Stimulation of High-Temperature Sandstone Formations from West Africa with Chelating Agent-Based Fluids

Abstract

Abstract Fluids based on chelating agents have been developed for matrix stimulation of high-temperature sandstone formations. These fluids dissolve sizeable amounts of calcite and clays and maintain high levels of dissolved metal in solution over time with minimal precipitation. A series of field samples from high-temperature (149°C) sandstone reservoirs in a West African formation bear carbonate concentrations ranging from 2 to 37% (w/w). The effects of matrix treatment using a chelating agent-based system on these field samples were studied using coreflood and slurry reactor experiments. Linear coreflood test data show dramatic increases in the formation permeability after treatment with the chelating agent-based fluid. The improvement in permeability is ascribed to the removal of carbonate minerals and soluble clays, without secondary metal-precipitation. Slurry reactor tests elucidated the kinetics of mineral dissolution in mechanically ground field samples. Treatment with acidic chelant fluids generated high levels of dissolved calcium, silicon, and aluminum that remained in solution over time. For comparison, conventional mineral acid treatment of the field samples generated high levels of metals in solution that declined over the same period of time, indicative of secondary precipitation. The effectiveness of the chelant fluid for stimulation of this high temperature formation was confirmed through increased formation permeability and high levels of dissolved minerals. Introduction Typically, the purpose of acidizing a carbonate formation is to remove near-wellbore damage and to produce "wormholes" to increase the permeability of the critical matrix. However, owing to the rapid reaction between hydrochloric acid (HCl) and carbonates, diverting agents such as ball sealers, viscoelastic surfactant diverters, and foams[1] are used to direct some of the acid flow away from large channels that may form initially and take all of the subsequent acid volume. Fredd and Fogler[2–4] have proposed the use of ethylenediaminetetraacetate (EDTA) chelating agents as the primary active components in fluids used to stimulate limestone and dolomite formations. By adjusting the composition and pH of these fluids, it is possible to customize the chelant solutions and target specific well conditions to achieve maximum wormhole formation with a minimal volume of solvent. Control over reaction kinetics is vital when acidizing carbonate formations at high temperatures in which high reaction rates can overwhelm some treatment fluids. For comparison, sandstone reservoirs undergo matrix acidizing treatments to remove damaging aluminosilicate minerals and reduce the skin value. The precipitation of silica is thought to be the major reason that sandstone-acidizing jobs fail to produce the anticipated decrease in skin, especially at temperatures > 150ºF or in the presence of acid-sensitive clay. Acid treatment of sandstone at high temperatures, therefore, requires a retarded acid. Additionally, conventional acid treatment of sandstone formations (such as a mud acid treatment) involves many stages of fluid, thereby increasing the complexity of the treatment. An alternative approach uses chelating agents combined with acids as the main treatment agent. Chelating agents are materials that are used to control undesirable reactions of metal ions. In oilfield applications, chelating agents[5] are frequently added to acidic stimulation fluids to prevent precipitation of solids as the acid spends on the formation. The use of chelating agents is one proposed approach to stimulation because they can complex many of the metal ions found in sandstone formations. Chelating agents are also used as components in many scale-removal and scale-prevention formulations.[6] EDTA fluids have been used extensively to control iron precipitation and to remove scale. For example, disodium EDTA has been used as a scale-removal agent in the Prudhoe Bay field of Alaska.[7] In this application, calcium carbonate scale had precipitated in the perforation tunnels and in the near-wellbore region of a sandstone formation. High decline rates followed conventional HCl treatments, but 17 wells treated with disodium EDTA maintained production after these treatments.