Oil production in southern Mexico is mainly from deep (>16,000 ft) high-temperature (> 300°F), naturally fissured dolomitic limestone reservoirs with clays present. With increasing depth, the dolomitization increases and the porosity and permeability decrease. These reservoirs must be stimulated to produce, and treatments are traditionally repeated to maintain production.
Although there is a substantial body of published research on stimulating carbonate formations, there is little on treating dolomites specifically. In general, the smaller the volume of reactive fluid required to penetrate into the formation to create a wormhole—a function of reactive fluid used, injection rate, and temperature—the more efficient the treatment. Hydrochloric acid (HCl) has both a high dissolution rate and dissolution capacity which, at high temperature, results in dissolution of the face of the formation, as opposed to wormholing, and uncontrollable corrosion rates. Organic acids have lower corrosion rates and limited dissolution capacity, but have a relatively high rate of dissolution, which also results in face dissolution at high temperature. Although, clays present in the formation may slow the rate of dissolution.
Chelant-based fluids are less corrosive, have lower dissolution rates, and are capable of creating effective wormholes at temperatures up to 400”F in carbonates. Although dolomite retards the dissolution rate of both acids and chelants, the difference is less with chelants. The wormholing efficiency of chelants in dolomites is also less rate dependent than is that of inorganic acids.
Acidic hydroxyethylethylenediaminetriacetic acid (HEDTA) fluids were developed to stimulate high-temperature carbonates and dolomites. These fluids have been highly effective in stimulating both carbonate, and even sandstone, formations worldwide. However, the optimum pH and injection rate for wormholing is markedly different for carbonates and dolomites. In addition, a low pH may result in emulsions and corrosion downhole or at surface when flowing back the treatment. Recent studies show that the efficiency of diammonium EDTA (DEA) with the correct pH is similar whether stimulating a high-temperature limestone or a dolomite formation, at similar injection rates.
A direct comparison has been made by performing treatments in the same reservoirs using the two chelant systems. In every case, the initial post-treatment production with the DEA-based fluid has been significantly higher and the production decline slower. This is attributed to stimulating both the dolomite and limestone intervals and removing the potentially damaging clay and fines. The fluid also eliminates the possible issues with using a low-pH fluid.