Cement is the main material used to seal wells which are drilled for the production of oil and gas. However, many well operations could induce cement sheath to crack and hence, lose zonal isolation. The research objectives in this paper are designed to realize the application of superabsorbent polymer (SAP) in well cement to plug the microcrack.
Since SAP is highly reactive with water, if SAP is used directly, the cement slurry will have a too high viscosity for pumping in favorable conditions. Therefore, SAP microspheres which were coated were applied. The method of manufacturing quantitative microcrack was established by using the chisel and transparent plastic tube, and the self-designed instrument was taken to test the self-plugging performance of microcrack. Based on the percolation theory, the percolation analysis of microcrack plane flow was carried out. The distribution model of coated SAP microspheres in cement paste was established, which could ensure SAP microspheres exist in any microcrack plane.
Finally, the gypsum-chitosan-gypsum coated SAP is proposed. Microscopic analysis and percolation theory are combined to research the plugging microcrack mechanism. And according to the exact analytical solution of plane percolation threshold, the dosage threshold of coated SAP microspheres which can realize the self-plugging performance of microcrack is 28.40%. The cement slurry whose basic performance met the demand of cementing engineering was developed to test the self-plugging performance of microcrack with water, and the results show that when the dosage of coated SAP microspheres is higher than the threshold, microcrack could be plugged.
In this paper, the percolation theory was first used to design the cement slurry. The application of coated SAP can effectively seal the microcrack in cement sheath, which is of great significance to guarantee the long-term sealing performance of cement sheath.