Diverting Agents-History and Application

Abstract

When porous zones or perforations in porous zones are opened for production, the stimulation fluids must be diverted if proper production, the stimulation fluids must be diverted if proper treating is to be assured. It seems that from the time the need for diverting agents was first recognized in the early 1930's, everything has been tried from soup to nuts. Introduction In 1932 the oil industry started using hydrochloric acid to stimulate oil wells. Immediately the problem arose of diverting the acid treatments into a desired zone, and over the years the diverting methods used have been dictated by the reservoirs being developed. In the period from 1932 to about 1945, the Permian Grayburg-San Andres carbonate reservoirs were primary drilling objectives in the Permian Basin. primary drilling objectives in the Permian Basin. These reservoirs were from 3,000 to 5,000 ft deep and were generally prolific. Stimulation treatments were usually small in volume (less than 2,000 gal) and the major concern was to avoid acidizing into bottom water. During the period from about 1946 to 1960, Permian reservoirs such as the Clearfork, Tubb, and Spraberry were exploited. These 4,000- to 8,000-ft-deep reservoirs were stimulated with large volumes of acid or fracturing fluids, or both. During the 1960's, deep gas fields (15,000 to 22,000 ft), principally Ellenburger, required new diverting principally Ellenburger, required new diverting materials to achieve effective stimulation at high reservoir temperatures and pressures. 1936 to 1946 The earliest documentation of a diverting agent was in 1936 when a patent was issued to Halliburton Oil Well Cementing Co. for the use of a soap solution that reacted with calcium chloride to form a precipitate. This was a water-insoluble, oil-soluble calcium soap that acted as a diverting material for the acid. One year later Halliburton was issued a patent for a diverting material that utilized locust bean gum to gel calcium chloride and sodium chloride for blanking off a zone and thus diverting acid into untreated intervals. Sulfuric acid was used as a diverting agent in connection with a conventional hydrochloric acid treatment. After the sulfuric acid was pumped into the wellbore, the pumps were shut down for a short time and then pumping of the hydrochloric acid was resumed. When in contact with the calcium carbonate, the sulfuric acid formed insoluble calcium sulfate, which was the diverting agent. This system was not widely accepted, owing, it is assumed, to the recognition that it could cause potentially permanent productivity damage. productivity damage. A major concern in the development of the Grayburg-San Andres carbonate reservoirs was that treatment could extend into the bottom water, particularly since the wells were open-hole completions. particularly since the wells were open-hole completions. Beginning in the late 1930's, various materials were designed to avoid this problem. Dowell developed "Blanket," which was a heavy calcium chloride solution that depended on its viscosity to divert the acid up the hole. Dowell later introduced "Jelly Seal" as the second-generation blocking material. This was a natural gum (locust bean gum) that when mixed with water formed a viscous, gelatinous fluid. When difficulty was encountered in obtaining a plug because of high porosity and permeability, "Jelflake," or cellophane, was mixed with the Jelly Seal to accomplish the shut-off. An internal breaker had to be used to lower the viscosity of Jelly Seal after treatment. For this purpose, a bacteria mixture was prepared by exposing a Jelly Seal mixture to air circulating in a warm room. JPT P. 593