D气田衰竭疏松石英砂岩浅气层漏失机理及防漏技术

衰竭气层具有气层孔隙压力、坍塌压力、漏失压力、破裂压力等较原始值明显降低的特点，并且钻井施工压差相对较高，井漏易发和气层损害是困扰施工安全和工程质量的主要难题，而搞清漏失机理及漏失性质是解决该难题的关键。为此，结合南海北部D气田大位移水平井钻井实践，研究提出了一套衰竭疏松石英砂岩浅气层井漏诊断及防控配套技术，该技术基于衰竭气层现今四压力剖面及水力学分析，结合漏失特征，首先对漏失性质准确研判；在此基础上，兼顾防漏及气层保护需要，从减小循环压耗及ECD值、降低施工压差入手，研发形成无固相返排解堵弱凝胶钻井液EZFLOW-Ⅱ，并依据衰竭气藏现今四压力剖面，大位移水平井井眼清洁、保护气层需要，对钻井液密度及流变性、封堵性和润滑性进一步优化，配合控制钻速、定期短程起下钻、强化固控等工程措施，实现了良好的防渗漏、防压差卡钻以及气层保护效果。该技术解决了南海北部D气田衰竭石英砂岩浅气层大位移水平井井漏及其引发的气层损害难题，相关成果对于类似低压衰竭疏松砂岩浅气层防漏、保护气层工作具有指导意义。 

Mud Losses into Depleted Loose Quartz Sandstone Shallow Gas Zones in Gas Field D: Mechanisms and Preventing Technology

The pore pressure, collapse pressure, leaking pressure and fracture pressure of a depleted gas zone declined remarkably with time. When drilling into a depleted gas zone, the differential pressure between the pressure exerted by the mud column and the pore pressure of the formation is high, resulting in mud losses and formation damage. Understanding of the mechanisms and nature of mud losses is key to solve these problems. Based on the extended reach horizontal drilling practice in the D gas field in the north of South China Sea, a set of technologies was established for analyzing, preventing and controlling mud losses into shallow depleted loose sandstone gas zones. Using these technologies, the nature of the mud losses was first accurately determined by analyzing the current four-pressure profile and mud hydraulics. A solids-free weak-gel drilling fluid EZFLOW-II was then developed. Using EZFLOW-II, the circulation pressure and ECD can be reduced, thereby minimizing the risk of mud losses. In developing the fluid EZFLOW-II, the density, rheology, plugging performance and lubricity of the drilling fluid were further optimized based on the current four-pressure profile of the depleted gas zone to satisfy the needs of hole cleaning in extended reach well and reservoir protection. In drilling operation, several measures, such as ROP control, periodic short trip and using of good SCE, were taken to prevent mud losses and pipe sticking, and to protect the gas reservoir. The use of these technologies helped solve mud losses in extended horizontal drilling and the resultant formation damage in the shallow depleted sandstone gas reservoir in Gas Field D in South China Sea. The work is of guiding significance to the prevention of mud losses into low pressure shallow depleted loose gas reservoirs of similar characteristics and to the protection of the gas reservoirs.