Producing Mississippi's Deep, High-Pressure Sour Gas

Abstract

Introduction Shell Oil Co. has three deep, sour gas fields located about 20 miles southeast of Jackson, Miss. The wells in these fields are completed in the Smackover formation of (Jurassic age) between 19,700 and 22,250 ft; bottom-hole pressures range from 17,500 psi at the Southwest Piney Woods field. Bottom-hole temperatures range from 365 to 385F. The well fluids consists of 27 - to 16 percent H2S, 3-to 9-percent CO2, 45- to 65-percent methane, and no condensate. This gas is saturated with water at bottom-hole conditions, and each well appears to produce small amounts of salt based on the chloride content of the produced water. Water production is about 8 to 10 bbl per million cubic feet of gas. Sour gas (hydrogen sulfide) in very low aqueous concentrations can cause failure of some metals, but it can be safely contained with conventional oilfield tubulars such as C-75 and J-55 grades at moderate pressures. Likewise, high pressure (greater than pressures. Likewise, high pressure (greater than 10,000 psi) sweet gas has been safely handled with conventional P-grade tubulars. But combination of sour gas at very high pressures and temperatures cannot be safely contained with conventional tubulars; if CO2 and salt water are present the problem becomes acute. At the time of discovery of Thomasville field in 1969, sustained production of high-content H2S sour gas under conditions of very high pressure and temperature had not been proven or attempted beyond a short field test by Phillips Petroleum Co. in their Josephine "A" well in 1965. The 13,830-psi bottom-hole pressure encountered in this well was considerably below the 17,300 psi found at Thomasville. Testing of the Thomasville well revealed many technical problems that confirmed the need for high-strength tubulars resistant to stress corrosion cracking, high-pressure wellheads, environment-resistant seals, corrosion protection, and the prevention of sulfur plugging. plugging. Because of the toxicity of the produced gas, and the possibility of failure caused by stress corrosion cracking possibility of failure caused by stress corrosion cracking and weight-loss corrosion of metals, safety is the primary concern in the development of these deep, sour gas reserves. Detailed contingency plans are prepared for the entire complex and great care is taken the design, materials selection, and quality-control procedures. In preparing or executing a work plan, careful attention is preparing or executing a work plan, careful attention is given to all aspects of the operation and possible consequences are discussed in detail with all concerned, including service companies. Well Completion Practices 20,000-psi Systems A temporary completion was made in the Thomasville discovery well during March 1969 to permit production testing. The well tested at rates up to 10 MMcf/D of sour gas before completion-equipment failures resulted in temporary abandonment of the well. JPT p. 629