Enhancing chalk formation integrity by diammonium phosphate treatment.

Fine-grained chalk reservoirs in the North Sea (NS) commonly experience fines migration or particle production, or both, leading to production issues like formation damage and reduced production. This study aims to improve the strength of chalk to mitigate fines migration and particle production using a novel chemical consolidation technique, utilizing Austin chalk (AC) outcrop samples and samples from producing reservoir chalk units from the Danish NS. Two cylindrical AC plugs and four downhole cores from the NS, each measuring 3.8 cm in diameter, were treated with a 1 M diammonium phosphate ((NH4)2HPO4) solution at elevated temperatures (75 °C). The samples were placed in a vacuum-sealed steel cell, pressurized to 6.9 MPa, and aged for 72 h to transform calcite (CaCO3) into the harder hydroxyapatite (Ca5(PO4)3OH). Rock strength was assessed before and after treatment using a non-destructive impulse hammer. Mineralogical changes were examined via Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS), and X-ray Diffraction (XRD). Permeability and porosity were also measured pre-and post-treatment. In addition, Total Inorganic Carbon (TIC) analysis was performed on the solution, and Gas Chromatography (GC) was utilized to analyze the gas phase, providing insights into the interactions between calcite and diammonium phosphate (DAP). DAP treatment markedly enhanced the stiffness of AC by up to 312% and of NS chalk by up to 53%, primarily due to the formation of hydroxyapatite (HAP) micron-sized crystals forming interparticle cements. SEM, EDS, and XRD confirmed HAP formation, leading to hardening by altering calcite. While effective porosity remained unchanged, permeability decreased after treatment. The reaction was markedly more effective on AC samples due to their overall higher permeability and lower initial strength than reservoir samples. These results, if applied within a reservoir, can help mitigate particle production and fines migration issues, improving operational efficiency, reducing production challenges and may be applicable to other fine-grained carbonate reservoirs.