Design and Implementation of a 22,600 ft Underbalanced Coiled Tubing Scale Cleanout in the Gomez Field, Pecos County, Texas

Abstract

Abstract This paper reviews the design, implementation, and results of a 22,611 ft underbalanced coiled tubing scale cleanout project for the wellbore of the Fort Stockton Gas Unit 5–1. The Fort Stockton Gas Unit 5-1 is a deep, low pressure, high temperature, Gomez (Ellenburger) Field gas well. Ellenburger formation is a fractured dolomite of Ordovician age. The formation has an average gross thickness of 1,600 ft with an average reservoir temperature of 335 F. No remedial operations have been performed on the FSGU 5-1 since its completion in March 1970 (Figure 2). In July 1993, during bottomhole pressure data acquisition, an obstruction was tagged at 21,164 ft from surface. An attempt using a 1.5 in. bailer deployed on slickline was unsuccessful in obtaining any type of sample. However, on the basis of offset well data, an assumption was made that scale with major physical characteristics of calcium carbonate and iron oxide was causing the restriction. In August 1993, the well was acidized for the first time since initial completion with no effect to its production characteristics. Because the FSGU 5-1 was considered a prolific producer relative to offset production, it was determined to remain risk adverse and not attempt any further remedial activity. Introduction The recent development of reliable 100,000 psi minimum yield strength coiled tubing has provided an economic avenue for performing deep underbalanced workover operations. Coiled tubing technology can provide significant economic and technical enabling qualities versus conventional workover methodology. Major areas of impact are, lower transportation cost, reduced rigup & teardown expense, less logistical coordination, reduced trip time of tubulars, elimination of kill fluids, reduction/elimination of reservoir damage, and elimination/reduction of safety hazards. The Fort Stockton Gas Unit No. 5–1 (FSGU 5–1) produces from the Gomez (Ellenburger) Field located in the Delaware Basin on the southwest flank of the Central Basin platform (Figure 1). Development of the Gomez (Ellenburger) Field was initiated in 1963. The Average daily production from the FSGU 5–1 was 4.7 MMcf/D during the first quarter of 1996 with 900 psig well head backpressure Based on a detailed nodal analysis simulation study performed in August 1996 on the well, wellhead compression was installed reducing wellhead backpressure from 900 psig to 500 psig. Although a 1.6 MMcf/D production increase was realized from this compression project, the nodal analysis simulation study indicated the well's production could still be optimized by approximately 2.0 MMcf/D. Based on offset well information, a 94% soluble scale interval beginning at 21,164 ft from surface existed and was restricting the well's inflow performance. From historic data, the probability that a surface acid treatment contacting enough scale surface area to dissolve it was minimal. P. 207^