This paper was prepared for the Oilfield Chemistry Symposium of the Society of Petroleum Engineers of AIME, to be held in Denver, Colo., May 24–25, 1973. Permission to copy is restricted to an abstract of not more than 300 words. Illustrations may not be copied. The abstract should contain conspicuous acknowledgment of where and by whom the paper is presented. Publication elsewhere after publication in the JOURNAL paper is presented. Publication elsewhere after publication in the JOURNAL OF PETROLEUM TECHNOLOGY or the SOCIETY OF PETROLEUM ENGINEERS JOURNAL is usually granted upon requested to the Editor of the appropriate journal, provided agreement to give proper credit is made. provided agreement to give proper credit is made. Discussion of this paper is invited. Three copies of any discussion should be sent to the Society of Petroleum Engineers office. Such discussions may be presented at the above meeting and, with the paper, may be considered for publication in one of the two SPE magazines.
Arsenic-containing inhibitors are highly effective in preventing acid attack on steel during acidizing of oil wells. For many years, these arsenic-containing compounds were the only effective chemicals available for this purpose. It was only recently that they were replaced by organic inhibitors containing no arsenic at all.
The main reasons for replacing the arsenic in the field were the hazards to operating personnel and the likelihood that it would enter the crude oil stream, thus possibly endangering arsenic-sensitive refinery catalysts. There were, of course, more potential risks connected with the use of these potential risks connected with the use of these compounds: the ecological problems can be large, and the reaction between the arsenic of the inhibitors and H2S in the treated formation can lead to plugging of the formation by solid sulfides. These are reasons enough for many oil companies to establish a strictly enforced rule against using arsenic in the field. Therefore, very few arsenic inhibitors are used today.
The reaction of arsenic with H2S in the oilbearing rock, particularly the formation of water-insoluble arsenic sulfides and their behavior underground, is of great academic interest. The chemical and thermodynamic behavior of some of these sulfides is very obscure in many respects. However, our field and laboratory investigations, which were done in connection with well treating problems, have shed some light on arsenic sulfide behavior.
During the mid-60's, quite a few wells were drilled and completed in the Hunton Zone of the Anadarko Basin in Oklahoma. The Hunton Zone consists of dolomite, and lies at depths between 20,000 and 30,000 feet.
Drilling and completing the wells in the Hunton Zone presented many problems. The bottom-hole temperature was approximately 35 degrees F, and the gas contained large concentrations of H2S. Because of the low permeability of the dolomite (less than 1 millidarcy), the gas could be produced economically only through fractures. Therefore, it became routine in the field to give each well a large "acid-frac" treatment immediately after drilling to fracture the rock hydraulically and simultaneously to etch the faces of the fractures. Thus, the acid creates many channels in the walls through which the gas can later reach the wellbore at an economically desirable flow rate.
