Enhancing Fracture Conductivity in Soft Carbonate Formations through Tribasic Potassium Phosphate and Sodium Fluoride Treatment

Summary Hydraulic fracturing is still one of the most commonly used methods to stimulate hydrocarbon production. However, in soft carbonate rocks, long-term efficiency of such a technique is usually compromised by proppant embedment that diminishes fracture conductivity due to high in-situ stresses. One of the promising methods to counter such a phenomenon is to increase rock matrix mechanical strength through chemical consolidation. In this research work, tribasic potassium phosphate (K3PO4) was tested as a chemical treatment for enhancing the surface hardness of two exemplary carbonate rocks: soft chalk outcrop and Indiana limestone. To assess this treatment’s performance level, a total of four propped fracture conductivity experiments are carried out, with two experiments on chalk slabs (untreated and treated) and another two experiments on limestone slabs (untreated and treated). The treated slabs would be subjected to 1 M solution of K3PO4 under 1,000 psi and 160°F for 72 hours to mimic realistic reservoir conditions. Mechanical characterization was performed with an impulse hammer device in measuring Young’s modulus on dry slabs both before and after treatment. Fracture conductivity was also assessed through the use of the API ACM-3000 system at varying closure stresses up to 2,500 psi in increments. The outcome indicated a significant increase in rock stiffness, translating to increased fracture conductivity after treatment. Under 2,500 psi of applied load, the treated limestone sample improved its fracture conductivity by 36% with reference to its untreated control sample. The increase was even higher in chalk, where the conductivity increased by 71% under the same stress. Furthermore, an extra test was conducted on chalk slabs by the same test conditions, where sequential treatment was given by exposing the specimens to K3PO4 for 68 hours and then to sodium fluoride (NaF) for 4 hours. This dual-step treatment resulted in a 214% improvement in hardness and a 151% enhancement in conductivity, surpassing the single-step tribasic treatment. The results of scanning electron microscopy (SEM) substantiated the formation of the dense fluorapatite-rich coating, and hence, the inclusion of fluoride increased the crystal shape of the surface, making it stronger and resistant to deformation upon applied stress. The results confirm that K3PO4 and the combined phosphate and fluoride treatment can markedly strengthen weak carbonate rocks and maintain fracture conductivity under high stress, offering a promising chemical approach to enhance stimulation performance in challenging carbonate formations.