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.