Experimental study on in-situ self-generating proppant fracturing prepad fluid system for micrometer-scale fractures in calcium-rich reservoirs

Conventional proppants effectively maintain primary and near-wellbore fracture apertures, but cannot access far-field micron-scale fractures (≤150 μm), which are prone to closure leading to reduced fracture conductivity and impaired hydrocarbon recovery efficiency. This study proposes an innovative system based on the injection of a phosphate-based prepad fluid into reservoir fractures, where it hydrothermally reacts with calcareous minerals to form hydroxyapatite (HA). This technology aims to overcome propping challenges in far-field micron-scale fractures of calcium-rich reservoirs. This study considers the design and optimization of the proppant system, evaluates the factors influencing proppant growth, and uses lab-scale experiments to assess proppant strength and performance. Reaction mechanisms were elucidated using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results showed that hydrothermal reactions between calcareous minerals and phosphate systems ((NH4)2HPO4, NH4H2PO4, NaH2PO4) achieve HA yields exceeding 90 %. Orthogonal experiments revealed that the order of significance for factors affecting self-growth performance is: reaction temperature > initial pH > phosphate concentration > reaction time. The optimal set of conditions to achieve maximum HA yield was determined to be a phosphate concentration of 5 wt%, reaction time of 48 h, reaction temperature of 120 °C, and initial pH of 2. Fracture flow conductivity experiments showed that rational optimization of the system formulation can lead to enhancements of up to 404 times in the short-term fracture flow conductivity, as well as enhancements of 80–110 times in long-term fracture flow conductivity. This significant improvement in conductivity validates the technical feasibility of this novel method.