漏失压力，探讨预防钻井液漏失方法。研究结果表明:膏盐岩段盐岩夹含盐泥岩、含膏泥岩，膏质泥岩夹泥岩、灰质泥岩是主要漏层岩性组合，宏、微观分析可见天然裂缝不发育，物性测试表明孔渗较低，不具备直接发生漏失的原有物质基础；含膏泥岩、含盐泥岩抗拉强度0.61~1.12 MPa，平均值0.946 MPa，在压力作用下易产生裂缝；钻井液平均漏速小于20 m3/h，具有微漏~小漏特征，通过降排量降密度可有效缓解漏失。通过盐间漏失层地质、地质力学、工程特征分析，认为复合盐层漏失为诱导破裂型漏失，漏失压力为地层破裂压力，约等于水平最小地应力，并通过模拟应力场数据与实钻漏失点数据对比进行验证。综上分析，明确膏盐岩段易漏岩性组合分布和建立复合盐层黏弹性三维地质力学模型求取水平最小地应力是预防复合盐层漏失的关键，对减少盐间漏失具有重要意义。

Geology-engineering integrated investigation of leakoff mechanisms and prevention measures: A case study of the Palaeogene composite salt layer in the Kuqa piedmont zone,Tarim Basin

The severe leakoff in the Palaeogene composite salt layer of the Kuqa piedmont zone, the Tarim Basin, brings about tremendous challenges to drilling safety. The geological and geomechanical characteristics of key thief zones of the Kuqa piedmont composite salt layers were investigated. Moreover, in accordance with drilling operation parameters, the leak-off types were identified, the leakoff-prone lithologic association and leakoff pressure were analyzed, and the leakoff prevention method was developed. The research showed that the gypsum-salt rock with interbeds of salt-bearing and gypsum-bearing mudstone, and gypseous mudstone with interbeds of mudstone and limy mudstone are the main leakoff lithologic associations. The macro- and micro- scale analyses indicated no material basis for direct leakoff-natural fractures are underdeveloped, and the measured porosity and permeability are low. The tensile strengths of gypsum-bearing and salt-bearing mudstone are 0.61?1.12 MPa, averaging 0.946 MPa, and such layers are prone to fracturing via pressurization. The average leakoff rate of drilling fluids is less than 20 m3/h and represents minor-small leakoff, which can be effectively mitigated by reducing pump rates and mud density. The analysis of geology, geomechanics and engineering characteristics of inter-salt thief zones demonstrated that the leakoff of composite salt layers is attributed to induced fracturing, and the leakoff pressure is equal to the formation fracturing pressure (close to the minimum horizontal principal stress), which is validated via the comparison between the simulated stress field and the leakoff points during drilling. Given the above-mentioned, the key to preventing leakoff of composite salt layers is to clarify the leakoff-prone lithologic association of gypsum-salt rocks and calculate the minimum horizontal principal stress from the viscoelastic three-dimensional geomechanical model of composite salt layers. The findings of this research are of great significance for reducing inter-salt leakoff.