Maintaining Hydraulic Fracture Conductivity Under High Stresses through Consolidation with DAP

Abstract

Acid fracturing creates conductive fractures in carbonate reservoirs through acid-rock dissolution. However, over-etching or stimulating weak carbonate formation could result in a fast decline of fracture conductivity. In addition, most models in the literature showed that rock strength has an exponential impact on fracture conductivity. This work conducted experiments to mitigate this problem through rock strengthening with diammonium phosphate (DAP). Different fracture surfaces were tested using the ACM-3000 acid fracture conductivity system to compare the conductivity of intact samples with the treated ones. The fracture conductivity of 4 pairs of weak Austin chalk samples (7in × 1.34in × 3in) with flat surfaces was evaluated by flowing nitrogen gas (200-350 cc/min) at increasing stresses up to 1,500 psi. Half of the samples were kept intact to represent the control group to which treated samples were compared. The treatment consists of saturating vacuumed chalk samples with 1M DAP for 72 hrs at pressure and temperature of 1,000 psi and 75°C, respectively. In addition, the surface hardness of the samples was measured pre-and post-treatment, and conductivity measurements were performed to evaluate the treatment effect on the samples. The chalk samples used in this study are naturally weak with low average surface hardness (i.e., 3 GPa compared to 15 GPa for Indiana limestone). However, the DAP solution could significantly enhance the surface hardness of soft chalk up to almost 3.5 times its original value. The chalk hardness increase reduced the normal deformation and increased the yield stress of the treated samples. The untreated sample exhibited lower endurance to loading and developed cracks at lower stresses. Also, the results showed that the normalized conductivity of the treated flat samples is at least double the untreated ones. The chemical treatment makes the chalk stiffer with less deformation when stressed, which leads to enhanced conductivity at higher stress. In this paper, an additional stage is suggested in acid fracturing to harden the surface of carbonate rocks chemically after acid injection. Successful application of such treatment in the field can extend fracture life and substantially reduce the need for re-fracking jobs.