Wenxi Zhu

The Colloidal Gas Aphron (CGA) drilling fluid successfully solved the problems of serious lost circulation and reservoir damage that are faced by drilling in depleted oil/gas reservoirs and low-pressure areas. However, the lack of high-temperature resistance, salt and calcium resistance are the key problems that restrict its application in complex formations. This study provides an environmentally friendly and non-toxic CGA formula based on self-developed reagents. Microscopic tests showed that stable aphrons were successfully generated in 36%NaCl-CGA or 7.5%CaCl2-CGA aged at 150℃, with stabilization of ＞2 hours. The Herschel-Bulkely model accurately describes the rheological behavior of CGA fluids containing NaCl/CaCl2. The addition of NaCl increases CGA fluid viscosity, while CaCl2 is the opposite. However, CGA fluid maintains appropriate rheological parameters and shear thinning behavior, which means good cutting carrying capacity. With the addition of NaCl/CaCl2, CGA has low filtration volumes, which meets API requirements. NaCl/CaCl2 reduces the lubrication coefficient and increases the adhesion of the mud cake. Moreover, the anti-cuttings pollution ability of 150℃ aged CGA can reach 10%. CGA, 36%NaCl-CGA, and 7.5%CaCl2-CGA all have low linear expansion rates (84%). Therefore, the CGA system has good inhibitory performance and is compatible with easily hydrated formations.

Effective heat transfer between the working fluid and subterranean rocks is essential for producing green and low-carbon geothermal energy. As the primary thermal conductive medium, cement has low thermal conductivity, leading to high thermal resistance and significantly reducing geothermal wells’ efficiency. Therefore, high thermal conductivity cement has emerged as a widely anticipated new research area. The purpose of this research is to address the substantial harm of traditional carbon-based thermal conductive fillers to cement. A novel expanded graphite (EG)/epoxy resin (EP) composite additive (MEG) was designed to increase cement’s thermal conductivity while preserving its mechanical strength and pumpability. Firstly, the physicochemical properties of MEG were revealed by FT-IR, UV-Vis, SEM, and TGA. Then, the applicability of MEG cement in adverse geological environments (high-temperature 60–100 ℃, high-mineralization 5–36% NaCl) was evaluated through simulated maintenance experiments. Finally, the hydration products and pore structure of MEG-cement were analyzed by XRD/FT-IR and SEM/MIP, revealing the thermal conductivity enhancing mechanism. The results showed that: 1) MEG uses ZDMA as a bridge to promote the ring opening and curing of EP, and is formed by strong cation -π interaction with EG. 2)After curing at 60–100 ℃, MEG-cement exhibits a significant increase (46.6-182.1%) in thermal conductivity within the optimal dosage range of 5–10%, fully meeting the requirements for compressive strength (10.4–21.7 MPa) and fluidity (19.3–21.2 cm) of cementing. In addition, MEG-cement maintained stable density and significant high thermal conductivity advantage in high-mineralization environments (5–36% NaCl), with an increase in thermal conductivity of 23.8- 54.1%. 3) The mechanism of MEG promoting heat transfer in cement is summarized as the enhancement of the hydration process and the production of C-S-H gels. C-S-H gels filled the gel pores and transition pores in the cement skeleton and formed a dense, high thermal conductivity network, which shortens the heat transfer path and thus greatly improves the thermal conductivity of cement. In summary, this study has successfully developed a MEG geothermal cement with independent intellectual property rights that provides reliable technical support for the efficient development of geothermal resources and has important engineering application value.

Guided by the goal of improving the heat transfer efficiency of geothermal cement, the development of high thermal conductivity fillers is urgently needed. Considering the drawbacks of carbon-based fillers in deteriorating the strength and fluidity of cement, this study proposes the use of green and low-cost deep eutectic solvents (DES) loaded with graphene oxide to form nanofluids (GONF) to improve the connection between graphene oxide and cement. The thermal conductivity and basic properties of GONF-cement under thermal storage conditions (60–100 °C, 0–36%NaCl) were reported, and the influence mechanism of nanofluids was analyzed. The following interesting conclusions were found: (1) The oxygen-containing functional groups on the surface of graphene oxide were connected to active sites of DES through the formation of the hydrogen bond network, forming thermal stable nanofluids. (2) Nanofluids was proven to be a low-cost (reduce material costs of graphene oxide by 90%) and effective material for enhancing the thermal conductivity and mechanical properties of geothermal cement while maintaining appropriate fluidity and low variation of density. (3) The synergistic mechanism between nanofluids and cement is due to the fact that nanofluids promotes hydration process by providing stable reaction sites, significantly reducing the porosity and pore size of cement. The dense structure enhances compressive strength and shortens the heat transfer path. This study enriches geothermal cementing technology and provides support for the low-cost application of graphene oxide in exploration engineering.

Polyvinyl chloride (PVC) releases hydrochloric acid (HCl) during its thermal degradation, and hydrochloric acid can react with hydration products of alkali-activated binders. According to this characteristic of PVC and the temperature change that occurs during the development of a geothermal well, the PVC was added into slag/fly ash binder to develop self-degradable materials. The thermal degradation properties of PVC, compressive strength, hydration products, and microstructure of binders at different stages were tested, in order to study the degradation mechanism of the material. It was found that 20% PVC reduced the compressive strength, decreasing the level of binder from 13.95% to 76.63%. The mechanism of PVC promoting the material degradation mainly includes the following: (1) the thermal degradation of PVC increases the number of multiple damage pores in the material, at a high temperature; (2) HCl generated by the PVC thermal degradation reacts with the binder gels, and breaks them into particles; and (3) HCl also reacts with other substances in the binder, including CaCO₃ and NaOH in the pore solution.