Summary This case history details the use of Sulfa-Check*, a sour-gas sweetener, on two exploratory well drillstem tests (DST's) offshore California. Also included is a brief description of the Sulfa-Check process and comments on future plans. This case history is intended to process and comments on future plans. This case history is intended to show that Sulfa-Check can be applied effectively in a sour-gas offshore DST. It will show Sulfa-Check's performance on each of the exploratory wells and explain the solutions initiated to mitigate problems incurred in subsequent DST'S. This information is presented for future operators' use in preparing an economical an safe sour-gas DST.
Introduction Sulfa-Check, a liquid product developed and manufactured by Exxon Chemical Co. to sweeten sour gas, is a yellow alkaline aqueous oxidizing solution first marketed in 1984. Its use requires a sweetening tower with an inlet sparger on the bottom and a mist eliminator and outlet at the top (Fig. 1). The tower is usually filled 75% full of Sulfa-Check, and the sour gas is bubbled up from the bottom. The H2S in the gas reacts with the Sulfa-Check will ultimately is converted into a slurry of pure sulfur and sodium bicarbonate while the remaining sweet gas exits through the mist eliminator and outlet. One gallon [0.0038 m3] of Sulfa-Check removes more than 2 lbm [907 g) of H2S at ambient temperature; the life of a given volume, however, depends primarily on the concentration of H2S in the gas stream and gas throughput. Sulfa-Check costs approximately $5/gal [$1320/m3]. Spent Sulfa-Check results in about 70% liquid waste and 30% solid waste.
Initial Considerations Sour-gas production can be a problem when exploratory wells are flow tested during DST operations. Three obvious considerations with producing sour gas are the safety of the rig personnel, the safe ty of the sea life, and compliance with regulatory agencies. The sweetening process selected must be compatible with the manual and automatic safety systems on the rig and must provide sweet flare gas that is consistently within all regulatory requirements. The California Air Resource Board, for example, requires that the maximum H2S concentration in natural gas to be flared does not exceed 15 grains/100 ft [343 Mg/m3], or 238 ppm. Various sweetening process schemes were initially compared with Sulfa-Check. Zinc oxide was considered but found to be better suited for treating sour crude than sour gas. An iron-sponge process was considered but not accepted because it generates a hazardous waste. The Sulfa-Check process was selected because it was expected to develop the best overall performance and did not generate a hazardous waste. After this determination, the sweetening-tower design stage began. Using the recommended design nomographs (Figs. 2 and 3) and the anticipated gas flow rates and H2S concentrations, we calculated the proper number of towers and tower sizes. The contracted well-test service company then built two twin NACE MR-02 certified, 4 × 20-ft [1.22 × 6.1-m], 600-psi [4.14-MPa] -working-pressure (WP) sweetening towers and one NACE MR-02 certified, 4 × 20-ft [1.22 × 6.1-m], 300-psi [2.07-MPa] -WP sweetening tower. The two 600-psi [4.14-MPa] -WP towers were used as the high-pressure sweeteners processing the main flow stream of gas downstream of the primary gas scrubber (Fig. 4). The 300-psi [2.07-MPa] -WP tower was used as the low-pressure sweetener processing residual sour gas that broke out from the sour crude oil in the surge tanks (Fig. 4). Incorporated in the skid and piping of each tower was a 1 1/2 × 20-ft [0.41 × 6.1-m] vertical piping of each tower was a 1 1/2 × 20-ft [0.41 × 6.1-m] vertical scrubber designed to catch any liquid carry-over from the outlets of the sweetening towers. All the interconnecting piping upstream of the towers was high-pressure welded line pipe that met all requirements for sour service. Arrangements were made to transport fresh Sulfa-Check from the beach to the drilling rig by 25-bbl [3.97-m3] marine transport tanks. The spent Sulfa-Check slurry was disposed of in separate compartments of a 50,000-bbl [7950-m3] barge (Fig. 3) moored to the rig.