Light-weight temporary-sealing cement for medium temperature geothermal wells by using polyvinyl alcohol fibers and fly ash cenospheres

Abstract

A new lightweight temporary-sealing cement (LTC) is developed in this study to prevent lost circulation and simultaneously protect target formation during drilling. Water-soluble polyvinyl alcohol (PVA) fibers and fly ash cenospheres (FACs), were added to class G cement to develop the LTC for 90–150 °C geothermal wells, on the basis of the temperature change process related to cement plugging. Fourier transform infrared (FT-IR) spectroscopy, X-ray diffraction (XRD), mercury intrusion porosimetry (MIP), scanning electron microscopy/energy dispersive spectroscopy (SEM-EDS) and total organic carbon (TOC) tests were conducted to investigate the chemical composition, crystalline phases, pore diameter distribution, permeability, microstructure of the specimens and the dissolution rate of PVA fibers in hardened cement. The following conclusions can be drawn. 1) The addition of 10% FACs and 3% PVA fibers optimized the cement slurry density to 1.54 g/cm3 2) The addition of PVA fibers enhanced the cement flexural strength. And all the compressive strengths of the samples were greater than 6.5 MPa 3) The fibers in hardened cement could maintain integrity in 60 °C water, and remain soluble under 120 °C dry condition, demonstrating that they could work with cement and FACs to seal the fracture for drilling at lower temperature, and dissolve out from hardened cement in ≥90 °C water. 4) Most of the PVA fibers dissolved and leached out of the cement within 4 h in 90 °C water immersion. The elongated holes formed by fiber dissolution, interconnected with other pores in cement matrix, contributing to the less tortuosity and substantially increased permeability of LTC. In general, the enough strength and high permeability of degraded LTC prove that the 90 °C water-soluble PVA fibers and FACs could be used to prepare the partially degradable LTC for 90–150 °C geothermal wells.