Efficient access to multiple permeable natural gas zones located in deep, over pressured, and extremely corrosive environments presents a challenge for South Texas operating companies. An innovative completion technique, featuring corrosion resistant alloy (CRA) tubulars has been effectively implemented by blending features of monobore, slim-hole, and conventional completion geometries.
Introduction
After attempting to produce deep, hot, corrosive, high-pressure Wilcox gas in Duval County, Texas efficiently, operators were faced with engineering and economic challenges. One operator decided to rethink the entire well planning process and asked, "What's important to produce these wells in the most economic and safe manner?" Casing cementing success, high rate stimulation, and high production capacity in a corrosive environment are the ingredients that must be blended into a successful well. High rate production (+/–30 MMcf/D) and the need for high rate (30 to 35 bbl/min) fracture stimulation, due to 50 to 300 ft thick reservoirs, dictated the use of 3 1/2-in. production tubing or larger.
Operators face a number of challenges as they attempt to produce natural gas from the deep Wilcox formations of South Texas. A successful prospect can yield 15 to 30 MMcf/D making the rewards substantial. However, the increased costs of CRA tubulars jeopardize the economics of a project if these rates are not achieved. As a consequence, drilling and completion technology is routinely tested before the rewards can be realized. Chief among the problems to overcome are environmental conditions encountered at the producing interval and at total depth (TD). Hurdles that stand between the operator and completion of a profitable well include:Temperature - The South Texas thermal gradient is among the steepest in the world. Temperature changes in excess of 2°F per 100 ft of well depth are common throughout the area. Bottomhole temperatures of 420°F at depths as shallow as 17,000 ft contribute to chemical inhibition, material selection, and yield strength derating problems.Pressure - Not only is the temperature gradient abnormally steep, but so is the pressure advancement with well depth. An 18.0-ppg mud at 17,000 ft is required to drill the deeper Wilcox formation wells.Acid Gas - South Texas gas composition varies greatly across the region. Many of the wells carry some component of both hydrogen sulfide (H2S) and carbon dioxide (CO2). The wells targeted by this study are "mildly" sour, 30 to 60 ppm, or approximately 1 psi partial pressure H2S and produce 11 to 12% CO2, approximately 1,900 psi partial pressure.Chlorides - Chlorides in produced water range from 30,000 to 100,000 ppm, which is high enough to affect material selection. Combined with several hundred barrels of formation water produced per day, the production flowstream can be termed extremely corrosive.Cost - A difficult economic consideration of the Wilcox trend is that while individual wells may have high rates of return, total project economics may often be much lower. As a consequence, the success or demise of any well is dependent on the relative cost of the hydrocarbon producing installation. Maximum income or revenue from deep, hot South Texas wells is naturally fixed by the producing formations. Significant reduction in cost directly increases the profit margin. Initial cost must be weighed against risk in order to fully appreciate value. No completed well or field economics are addressed herein, but a summary of direct costs and associated risks is explored further in the body of this technical treatise.
The optimum answer to deep, hot, sour gas production in South Texas is to provide a corrosion resistant conduit for high volumes of gas and to make it equal to or cheaper than a conventional well. Safety compromises are not allowed.
