Formation Screening to Minimize Permeability Impairment Associated With Acid Gas Or Sour Gas Injection/Disposal

Abstract High acid gas content streams, consisting primarily of carbon dioxide, hydrogen sulphide or a combination of both are commonly generated as by-products of the sweetening process used to bring many produced gases and solution gases to pipeline specifications for sales and transport. Typically, sour gas has been extracted from acid gases through the use of Claus or other types of elemental sulphur reduction processes, the sulphur sold or stockpiled, and the residual carbon dioxide vented to atmosphere. With depressed prices for the commercial sale of sulphur and environmental concerns with the emission of large volumes of greenhouse gases, industry has shown considerable interest in the feasibility of re-injecting acid gas from sweetening processes, either back into the original producing formation, or into selected disposal zones which may consist of aquifers or previously depleted oil or gas zones. A major concern with the reinjection process is the potential for formation damage and reduced injectivity in the vicinity of the acid gas injection/disposal wells. This paper discusses screening criteria for reservoir selection for zones suitable for acid/sour gas re-injection or disposal, and highlights potential areas of concern for reduced injectivity. Such phenomena include acid gas induced formation dissolution, fines migration, precipitation and scale potential, oil or condensate banking and plugging, asphaltene and elemental sulphur deposition, hydrate plugging and multiphase flow associated with acid gas compression. Variations on acid gas injection schemes, such as concurrent contacting with produced water at elevated pressures and subsequent disposal of the sour water, will also be discussed and potential damage concerns highlighted. A variety of screening and laboratory tests and results will be presented which illustrate the various damage mechanisms outlined and provide a specific set of design criteria to evaluate the feasibility of an acid gas injection/disposal operation. Introduction Acid gases [gases which contain carbon dioxide (CO2) and hydrogen sulphide (H2S)] are produced from many formations as either free gas or liberated solution gas from sour oils. These gases must be "sweetened" to selectively remove the acid gas components before the gas can be transported and sold for commercial use. A variety of sweetening processes are used to remove acid gas components (amine extraction being the most common). The sweetening process results in the production of acid gasfree "sales" gas, and a rich waste gas stream consisting of virtually pure CO2 and H2S (commonly referred to as concentrated acid gas). In the past, a variety of techniques have been used to handle acid gas streams, most of them primarily concerned with the reduction of the extremely toxic hydrogen sulphide to an inert/non-toxic reaction product. The most common technique is the Claus reaction process where the H2S gas in the acid gas stream is catalytically converted to elemental sulphur. This process was an economic one in the past, particularly in regimes of good sulphur commodity prices. Many operators deliberately attempted to exploit reservoirs containing high concentrations of H2S with sulphur recovery as the primary motivating factor.