超深层裂缝型储层最佳井眼轨迹量化优选技术与实践--以克拉苏构造带博孜A气藏为例

Technology and practice of quantitative optimization of borehole trajectory in ultra-deep fractured reservoir:a case study of Bozi A gas reservoir in Kelasu structural belt,Tarim Basin

利用定向井眼轨迹穿越有效天然裂缝发育区,是超深层油气藏高效开发的重要手段。然而,超深层气藏处于高压、高温、高地应力的环境中,钻井过程中溢流、垮塌、漏失事故频发,同时裂缝发育和地应力集中共同导致地层各向异性极强,不同方位井眼轨迹上的钻井风险不同。以库车坳陷克拉苏构造带博孜A气藏为例,兼顾定向井安全钻井并尽可能多穿裂缝带,建立研究区的储层三维地质力学模型,分析活动性好的裂缝优势方位和产状特征,考虑强地应力、裂缝弱面评价以及与垮塌、漏失相关的安全钻井密度窗口,为定向井钻前设计提供最优化的井眼轨迹方案。研究结果表明:(1)天然裂缝走向与最大水平主应力方向夹角小、裂缝面剪应力与正应力比值较高时,裂缝有效性较好;(2)钻井井眼垂直于有效裂缝面时,流体从储层向井筒的导流能力最大;(3)储层处于走滑应力机制时,钻井井眼在沿着最大水平主应力方向上的一定扇形区域内稳定性较好,且井斜角越大,钻井相对越安全;(4)天然裂缝发育时,井眼稳定性整体降低,但钻井井眼垂直于有效裂缝面时,稳定性相对较好;(5)根据裂缝剪切变形能力,确定有效裂缝的发育位置和产状,结合裂缝性地层的井壁稳定性评价与漏失压力预测,可定量预测最优井眼轨迹。实践检验表明,预测结果与钻井实践吻合较好,证明了井眼轨迹在优质裂缝体钻遇和安全钻井中的可靠性,为超深层裂缝型储层大斜度井开发奠定了地质力学基础。

The directional well trajectory is an important approach to efficiently produce ultra-deep fractured oil and gas reservoirs by drilling through effective natural fracture development zone.However,complex downhole accidents,such as overflow,collapse and leakage,occur frequently during drilling process due to the high pressure,high temperature and high in-situ stress of ultra-deep gas reservoir.In addition,the combination of fracture development and stress concentration leads to the extremely high anisotropy of the target formation,as a result,various drilling risks exist in borehole with different azimuths of well trajectory.In this paper,Bozi A gas reservoir in Kelasu structural belt of Kuqa Depression is studied as an example.By considering both safe drilling and intersecting with fracture zone as far as possible of the directional well,a 3D geomechanical model for the target reservoir has been established in the study area,so as to analyze the dominant orientation and occurrence of active fractures,evaluate strong in-situ stress and fracture weak plane,determine the mud density window of safe drilling for preventing downhole collapse and leakage,and provide an optimized well trajectory scheme for pre-drilling design of directional well.The results indicate that:(1)The fracture effectiveness is better when the angle between natural fracture strike and maximum horizontal principal stress is low and the ratio of shear stress to normal stress on fracture plane is high;(2)The fluid conductivity is the largest when borehole is perpendicular to the effective fracture plane;(3)When the reservoir is in strike-slip stress state,borehole is generally stable in a certain fanshaped area along the maximum horizontal principal stress,and the larger the deviation angle is,the safer the drilling operation is;(4)When natural fractures are developed,the borehole stability decreases as a whole.While the borehole is relatively stable when it is perpendicular to the effective fracture plane;(5)The development position and occurrence of effective fractures can be determined based on shear deformation capability of fractures.Finally,the optimized wellbore trajectory can be quantitatively predicted by considering both wellbore stability and leakage pressure in fractured reservoir.The practice shows that the prediction is consistent with drilling results,which proves the reliability of the optimized borehole trajectory in high-quality fractured reservoir and safe drilling,providing geomechanical foundation for the development of highly deviated wells in ultra-deep fractured reservoir.