Enhancing fracture conductivity in carbonate formations through mineral alteration

Abstract

In geothermal, oil, and gas reservoirs, the conductivity of hydraulic or acid-etched fracture determines efficient and economical resource exploitation. Proppant embedding or acid-rock reaction weakening rock leads to a sharp decline in fracture conductivity. Mineral alteration is a technique of in-situ transformation of existing minerals into harder minerals to improve rock strength, and diammonium hydrogen phosphate (DAP) has been shown to be an effective mineral alteration agent for high-porosity and high-permeability carbonate rocks at low temperatures. This work studied the effect of 10 wt% HCl and 0.8 M DAP solution on the hardness of rock samples (0.36%–1.31 % porosity and 2.72–17 × 10⁻⁶ μm² permeability) at 25, 80, 140 and 200 °C. The experimental results proved the weakening effect of acid and the strengthening effect of DAP on rock. The mechanism of hardening caused by DAP treatment and the positive effects of high temperature were revealed. As the temperature increased, the chemical reaction between DAP and rock accelerated, resulting in an increase in the amount of reaction products (calcium phosphate) and higher crystallinity, which made the rock harder. Even at ultra-high temperatures (200 °C), DAP treatment remained remarkably effective for very dense rock samples. In addition, the relationship between rock hardness and rock embedding strength was established. The fracture conductivities under different rock hardness were calculated by Nierode-Kruk correlation and numerical method. The results indicated that it was feasible to improve the fracture conductivity through DAP treatment at high temperatures. This study provides a theoretical basis for creating high-conductivity fractures through mineral alteration in deep carbonate reservoirs.