Use of Acid Precursor as Alternative to Acid Treatment to Drill-In Fluid Filter Cake Removal: FN Case Study (2014)

Abstract The FN field is a carbonate reservoir contracted to deliver 250 MMscf/D gas for liquefied natural gas (LNG). Three horizontal wells were drilled and completed to meet the gas demand. The reservoir thickness ranges from 45 to 75 m, and the average permeability distribution is 35 to 104 md with a weak aquifer drive. During the drilling phase, potential well damage can result from drilling fluids and carbonates solids, which form thin, firm filter cake that is used to help minimize fluid loss into the formation. However, after drilling concludes and before production begins, the filter cake must be removed or dissolved to help ensure good well productivity. As a standard practice, 15% hydrochloric (HCl) acid is used to acidize the formation face, either bullheading at high rates with chemical diverters or using coiled tubing (CT). Either of these options requires the well to be displaced with clean filtered brine, and the well is flowed clean before acidizing begins. Removal of filter cake, carbonate debris, and spent acid must be performed again following the acid treatments. The primary challenge when using these methods is effective openhole coverage (especially in long horizontal wells, as in the case of the FN reservoir) during treatment. Even when chemical diverters are used, some well sections can be left untreated because of the fast reaction of the carbonate with acid. To optimize stimulation treatments in the FN field, a new acid precursor (AP) (ester of an organic acid) treatment was designed to dissolve the drilled-in fluid (DIF) filter cake; the acid treatment was used as a backup in the event the AP treatment was unsuccessful. APs undergo hydrolysis at bottomhole temperature (BHT) to produce in-situ weak acid at a controlled time. The AP used in this study was selected for treatment based on extensive laboratory testing. The lower completions of the FN wells are 7-in. predrilled carbon steel liners in the openhole section. After installing the liner, the mud in-hole was displaced with clean filtered brine. The AP was mixed with the completion fluid and pumped through the wash pipe of the liner setting tool to cover the openhole section before running the completion string. This paper discusses the effective application of the AP in the FN wells to remove formation damage from DIF filter cake, thereby eliminating the use of acid to improve well productivity in carbonate reservoirs. Introduction/Background The FN field is an offshore gas field located at a water depth of 96 m and 18 km from the central processing platform (CPP) in the KN field. The FN field is a large Cycle VI/V carbonate platform that is overlaid by Cycle V/VI clastics. It is a high relief organic buildup, steep flanked, and generally flat topped, with a maximum thickness of approximately 1200 m. Two small pinnacles cap the buildup but are confined to the crestal part of the structure in the NE area. It was mapped using vintage three-dimensional (3D) seismic survey equipment. The seismic reflection quality is good down to the top of the carbonates. The structure is composed of stacked carbonate zones with good porosity carbonate development. The field was discovered by Exploration Well FN-1 in August 1969; the FN-2 appraisal well was drilled in 2001, and the FN-3 well was drilled in 2006 to appraise the lateral extent of gas accumulation and reservoir quality. The field development plan (FDP) recommended drilling and completing three horizontal wells, and the FN field is planned to deliver 250 MMscf/D of non-associated gas (NAG), with each well having a deliverability close to 88 MMscf/D. All three wells are monobore completions with 7-in. tubing in the upper completion and a 7-in. predrilled liner in the lower completion.