Chemical Treatment for Sustainable Acid Fracture Conductivity of Weak Carbonates

Abstract

ABSTRACT Acid fracturing is an essential stimulation technology to increase the production of carbonate reservoirs. However, acid usually softens the rock, and the stability of the fracture hydraulic conductivity is hardly maintained. Therefore, rock strength after an acid treatment is crucial in controlling the emerging hydraulic conductivity. Consolidation agents like diammonium phosphate (DAP) could enhance the mechanical properties of damaged or weak carbonate rocks. This study investigates the DAP consolidation effect on weak carbonate fracture hydraulic conductivity. Weak Austin chalk samples were soaked in 1 M DAP for 72 hrs. under high pressure and temperature. The surface hardness of weak Austin chalk samples was evaluated nondestructively using impulse hammering. The sample's surface hardness was greatly improved upon the DAP treatment. The treated samples’ hydraulic fracture conductivity and compaction were measured and compared with similar untreated samples. The treated samples exhibited higher and sustained hydraulic conductivity and less compaction under higher stress. The proposed solution could be particularly useful in soft carbonate formations, where induced acid softening is a common problem. Boosting the fracture surface hardness with DAP can improve the efficiency of the acid stimulation process and increase the amount of hydrocarbons that can be extracted. INTRODUCTION While acid fracturing manifests itself as one of the most deployed stimulation methods in carbonate reservoirs, its success is limited by the carbonate strength after the operation. The asperities that keep the fracture open are more vulnerable to failure because acidizing decreases the compressive strength of the carbonate (Gong et al., 1999). Likewise, (Nierode & Kruk, 1973) in Equations 1&2 related the hydraulic conductivity (kfwi) of the fracture to the rock embedment strength (SRE), e.g., the lower the embedment strength, the lower the hydraulic conductivity. Moreover, the rock embedment strength determines the rate at which the conductivity falls under stress (σC) (Gomaa & Nasr-El-Din, 2009). Also, soft carbonates like chalk exhibit more time-dependent deformation (creep) than harder carbonates like dolomite (Abass et al., 2006). (Equation) (Equation) In carbonate gas wells post-acid fracturing jobs, a severe production loss of more than 60% within 6 months was reported (Bartko et al., 2003). The reasons behind the sharp decline should be understood before considering restimulation jobs which are not usually successful (Abass et al., 2006). However, the best scenario would be to have a sustainable acid fracture at the first trial.