Open Hole Fluid Displacement Analysis - Forward vs. Reverse Circulations

Abstract

Angola is home to two of BP's largest subsea developments, Greater Plutonio and PSVM. High rate production wells have been achieved through the use of long open hole gravel pack completions, with alternate path screen design. Since the screens are deployed in conditioned oil based mud, the open hole is displaced to aqueous fluid prior to gravel packing the well. The early Greater Plutonio wells extensively utilised what is referred to as the 'conventional,' 'forward,' or Heel-to-Toe technique to displace the oil based fluid to water based. However, depletion of some of the reservoir sands increased the likelihood of screen plugging in wells with long open hole sections, as demonstrated by a handful of incidents. Also, the tight pore pressure to fracture gradient (PPFG) windows and small screen gauge sizes in the PSVM development made the 'forward' technique less suitable to adopt as a basis of design. As such, both projects developed a different displacement technique referred to as 'reverse' or Toe-to-Heel circulation to overcome the issues of sand control screen plugging and small PPFG windows. Work was performed to confirm that service tools could maintain full functionality, the fluid pill train properties were appropriate, and that hydraulic models could adequately predict the ECDs generated during the displacement. Eight open hole gravel pack wells, across both projects, have now been successfully completed using the Toe-to-Heel technique. Using the data from these wells, along with the existing data set from the earlier Greater Plutonio wells, it has been possible to determine a set of guidelines which indicate which technique is best suited under certain conditions. This paper briefly describes the history of the two projects, details the operational differences between both methods, describes the evolution of the modelling work done for ECD prediction, including several pros and cons associated with each method. The actual displacement data is also presented, and is compared with the predictions from the initial modelling simulations. The development of even more accurate modelling techniques is also discussed.