Formation Pressure While Drilling, Wireline Formation Testing, and Fluid Sampling in a High Pressure/High Temperature Exploration Well using Oil Based Mud: A Case History

摘要

An exploration well was evaluated offshore in the Norwegian Sea. The temperatures of the deepest reservoirs exceeded 180 deg C and pressure over 800 bar. The well was drilled with synthetic oil-based mud. After penetrating top reservoir, a formation pressure while drilling (FPWD) tool was deployed. This application was the first in a vertical well from a floating rig. The objectives for running the FPWD tool were to measure formation pressure to within +- 72.5 psi (5 bars) in order to optimize mud weight. In order to restrict tool motion downhole, the tool was deployed with the bit on-bottom. This resulted in a short formation exposure time and a dynamic wellbore situation. Repeat points were therefore acquired, subject to variations in circulation rate, in order to investigate diffusion due to incomplete filter cake development, and the rate of healing processes in the formation. The effect of circulation (annular pressure) on FPWD measurements was observed on some pressure build-up plots that reflected the dynamic effect on the near well bore region. The data obtained demonstrated an approximately 83 psi decrease in stabilized (not building) pressure, at the same depth, with decreasing circulation rate and increasing exposure time. This reflected the equilibration process that diffuses the mud invasion with time. The last FPWD point obtained with no circulation fell within 3 psi of the shallow extrapolation of the wireline gradient obtained subsequently. Attempts to obtain wireline formation pressures failed due to tight tests in the region where FPWD data was successfully acquired. It was thought that the small drawdown and storage volume of the FPWD tool facilitated the acquisition of pressure data in low reservoir quality environments. Also, wireline mobilities were reduced due to particle plugging from CaCO_3 additives and other low gravity solids generated from drilling through hard, quartz-cemented sandstone.