Hydraulic Fracture Conductivity Sustenance in Carbonate Formations Through Rock Strengthening by DAP Solution

Hydraulic fracturing operations involve generating conductive pathways in low permeability formations to maximize hydrocarbons production. One of the main challenges is maintaining fracture conductivity under high closure stresses, especially in soft formations. However, long-term fracture conductivity can be sustained through fracture surface hardness improvement. This research targets the strengthening of carbonate rocks via the transformation of calcite into the harder hydroxyapatite mineral. In this study, limestone, chalk, and dolomite rock specimens were treated with 1M solution of diammonium phosphate (DAP) for 3 days at room temperature conditions. Rock samples' hardness was measured by indentation (Brinell hardness) technique before/after the treatment to assess the strengthening effect of DAP. The changes in the mineralogy in treated samples were studied by SEM-EDS technique. The formation of phosphate minerals was achieved in treated samples, and they were clearly seen in the SEM images. The results have shown that both limestone and chalk samples reacted strongly with DAP solution, which was expressed in terms of rich abundance in newly formed minerals inside rock specimens. The reaction between dolomite and DAP solution was observed to be weak which resulted in generation of isolated phosphate minerals. The formed minerals were identified as hydroxyapatite (5 hardness in the Mohs scale) after comparing their morphology with other phosphate minerals reported in the literature. Treatment of the rocks by DAP solution resulted in improvement of their strength. The Brinell hardness of the chalk specimen was increased by 30% after the treatment, whereas in the case of the limestone sample, a 13% increment in hardness was achieved. The proposed carbonate rock strengthening technique can be applied in hydraulic fracturing It is intended to solve common soft formations problems (e.g., asperities failure, proppant embedment) causing acid/propped fractures' conductivity reduction.