川西南部钻井井下复杂的地质原因

Geological causes of drilling complexity in southwestern Sichuan Basin

四川盆地川西南部地区井漏、井壁垮塌、高压盐水侵等钻井复杂频发,严重影响井下作业安全和工程进度。为了减少钻井复杂故障率、为工程制定应对措施提供地质依据,采用地震、测井以及录井等多专业信息融合分析的方法,综合区域构造背景、地层岩性特征、地层高压流体特点对钻井井下复杂的地质原因进行了分析研究。研究结果表明:①断层和裂缝是引起研究区井漏的主要原因,其次是采空区地层压力下降导致压张性井漏和平衡高压盐水层压力的高密度钻井液引起盐水层上部低压地层压张性井漏;②井壁垮塌的主要原因是断裂和裂缝导致井壁围岩力学强度降低以及泥页岩水化导致泥岩基质分散;③盐水侵主要发生在三叠系雷口坡组、嘉陵江组等蒸发相的碳酸盐岩地层中,具有盐水矿化度高和异常高压等特征,以局部封存水和运移水两种富集形态存在,水层的位置难以预测,膏盐层应以整体井控安全防控为主。

In southwestern Sichuan Basin,some drilling complexity such as lost circulation,borehole collapse,and high-pressure brine invasion often happens to seriously affect downhole safety and drilling schedule.So,in order to reduce failure and provide geological evidence for making engineering countermeasures,a method to integrate multi-disciplinary information including seismic exploration,well logging,and mud logging was used to analyze the geological causes of downhole complexity from aspects of regional structural setting,lithology,and high-pressure fluid.Results show that(1)both fault and fracture are the primary causes of lost circulation,whereas the secondary ones are two kinds of compressional-tensional lost circulation in the mined-out area and in the upper low-pressure layer of brine stratum individual resulting from formation-pressure decrease and high-density drilling fluid to balance the pressure in high-pressure brine stratum;and(2)not only the decrease of surrounding-rock mechanical strength induced by fault and fracture but also the dispersion of shale matrix due to hydration are the main causes of borehole collapse;and and(3)brine invasion mainly occurs in some evaporative-facies carbonate rocks with high salinity and abnormal high pressure,such as Leikoupo Formation and Jialingjiang Formation,and brine exists in the form of local storage water and migrating water,which brings about much difficulty to predict the specific position of aquifer.Thus,for safe drilling the gypsum salt layer,both prevention and control should be emphasized.