A formation pressure while drilling tool (FPWD) was deployed in a recent vertical HT/HP exploration well (c. >180Degc / 830 bar). Subsequently conventional wireline formation pressures and samples were also acquired. Procedures were established to restrict tool motion, which was a key risk to obtaining a sustained pressure seal with the borehole wall. The objectives for running the FPWD tool were to measure formation pressure to within +/- 72.5 psi (5 bars) in order to optimise mud weight for coring. At the time of acquisition the well was approximately 690 psi overbalanced with respect to formation, this overbalance was subsequently dropped to approximately 320 psi. In addition the data helped to confirm the lack of reservoir quality in one prospective interval. In the planning stages the dynamic wellbore situation was acknowledged. It was planned to acquire repeat points subject to variations in circulation rate and, inherently, formation exposure time. This was to investigate diffusion and healing processes in the formation. 12 FPWD measurements were obtained at 5 separate depths. The data obtained demonstrate an ~ 83 psi decrease in recorded pressure at the same depth with decreasing circulation rate and increasing exposure time. This applies to pressure build-ups that were classed as stabilised (not building) and reflects the equilibration process that diffuses the mud invasion with time. The effect of circulation (annular pressure) can also be observed on some pre-test pressure build-up plots, reflecting the dynamic effect on the near well bore region. The last FPWD point obtained was acquired with no circulation. This point falls within 3 psi of the extrapolation to shallower depth of the fluid-sample calibrated gradient obtained from later wireline formation tester acquisition. The data clearly shows the dynamic effect on formation pressure while drilling data. Subsequent attempts to obtain wireline formation pressures failed (tight tests) in the region where FPWD data was successfully acquired. Although the heterogeneous nature of the reservoir could be responsible (slight differences in the probe setting depth), it is thought that the small drawdown and storage volume of the FPWD tool facilitates the acquisition of pressure data in low reservoir quality environments, although that data may be affected by the dynamic situation in the borehole environment.
Alan is a Petrophysical Engineer in Norske Shell. He joined Shell in Aberdeen in 2001 where he worked until March 2005. He moved to Norway permanently on April 1st 2006 and is living in Stavanger. He has a BSc in Geology and Physical Geography from the University of Edinburgh and an MSc in Petroleum Geoscience from The University of Aberdeen