Low-Density High-Fluidity High-Strength Epoxy Resin Cement Slurry Technology for Wellbore Reconstruction

The production rate of a shale gas well generally declines fast, resulting in a low average recovery of the well. Refracturing of an old well with shale gas production pressure depletion can effectively enhance the ultimate recovery of the well, and wellbore reconstruction, that is, re-cementing the well inside the existing casing, is the key to the successful implementation of wellbore refracturing. Researches were conducted on several technical methods, such as the control of the rheology of the cement slurry for wellbore reconstruction, the control of the strength of the set cement and the setting time adjustment and control of the cement slurry, to deal with the technical difficulties that are encountered in shale gas well reconstruction, such as cementing in annular spaces with extremely narrow clearances and extremely high requirements on the fluidity of the cement slurry in the narrow clearance as well as the long-term zonal isolation with the ultra-thin cement sheaths. Based on the close packing theory, the compressive strength, fluidity and high temperature high pressure thickening time of cement slurries at different lengths of aging time were measured through laboratory experiments, and a low-density, high-fluidity high-strength cement slurry system for wellbore reconstruction was designed. The experimental results show that by adding 8%-10% epoxy resin into a cement slurry, the compressive strength of the set cement can be enhanced and the cement slurry still retains good fluidity. The cement slurry has stable properties, and has been used in wellbore reconstruction for 9 well times. In practical engineering application, the cement slurry has a density of 1.63 g/cm³, a fluidity of 27 cm, a flow index of greater than 0.8, a 24 h- and a 48 h-compressive strengths of 16.23 MPa and 20.50 MPa as well as a thickening time that is controllable, satisfying the operational requirements of wellbore reconstruction. The well Jiaoye 5-1HF was the first reconstructed well in which refracturing was conducted, with all technologies being domestically developed. The cementing job quality in the reconstructed well intervals was excellent, and staged refracturing operation was successfully conducted after reconstruction of the wellbore. The shale gas production during well testing has recovered to 88.10% of the production after initial fracturing operation. These researches provide a reference for the reserve growth and production increase in the development of mature shale gas fields in China.