Diammonium Phosphate-Induced Strengthening of Dolomite-Rich Carbonates: Insights into Mineralogical Reactions

Summary Acid fracturing is a widely used stimulation method for carbonate reservoirs, aiming to enhance productivity by creating conductive fractures through mineral dissolution. However, acid-rock interactions can significantly weaken the mechanical integrity of the formation, leading to a rapid closure of fractures and loss of conductivity over time. Preserving the mechanical stability of acidized zones is critical for maintaining long-term well productivity. Chemical consolidation techniques, such as treatment with diammonium phosphate (DAP), are now used as potential solutions. DAP treatment results in the formation of harder mineral phases, including hydroxyapatite (HAP), on carbonate surfaces, restoring stiffness and maintaining fracture conductivity under stress. While successful results have been reported for acidized chalk and limestone, the reactivity of dolostone and other downhole minerals, such as anhydrite, with DAP remains less well understood. Dolomite reacts more reluctantly than calcite with DAP, producing poorly crystalline HAP and magnesium-rich phosphates. At the same time, anhydrite can dissolve under high-temperature DAP treatments, contributing calcium for secondary mineral precipitation. This study investigates the use of DAP to consolidate acidized dolomitic carbonate samples under downhole conditions. Although acidizing successfully etched the rock, it also led to channel formation that prevented measurable pressure drops during conductivity coreflood tests. Characterization methods including mechanical stiffness mapping, X-ray diffraction (XRD), energy-dispersive spectroscopy (EDS), wavelength-dispersive X-ray fluorescence (WD-XRF), Fourier-transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), and gas chromatography (GC) were used to evaluate mineralogical transformations and mechanical strength changes. Stiffness measurements revealed that acidization significantly weakened carbonate-rich samples (up to 71% reduction), whereas DAP treatment partially restored stiffness, with recovery as high as 34%. The results show that increasing the surface area through acid etching enhances reactivity, while DAP treatment improves mechanical strength by forming cement-like phases such as struvite and whitlockite, even in compositionally complex substrates. This highlights its potential as a promising approach to maintaining the efficacy of stimulation treatments over an extended period in carbonate reservoirs.