Impact of acid wormhole on the mechanical properties of chalk, limestone, and dolomite: Experimental and modeling studies

Abstract

Matrix acidizing is a widely applied stimulation process in carbonate reservoirs to remove damaged parts of the formation and enhance wellbore productivity. Few studies considered the impact of acid on the deterioration of mechanical rock properties considering different carbonate types. Assessment of the wellbore stability and the sustainability of the wormhole structure depends on the mechanical rock properties after acidizing. In this study, three rock lithologies (chalk, limestone, and dolomite) were treated with acid at different injection rates to establish acid efficiency curves. The acid was injected in a coreflooding setup until wormhole breakthrough, indicated by zero pressure drop across the sample. The rock petrophysical (i.e., porosity, permeability) and mechanical properties were measured before and after the treatment. XRD was used to estimate the mineralogical compositions of each rock type. Then, the impulse hammer technique was implemented to measure the rock surface hardness at ambient pressure. Also, acoustic measurements were used to quantify the dynamic Young’s modulus and Poisson ratio at high confining pressures. To estimate the deformation in the samples with wormholes under varying confining stress, a finite element model was constructed. Results indicate that the acid treatment significantly reduces the rock surface hardness (by 47 to 77 %), while Young’s modulus decreases by 18 to 33 % at ambient stress. The treated rock samples showed higher Young’s modulus under high confining stress. Chalk samples were the most impacted by the treatment, while dolomite specimens were the least. The finite element numerical model indicates that rock deformation is more significant in chalk samples, especially at high confining stress that acts perpendicular to the wormhole.