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Matrix acidizing is vital to enhance the production of tight carbonate formations. In weak carbonate rocks or acid over-dissolution, the rock is prone to failure upon developing wormholes. Strengthening agents can play a significant role in mitigating failure and supporting more extended productivity. In this study, we tested 0.1 M and 0.2 M zinc sulfate (ZnSO4) and 0.5 M diammonium phosphate ((NH4)2HPO4) as strengthening agents, which change the dominant host mineral into harder ones. Both agents were tested on two different types of acidized (i.e., wormholed) carbonate rocks, specifically Austin chalk and Indiana limestone. The impulse hammer and acoustics measurements evaluated the carbonate rocks' mechanical properties before and after the treatment. In addition, scanning electron microscopy energy-dispersive X-ray spectroscopy (SEM–EDS) and scanning electron microscopy (SEM) were used to affirm the change of rock mineralogy into harder minerals after the treatment. The chemical treatment caused no noticeable change in the wormhole size and shape, as revealed by the scanned micro-computed tomography (micro-CT) images. The chemical treatment of the samples was conducted by core flooding the plugs at high pressure and temperature with 4–5 pore volumes of the chemicals and leaving them in the treatment solution for 72 h. As a result of the formation of harder minerals, the mechanical properties after the treatment showed improvement. Zinc sulfate raised the surface rock hardness of limestone by 16%, while diammonium phosphate showed a 30% increase when evaluated with the same technique. The most noticeable improvement happened to chalk after treatment with diammonium phosphate. Chalk impulse hammer hardness almost quadrupled with the diammonium phosphate. The acoustics measurements showed a similar result to the impulse hammer through an increase in the dynamic Young's modulus and a decrease in Poisson's ratio. [ABSTRACT FROM AUTHOR]

Fracture conductivity is a measure of the fracture ability to transfer fluids from a hydrocarbon-bearing zone to a well. Proppant embedment in soft formations and rock weakening due to the acid/rock reaction limit the stimulation success resulting in sharp fracture conductivity decline. The focus of this study is to increase the rock strength by mineralogical alteration aimed at sustaining long-term fracture conductivity. To achieve these, several standard cylindrical Indian limestone plugs with diameters of 1.5 and 2.5 in and length of 3 in were treated with different types of zinc solutions to substitute calcium with zinc in calcite crystals. The goal of these mineral alterations is to change the principal component of limestone rocks surface, calcite (CaCO 3), into smithsonite (ZnCO 3). Smithsonite is harder than calcite and its lattice system is similar to calcite. The treatment was carried out by full immersion of samples into zinc solutions at a predetermined time intervals. The rock strength was measured from samples surface (Young's modulus) before and after the treatment by a non-destructive technique using an impulse hammer. Zinc sulfate solution was found to enhance the rock hardness by values ranging from 25% to 35% via the formation of a layer of smithsonite on the sample's surface. SEM, XRF, XPS and FIB-SEM experiments were conducted to understand the exchange mechanism that eventually led to the hardening. Also, effective porosity and permeability were measured before and after the chemical treatment to investigate its impact. A field-scale modeling study of rock hardness impact on productivity is provided as a preliminary proof of concept. • Improving hydraulic fracture conductivity in carbonate minerals by forming harder ones was proved to be possible. • Rock surface Young's modulus improved significantly when calcite minerals were replaced with smithsonite. • Hi-Res XPS, XRF, SEM, and FIB-SEM experiments were conducted to understand the exchange mechanism. • Automated impulse hammer measurements were taken to prove the increase in rock hardness. • Numerical simulations proved that enhancing the rock hardness could improve the fractured well productivity. [ABSTRACT FROM AUTHOR]

Fracture conductivity is a fracture's capacity to deliver fluids from the hydrocarbon reservoir to the wellbore. Sustaining long-term fracture conductivity results in better well productivity and reserve recovery. Harder rocks tend to sustain conductivity better than do weak and ductile rocks because they are less likely to suffer from proppant embedment and the crushing and creeping of asperities. This study shows that the hardness of carbonate rocks can be improved significantly when they are treated with carbonate-consolidating chemicals. Ideally, this should be done without damaging the matrix permeability. This research tested the capacity of microemulsion, calcium hydroxide (Ca(OH) 2) nanoparticles, and tetraethyl orthosilicate (TEOS) to consolidate carbonate rocks. These are all believed to improve rock hardness by enhancing carbonate grain attachment. The treatments were applied to cores and slabs of Mississippian Indiana limestone and permo-carboniferous Khuff carbonate rocks. These chemicals were tested by either impregnation or complete immersion of the rock sample in a solution. SEM, X-ray fluorescence, porosity, permeability, and rock hardness were all measured before after the treatment. It was observed that the rock surface Young's modulus could be improved by 10%–38%. Matrix permeability was reduced by 50% when Ca(OH) 2 nanoparticles were tested. Nevertheless, the TEOS improved the carbonate rock's hardness and permeability 18% and 44%, respectively. A modeling study has also been provided to explain the impact of fractured rock strengthening on productivity improvement. • TEOS is an efficient chemical for strengthening of carbonates. • Ca(OH) 2 nanocrystals consolidate carbonates. • Treatments lead to an increase in Youngs modulus of up to 38%. • Permeability is improved in the case of TEOS treatments. [ABSTRACT FROM AUTHOR]