Dolomite Stimulation with Retarded Acids

Abstract

Globally, dolomite formations are important reservoirs for oil and gas. Acid stimulation is commonly used to extend the life of carbonate reservoirs, and a good understanding of the fluid performance is essential for effective treatment design. Three acids, hydrochloric acid (HCl), emulsified HCl, and a single-phase retarded acid based on HCl, were assessed for their ability to create wormholes in Silurian dolomite under laboratory conditions using a standard core flow experiment. Select cores were imaged by X-ray computed tomography to visualize the wormhole morphology. Similar experiments in Indiana limestone was used as a control. The core flow experiments showed that the pore volume to break-through (PVbt) values for the retarded acids in Indiana limestone were less sensitive to changes in temperature overall than unmodified HCl. For Silurian dolomite though, the opposite is observed. HCl has uniformly high PVbt values at lower (200 °F) and higher (325 °F). The emulsified acid and the single-phase retarded acid are more efficient than HCl, but the difference is smaller at 325 °F. Core images revealed that all three fluids had some degree of wormhole branching at 200 °F and much less branching at 325 °F. By visual inspection, the single-phase retarded acid has less ramification than HCl and the emulsified acid. Overall, the results show that retarded acids should make effective stimulation fluids for dolomite reservoirs.