Coalescence of a subcritical crack pair in carbonate rocks upon acidizing

For chemically assisted cracking in tight low-permeable carbonate-rich reservoirs, crack growth and coalescence are driven by a complex interplay between stress redistribution, mineral dissolution, elasto-viscoplastic deformation, damage-enhanced specific surface area, and evolving permeability. Albeit that extensive research exists on crack initiation and growth, little has been found focusing on the underlying mechanism of how two adjacent cracks interact and coalesce through their tips - driven by a combined effect of chemical erosion and internal pressurization. Here we adopt a fully coupled reactive chemo-mechanical model for investigating the coalescence of the propagating plasticity zones around two adjacent crack-tips that precede the subcritical growth of the cracks. The constitutive framework captures time-dependent processes including proton diffusion, dissolution-induced stiffness degradation, damage evolution, chemical alteration of the yield limits, and micro-cracking feedback. The implemented formulation is applied to a pair of internally pressurized blunt-tip collinear cracks exposed to a weak-acidic solution. Our results show that the crack pair coalescence undergoes a multi-stage subcritical development: (i) a quasi-linear mechanically dominated initial (incubation) phase, (ii) a dissolution-enhanced softening phase once an accumulated mass-removal threshold is reached, and (iii) a secondary acceleration phase upon the onset when the two propagating plasticity zones coalesce in the ligament between the crack-tips. It is illustrated that an intensified acidity, or a higher rock susceptibility to micro-cracking, amplifies positive feedback between damage evolution and chemical dissolution, markedly enhancing crack growth. Mild intrinsic heterogeneity seeds further accelerate the process zone interaction and crack coalescence, through forming networks of orthogonal micro-deformation bands in front of the crack-tips.