TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractWhen drilling wells in HTHP fields, the drilling fluid properties are of utmost importance. The difference between fracture and pore pressure, expressed as the hydraulic window, can be quite narrow. One of the remedies for making sure that one stays within this hydraulic window, is to design a drilling fluid with very low viscosity. This will reduce the Equivalent Circulating Density (ECD) effect from the fluid. While lowviscosity fluids have the ability to dramatically reduce frictional pressure loss compared to more viscous fluids, there is a limitation to the reduction in viscosity before conventional weight materials start to settle. In HTHP wells with extremely high pressures and temperatures, sag stability of the fluid is essential to ensure adequate well control. This paper describes the development and application of a novel technology that has resulted in a ten-fold reduction in the particle size of the weighting agent. With this development, invert emulsion drilling fluids can be designed with reduced viscosity in combination with minimal settling potential of the weighting agent.The paper further explains in detail the first field application of this specially treated micron-sized weighting agent used in a HTHP well. This successful field application has proven this technology to be a viable alternative to the use of high-density, high-cost brine-based drilling fluids.

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractKristin is a HP/HT field developed on Haltenbanken offshore mid-Norway. The Kristin development consists of 4 templates with a total of 12 subsea wells. The inclination through the reservoir ranges from 28 to 85 degrees. The reservoir at approx. 4600mTVD (15092 ft) has an initial pore pressure corresponding to 1.96sg EMW (16,36 ppg) and a temperature of 172°C (342°F). With a water depth of approx. 360m (1181 ft), a MW of 2.05sg (17,11 ppg) is needed to be able to maintain a riser margin. Three different drilling fluid systems have been used in the reservoir section: 1) Cs/K-COOH clear brine system. 2) Invert emulsion HP/HT OBM. 3) Invert emulsion HP/HT OBM with ultra fine weight particles. Challenges such as ECD management, hole stability, formation damage, weight material sag and operating on subsea HP/HT wells during harsh winter conditions had to be addressed both in the planning and the operational phase. In this paper the background for selection of the drilling fluid is briefly described and especially the rationale behind using three different systems. The paper highlights operational experiences to illustrate how the drilling fluid systems influenced and coped with the challenges of drilling subsea, high angle HP/HT wells. The paper provides a discussion of the pros and cons of the different fluids systems. Finally the paper identifies some of the challenges that lie ahead as the production has started and the reservoir starts to deplete.

The Wisting Field is located offshore in the Barents Sea north of Norway. The field development requires drilling and completion of horizontal wells in a narrow pressure window, which is challenging for conventional technology. The Reelwell Drilling Method (RDM) provides a solution for managed pressure drilling (MPD) to mitigate these challenges. Based on the use of dual-channel drillpipe, RDM enables the pressure gradient in the openhole section to be held constant and nearly independent of the drilling fluid flow rate. RDM provides low-energy drilling, i.e., efficient drilling and hole cleaning when using low flow and low-drillstring rotation speed. This allows drilling of horizontal sections within a small pressure window and limits drillstring fatigue in high-dogleg applications. In order to start qualification of RDM for the Wisting Field, a horizontal trial well was drilled in fall 2018 at the Ullrigg Drilling and Well Test Centre, located at the NORCE Norwegian Research Centre in Stavanger, Norway. The goal for this trial well was to prove the dual-channel drillpipe capability to perform low-energy drilling, i.e., low-rotary speed, low flow, and stable downhole pressure gradient. The trial well had a challenging geometry with up to 14°/30-m build rate, as required for the field. The 9½-in. horizontal section was drilled in order to check the performance of the RDM and start the qualification of the RDM for the Wisting Field. The trial well was drilled with a low-flow motor, i.e., with flow rates around 700 lpm. The string rotation speed was held lower than 5 rpm to limit drillstring fatigue in the high-dogleg section of the well. Efficient drilling and hole cleaning of the 9½-in. horizontal hole section was demonstrated. The results from the trial confirm the predicted performance for the RDM. The results indicate that RDM can keep well pressure gradient within the required window and mitigate the drilling challenges for the Wisting Field. The RDM architecture with return flow in the inner string represents a major advantage to avoid cuttings build up in the well, challenges with laminar return flow, formation wash-outs, mechanical down hole dynamics and ECD control by low energy drilling.