Use of a Novel Drill-In/Completion Fluid Based on Potassium Formate Brine on the First Open Hole Completion in the Gullfaks Field

Abstract

This paper describes the first use of a novel drilling fluid based on potassium formate brine and weighted with manganese tetraoxide. Also included is the completion technique used to open hole complete an unconsolidated reservoir. The fluid was selected for maximum brine density, and to facilitate removal of the filtercake criteria for fluid and completion selection are presented together with laboratory work conducted to optimise the fluid and the completion for a specific application. Potential problems related to operations and working environment were identified through a risk analysis approach. Predicted solutions are compared to actual field experience. Key factors included minimisation of ECD, monitoring of soluble barium in the drilling fluid, rig-crews' exposure to manganese tetraoxide and barium, and procedures to combat lost circulation. The filtercake removal and its transportation through the pre-packed screen completion were comprehensively investigated. The perceived benefit of the formate brine system was the possibility to drill, complete and produce an unconsolidated, pressure sensitive reservoir with the same fluid. This was achieved with minimal on-site fluid treatment. The fluid was significantly more expensive than conventional alternatives. However, based on drilling fluid performance, rig-time and well productivity, one can conclude that the field test was successful and the use of the fluid was justified. Introduction The Gullfaks field is located in block 34/10 in the Norwegian sector of the North Sea, 150 km west of the Sognefjord. Gullfaks is the first field developed by a fully Norwegian licence group consisting of Statoil as the operator, Norsk Hydro and Saga Paetroleum. The fields current production is more than 90,000 Sm3/D from three reservoirs. The field has been developed with three concrete gravity-base platforms: Gullfaks A, B and C, each platform with two drilling shafts. Hence, while drilling in one of the shafts, wire-line, snubbing or coiled tubing operations may take place in the opposite shaft. To date, more than 90 wells have been drilled from the three platforms. Further 25 platform wells are planned. The oil is located within three major sandstone formations of Jurassic age; the Brent Group, the Cook formation and the Statfjord formation. The Brent Group is further subdivided into two groups; P. 647

Characteristics and Removal of Filter Cake Formed by Formate-Based Drilling Mud

“…The highest removal efficiency was 90%, which was attained by potassium formate brine mixed with 10% citric acid. The other three fluids gave insufficient removing efficiency, which was below 70% (Svendsen et al, 1995).…”

“…It was shown by others that citric acid has the highest dissolving efficiency to the filter cake damage, which was generated by formate based fluid, compared to other breakers (Svendsen et al, 1995). The citric acid was mixed for 3 hours and a white precipitate was noticed during the mixing at low acid concentration.…”

“…Svendsen et al tested citric and acetic acids at a concentration of 10 wt% to remove filter cake damage that generated by formate based mud (Svendsen et al, 1995).…”

“…Traditionally, cleaning up a reservoir filter cake has been accomplished with conventional chemical and mechanical means. The chemical means include formate brines (Hands et al, 1998), live mineral and organic acids (Huang et al, 2000), enzymes (McKay et al, 2000;Al Otaibi and Nasr-El-Din, 2005), chelating agents (Bradshaw et al, 2006), or a combination of formate brines and organic acids (Svendsen et al, 1995). The combination of high reservoir temperature and long shut-in times after acid treatment lead to a high probability of severe corrosion of the well completion.…”

Formation Damage Caused by Improper Mn3O4-Based Filter-Cake-Cleanup Treatments

“…Svendsen et al (1995) introduced Mn 3 O 4 to potassium formate drilling fluid to increase its density to 106 lbm/ft 3 (1698 kg/ m 3 ). The recommended cleaning fluid to remove the filter cake was 10 wt% citric acid (Donovan and Jones 1995;Svendsen et al 1995;Downs et al 2005). Mn 3 O 4 provides low rheological properties and equivalent antisag behaviour (Kleverlaan and Lawless 2004;Carbajal et al 2009).…”