Pore Pressure and Stress Gradient Prediction in Geopressured Formations, South China Sea, PRC

摘要

One of the challenges in drilling wells in South China Sea, Peoples Republic of China (PRC), is the high-temperature/high-pressure (HTHP) environemnt. In HTHP, the range of safe mud-densities fall within a very narrow window between the pore pressure and formation breakdown. Planning the three wells drilled in South China Sea relied on pore presusre and fracture gadient predictions that used seismic and sonic input data for models of compaction and borehole stresses. Pressure and stress gradients can be successfully predicted by quantifying the compaction trends and burial rates of the formations, estimating variance from these trends, and then calculating formation strength and pre-drilling stresses. Abnormal pore presusres were deduced from deviations of sonic velocities from known trends based on seismic integrated travel-time data and wireline sonic data. Rock mechanical elastic properties and strength were estimated and then the equations of elasticity and strength-constrained stresses were applied to determine the onset of shear-failure and formation breakdown. The computed safe mud densities for wellbore elastic stability ranged from 16 to 18 ppg. Because of thermal gradients between the formation and circulating mud, calculated thermal-induced stresses may have drastically lowered the breakdown pressures by as much as 1,000 to 2,000 psi. Mud cooling was not advisable in this case.