New Advances in Single-Phase Retarded Acids for Stimulating Carbonate Reservoirs

Abstract

Single-phase retarded acid systems provide higher performance and operational efficiency benefits than systems based on unmodified HCl and emulsified acid. When combined with diverting fluid systems and modeling, they have proven to resolve many challenges operators experience with carbonate acidizing. Our work assessed a new advanced single-phase retarded acid. The new fluid can be formulated to reach the dissolving power of up to 28 wt% HCl and is stable at high temperatures. First, static carbonate dissolution tests were conducted to screen the retardation performance of numerous chemicals. The thermal stability of selected retarder packages was evaluated in low-pH environments at temperatures up to 350°F. Wormhole experiments were then performed in Indiana limestone and Silurian dolomite cores at temperatures up to 325°F and with fluids of HCl strength ranging from 15 to 28 wt%. The rate of wormhole propagation was used as a measure to compare the performance of the acid systems. The core flow testing showed that the new single-phase retarded acid is highly effective in forming dominant wormholes in both limestone and dolomite cores in a wide range of acid injection rate, temperatures between 125 and 325°F, and HCl concentrations ranging from 15 to 28 wt%. The new single-phase retarded acid has a low viscosity, close to the viscosity of unmodified HCl, which should result in low friction pressure in field applications. The pore volume to breakthrough data were incorporated into a model and allows predicting the radial performance of the new fluid in the field and examining different treatment scenarios. The results of the simulations show the significant improvement in achieved wormhole depth of the new fluid versus unmodified HCl. The novelty of the new low-viscosity single-phase retarded acid is in the ability to efficiently extend the application envelope of this type of fluid towards the high temperatures and high carbonate dissolving power.