Diammonium Hydrogen Phosphate Treatment for Sustaining Long-Term Acid Fracture Conductivity of Chalk Formations

Abstract

Abstract The reactive acid, injected into the formation during acid fracturing operations, etches the surfaces of the fracture, generating asperities that keep the fracture conductive. However, the stresses acting on the fracture tend to close it, destroying the asperities. Asperities' destruction is especially severe in soft carbonate formations such as chalk. This work focuses on improving the long-term acid fracture conductivity sustenance by hardening the chalk rocks through chemical treatment. Diammonium Hydrogen Phosphate (DAP) is applied to harden chalk slabs treated with 10% HCl acid in this work. Treated slabs are inserted in the API conductivity device that allows for measuring the conductivity of the fracture. The conductivity of the fracture treated by DAP after acid injection is measured at different stresses and compared with the conductivity of the acidized fracture with no DAP treatment. Furthermore, the additional analysis is conducted to observe the changes in the roughness and hardness of the samples before and after treatments. The results demonstrated that the hardness of the chalk samples was decreased by 30 - 40% after acidizing. However, the subsequent treatment of the acidized samples with the DAP solution restored their original hardness. Moreover, the hardened samples were more deformation-resistant to the applied stresses based on Linear Variable Differential Transformers'(LVDT) measurements. The conductivity of the DAP-treated fracture decreased less abruptly with applied stresses (up to 2,500 psi) compared to the acidized but untreated fracture. As a result, the ultimate conductivity of a DAP-treated fracture was 60 md-ft, which was significantly higher than the conductivity of the untreated fracture that constituted 8 mdft. Additional long-term conductivity experiments were conducted over time (6 days) while applying the fixed stress. These experiments have shown that the decline in the conductivity of treated fracture is less severe compared to untreated fracture. It was noted that the plateau value of long-term fracture conductivity for the treated samples was significantly higher than for the untreated case. This work presents a novel approach to sustaining acid fracture conductivity in soft chalk formations through chemical treatment with a DAP solution. This approach is validated through the API conductivity experiments that mimic the behavior of a fracture at reservoir conditions. The positive results obtained from this research allow suggesting that the technique will have great potential if deployed in the actual field operations.