The Stretford Process at East Wilmington Field

Abstract

An effective means of handling the accelerated production of hydrogen sulfide resulting from waterflooding the Wilmington oil field had become an absolute necessity. A feasibility study comparing possible systems showed the Stretford Process to hold great promise. The prediction has proved to be almost an understatement. proved to be almost an understatement. Introduction The Stretford Process has been increasingly accepted throughout the world during the last decade for treating coke oven gas, reformed petroleum products, and effluents arising in the manufacture of certain industrial products. As we shall explain, the process appears useful also for treating oilfield gases under certain conditions. In general, these conditions are: (1) that hydrogen sulfide is present in lower concentration, (2) that the ratio of CO2 to H2S is high, and (3) that the air content does not exceed a few percent. Where higher hydrogen sulfide concentrations and lower CO2/H2S ratios prevail, an amine process followed by a Claus system for the production of process followed by a Claus system for the production of sulfur might be more economical. Where hydrogen sulfide concentrations are extremely low, it may be more desirable to treat the gas with partly regenerable or nonregenerable chemicals, such as was done in the iron sponge purification system and the nonregenerable liquid chemical system employed in the Wilmington field before this Stretford unit was installed. One of the most important features of the Stretford Process is that it is almost entirely pollution free. Neither Process is that it is almost entirely pollution free. Neither gaseous nor liquid pollutants are discharged in the day-today operation. Sulfur produced is innocuous, and may have a market value, or it may be easily disposed of without producing surface pollution. pollution. In the Wilmington field the injection of large quantities of water from ocean-charged aquifers has been responsible for converting the normally sweet natural gas to sour gas, with hydrogen sulfide content increasing at compound growth rates in excess of 50 percent/year. As evidence of this, Fig. 1 shows the growth of hydrogen sulfide concentration in the older portion of the Wilmington field to the west. (Process Plants "A" and "B" are natural gasoline extraction plants that have processed the entire volume of Wilmington field gas. The two curves represent the intake concentrations of all gas processed, with the exception of the Long Beach Units - East Wilmington - gas.) Fig. 2 demonstrates the increased hydrogen sulfide content in the gas as the water cut increases for typical wells in the Ranger zone, Block VI, East Wilmington. The phenomenal increase in hydrogen sulfide quickly made the earlier sweetening units obsolete. It became necessary to decide on a long-term solution in the form of a process unit that could treat all of the gas of the East Wilmington field operated by the Long Beach Unit, and possibly also a large part of the gag from the older part of the Wilmington field to the west. The unit would have to have enough capacity to take care of growth for several years. Attractive features to look for were that the process be selective, that it employ a regenerable chemical, and that it produce sulfur rather than hydrogen sulfide as the effluent. produce sulfur rather than hydrogen sulfide as the effluent. The process had to be one in which risk was minimal, since the entire oil production of the Wilmington field depended on its reliability. P. 545