Low-Density Invert Emulsion Drilling Fluid Enables Recovery of Oil Reserves in Extremely Depleted Reservoirs: A Case History From Valhall, Norway

Abstract

Abstract A drilling fluid designed for drilling long horizontal wells in extremely depleted chalk reservoirs within the limitations of a narrow mud weight window, high overbalance, high solids contamination and static periods was successfully implemented when drilling a water injection well on Valhall Flank North in the southern Norwegian Continental Shelf. In parts of the Valhall reservoir the depletion is estimated to 313 bar (4500 psi). The estimated pressure gradients in the planned trajectory were pore pressures between 0.64 sg and 0.70 sg (5.3 ppg to 5.8 ppg) and with a corresponding fracture gradient of 0.95 sg (7.9 ppg). These conditions required a drilling fluid which could be maintained at a density as low as 0.78 sg (6.5 ppg) combined with a low viscosity, strict fluid loss control, tolerance towards contamination of chalk and high stability. No conventional drilling fluid would provide a sufficiently low density to drill this section, thus an unconventional solution had to be found. The proposed solution was to add hollow glass spheres (HGS) rated to withstand pressures up to 1310 bar (19,000 psi) to reduce the density of a conventional fluid. However, previous experience using HGS as an additive in drilling fluid was limited. Therefore, the design and qualification of this drilling fluid had to be conducted both in the laboratory as well as on a larger scale yard-trial. Laboratory testing and the yard trial verified the feasibility of mixing the HGS in a large-scale production and confirmed their ability to withstand the expected downhole pressure and mechanical strain while drilling. The 1750 m (5740 ft) 8.5" injection well on Valhall Flank North was drilled in one run, with no drilling fluid-related problems. The drilling fluid density was maintained between 0.78 sg (6.5 ppg) and 0.85 sg (7.1 ppg) while the maximum measured equivalent circulation density (ECD) increased from 0.83 sg (6.9 ppg) at start of the section, to 0.96 sg (8.0 ppg) at section TD. Completing this well was resulted in an increase of 7.5 mmboe recoverable oil demonstrating that implementing HGS in drilling fluids provides an expanded operational envelope and access to so far inaccessible oil reserves.