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Summary High-temperature (HT) deep carbonate reservoirs are typically drilled using barite (BaSO4) as a weighting material. Primary production in these tight reservoirs comes from the network of natural fractures, which are damaged by the invasion of mud filtrate during drilling operations. For this study, weighting material and drilling fluid were sampled at the same drillsite. X-ray diffraction (XRD) and X-ray fluorescence analyses confirmed the complex composition of the weighting material: 43.2 ± 3.8 wt% of BaSO4 and 47.8 ± 3.3 wt% of calcite (CaCO3); quartz and illite comprised the rest. The drilling fluid was used to form the filter cake in a high-pressure/high-temperature (HP/HT) filter-press apparatus at a temperature of 300°F and differential pressure of 500 psig. Compared with the weighting material, the filter cake contained less CaCO3, but more nondissolvable minerals, including quartz, illite, and kaolinite. This difference in mineral composition makes the filter cake more difficult to remove. Dissolution of laboratory-grade BaSO4, the field sample of the weighting material, and drilling-fluid filter cake were studied at 300°F and 1,000 to 1,050 psig using an autoclave equipped with a magnetic stirrer drive. Two independent techniques were used to investigate the dissolution process: analysis of the withdrawn-fluid samples using inductively coupled plasma-optical emission spectroscopy, and XRD analysis of the solid material left after the tests. The dissolution efficiency of commercial K5-diethylenetriaminepentaacetic acid (DTPA), two K4-ethylenediaminetetraacetic acid (EDTA), Na4-EDTA solutions, and two “barite dissolvers” of unknown composition was compared. K5-DTPA and K4-EDTA have similar efficiency in dissolving BaSO4 as a laboratory-grade chemical and a component of the calcite-containing weighting material. No pronounced dissolution-selectivity effect (i.e., preferential dissolution of CaCO3) was noted during the 6-hour dissolution tests with both solutions. Reported for the first time is the precipitation of barium carbonate (BaCO3) when a mixture of BaSO4 and CaCO3 is dissolved in DTPA or EDTA solutions. BaCO3 composes up to 30 wt% of the solid phase at the end of the 6-hour reaction, and can be dissolved during the field operations by 5 wt% hydrochloric acid. Being cheaper, K4-EDTA is the preferable stimulation fluid. Dilution of this chelate increases its dissolution efficiency. Compared with commonly recommended solutions of 0.5 to 0.6 M, a more dilute solution is suggested here for field application. The polymer breaker and K4-EDTA solution are incompatible; therefore, the damage should be removed in two stages if the polymer breaker is used.

Accurate prediction of the rock dissolution process is crucial for designing efficient acid stimulation treatments. At typical conditions, the dissolution of carbonates in most acids is limited by the rate of convective diffusion of reactive species to the surface of the rock. The experimental techniques used to determine the acid-diffusion coefficient are comparably well-understood by the research and engineering community. However, one important physicochemical phenomenon termed diffusion relaxation has not been studied in detail and accounted for in all the existing acid fracturing and matrix acidizing modeling software programs. The objective of this work is to address these gaps in research and optimize acid treatment designs. Diffusion relaxation occurs downstream of an inert or less reactive rock layer and results in higher mass transfer, i.e., dissolution rate of the rock located immediately downstream of an inert layer. To study the process of diffusion relaxation, 15 wt% hydrochloric acid at a temperature of 150°F was injected through a composite acid fracture model. This model was prepared by inserting 0.5 and 0.25 in.-long sandstone layers into a standard 7 in.-long fracture model made of Indiana limestone. Laser profilometry of the fracture surfaces after the experiment revealed the presence of 0.1 in.-deep channels of more etched limestone downstream of inert layers, as compared to the upstream of inert layers. The zone of an enhanced dissolution rate—termed diffusion relaxation zone—extends to a distance comparable to the length of an inert layer and appears because of the following. As soon as the acid flow encounters inert areas, the concentration of reactive species at the fracture surface starts to accumulate since there is no dissolution reaction. Right downstream the inert areas, the limestone surface contacts with the acid that has not been spent by the diffusion of reactive species. Because of that and an impact of tangential mass transfer in the diffusion boundary layer, downstream of inert areas the diffusional mass transfer significantly—often more than two times—exceeds the limiting mass transfer established upstream of the inert areas. Etched channels formed in diffusion relaxation zones contribute to the fracture conductivity, which is not considered in existing modeling software programs. Results indicate that the observed phenomenon is universal, i.e, it also occurs during dissolution of rocks with different reactivities. This research innovatively discusses the impact of physicochemical phenomena of diffusion relaxation on the dissolution of carbonate rocks, and formation of conductive flow channels. Presented results are integral for designing acid stimulation operations.

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