Completion Practices in Deep Sour Tuscaloosa Wells
Authors: Huntoon George G. 
Published: December 2007

Huntoon, George G., SPE, Amoco Production Co.

Summary
Successful development of the Tuscaloosa trend in Louisiana has required unique completion practices to produce the trend's deep sour formations. Amoco's operations in the Tuscaloosa formation are between 16,000 and 21,000 ft [4877 and 6400 m], and a range of pressure environments, high temperatures, and corrosive elements is encountered. Application of proved completion practices and equipment has resulted in several techniques that enhance the safety, longevity, and production capacity of these wells. The design of deep Tuscaloosa completions is assisted by a series of correlations developed to project bottomhole and surface shut-in tubing pressures, temperature gradients, and flow capacities for deep sour wells. This paper discusses material selection, completion practices, completion fluids, wellhead equipment, packer designs, corrosion-inhibition systems, and safety and monitoring equipment used in the Tuscaloosa trend. The design of a wellhead surface installation used to detect equipment failure, to pump kill fluids, and to circulate corrosion inhibitors is reviewed. A case study illustrates the methods used in completing a Tuscaloosa well with surface pressures exceeding 16,000 psi [110.3 MPa].

Introduction
The Tuscaloosa trend extends across southern Louisiana and is approximately 20 miles [35 km] wide and 200 miles [322 km] long (Fig. 1). The formation was deposited during the Cretaceous period when Louisiana was still submerged. Sediment flowed seaward over and through a carbonate reef shelf, leaving alternating deposits of shale and sandstone. The subsequent geological history resulted in the off-shelf deposits being buried to depths of 16,000 to 21,000 ft [4875 to 6400 m]. Interest in the deep Tuscaloosa was sparked by the discovery of the False River gas field in 1975 and development continues today. The principal production is gas, with associated condensate ranging from 160 bbl/MMcf [21.3 dm3/kmol] for some wells in the Port Hudson field to as low as 2 bbl/MMcf [0.27 dm3/kmol] in a Morganza well. Drilling and completion costs have risen sharply as deeper, higher-pressure formations are penetrated, resulting in a cost averaging between $10 million and $20 million per well, depending on location and depth. Careful consideration is given to the well design to maximize production capacity, minimize corrosion, and enhance safety. Development of higher-strength, corrosion-resistant tubular goods became necessary for the deep sour-gas production in Mississippi. Operators of the deep, high-pressure Tuscaloosa wells have benefited from this technology. As a result, a rational and sound completion design for high-pressure wells can be accomplished once all the environmental factors affecting the completions are identified. There are still limitations as wells are drilled deeper and into increasingly higher pressure and temperature environments. The harsh environment of the Tuscaloosa required development of an effective inhibition program as a part of the completion design to abate potential corrosion failures. An outline of steps and procedures used in developing a completion design and methods of corrosion treatment are discussed. Special considerations necessary to complete a 21,000-ft [6400-m] well with surface pressure greater than 16,000 psi [110.3 MPa] are reviewed.

Background
A viable and mechanically sound completion can be designed once the environmental elements that affect a completion are understood. Since the gas composition in the Tuscaloosa contains 5 to 8% CO2 and approximately 20 ppm H2S over a wide range of pressures (8,000 to 20,000 psi [55 to 138 MPa]), specific metallurgical problems exist. Careful consideration must be given to the effects of sulfide stress cracking (SSC) and weightloss corrosion in designing tubular goods. A guide has been established in the NACE Standard MR-01-75 (1980 revision), which shows that even a low concentration of H2S at sufficient pressure will result in potential stress cracking if materials are selected improperly. The next problem is understanding the temperature and pressure environment that can be encountered across the trend. Curves were constructed from representative Amoco wells to improve prediction of temperature and pressure environments in the Tuscaloosa. The temperature profile shows that temperatures above 180F [82C] start at approximately 12,000 ft [3658 m]. Since the effects of sulfide stress cracking are negated above a threshold temperature of 180F [82C], casing strings and production tubing can be designed to optimize performance and cost. An improved method of predicting surface pressures was needed to determine the ratings required for wellheads and surface equipment. Pressures in the Tuscaloosa range from a normal gradient 0.465 psi/ft [10.5 kPa/m] in Port Hudson to abnormal 0.81 psi/ft [18.3 kPa/m] in Lockhart and Moore Sams and approximately 1.0 psi/ft [22.6 kPa/m] in certain areas.
JPT
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