Seepage characteristics of thermally and chemically treated Mesozoic granite from geothermal region of Liaodong Peninsula

Abstract

The seepage characteristics of granite are complicated due to flow exchange between fractures and micropores, which are strongly heterogeneous and anisotropic. The potential geothermal reservoirs of hot dry rocks are often\xa0related to granite. The key to geothermal production from hot dry rocks is to generate an efficient fracture network through enhanced geothermal system (EGS) technologies. Although hydraulic fracturing technology is applied in most EGS projects worldwide, it has been indicated that thermal and chemical stimulation methods are also efficient. In this paper, different thermal and chemical treatments are applied to Mesozoic\xa0granite samples\xa0obtained from geothermal region of Liaodong Peninsula in northeastern China, and their impacts on the seepage characteristics of the granite\xa0are carefully analyzed. The thermal and chemical damage mechanisms of granite, in terms of the mineral components, microstructures and physical properties, are also studied. The results show that cooling in cold water results in a much larger permeability increase than that achieved with cooling in air after the granite is heated to a specific temperature. Thermal treatments cause mechanical damage\xa0(e.g., compressive strength, elastic modulus, etc.)\xa0of granite. However, the differences between the damage factors of various granite samples are large, which\xa0are highly dependent on the original mineral composition and microstructures. Thermal cracking induced by either thermal fatigue or thermal shock causes changes in the porosity and permeability of granite. The permeability of the granite changes substantially, i.e., 3–4 orders of magnitude, after soaking in mud acid solutions, while it changes by less than 1 order of magnitude after soaking in alkaline solutions. The dissolution\xa0ability of the mud acid solution with the highest concentration (12% HCl\u2009+\u20093% HF) is the strongest among the three types of mud acid solutions tested (S1, S2 and S3). But it mainly acts on improving the proportion of small pores with the size of 0.01–10\xa0μm. It has the least impact on the generation of large pores (10–50\xa0μm), which may be caused by the heterogeneity of mineralogy among different granite samples. Different chemical solutions infiltrate the interior of the granite through various micropores and fissures, enhancing the chemical interaction through the dissolution and precipitation of minerals, the porosity, and the permeability, destroying not only the grains and minerals but also the microstructures of the granite.