Laboratory Evaluation of Single-Phase Retarded Acid System Performance in Limestone and Dolomite Rock - Part 3: Learnings from 150+ Experiments

The objective of this work is to compare the performance of various commercially available Single Phase Retarded Acids (SPRA) on limestone and dolomite quarry cores with data obtained from reservoir cores. The work involves state-of-the-art equipment for the acidizing experiment at reservoir conditions and makes use of advanced high-resolution imaging to visualize the results of the pore-volume-to-breakthrough (PVBT) acidization experiments. This work follows up the results of systematic lab testing of SPRA systems in Indiana Limestone cores previously reported in SPE-222481 and Bonne Terre Dolomite cores previously reported in SPE-225375 and represents the first systematic comparison of multiple single-phase retarded acid systems at the same test conditions by an independent laboratory. The previous publications reported sets of 4 to 5 "top-of-class" SPRA systems based on their superior performances in PVBT experiments on quarry cores. The top-of-class limestone SPRAs and top-of-class dolomite SPRAs were used for experiments on reservoir cores in this work. The testing scope included two temperatures, a range of HCl concentrations, and multiple types of limestone and dolomite reservoir rocks. Ten single-phase retarded acid (SPRA) systems, with varying base hydrochloric acid (HCl) concentration, were evaluated against unretarded HCl on a series of reservoir limestone and dolomite cores. More than 150 individual experiments on quarry cores and reservoir cores have been performed to date. In some cases, the improved performance of the SPRAs in quarry cores was not observed in PVBT tests on reservoir cores. The following overall observations and hypotheses regarding the applicability of SPRAs in matrix acid stimulation of wells are made: Reservoir cores must be used in PVBT laboratory testing to evaluate the true benefits of SPRAs in reservoir rock types.SPRAs appear to be justifiable in nearly all high-temperature (≥ 250°F) reservoirs when low pumping rate during the stimulation job is expected (e.g. coil tubing stimulations in tight rock; low-rate bullheading or coiled tubing stimulation of extended-reach open hole laterals).The SPRAs do not appear to be justifiable for moderate and high-rate stimulation in the reservoir rock types studied here, in contrast to what was expected based on previously reported Indiana Limestone and Bonne Terre Dolomite PVBT testing.The variation in rock properties between rock types and between plugs from the same rock type need to be well documented and qualified to enable proper interpretation of the PVBT data.