Permeability and Porosity Changes in Sandstone Reservoir by Geothermal Fluid Reinjection: Insights from a Laboratory Study

Abstract

Geothermal energy is a clean and environmentally friendly energy source that can be used sustainably; however, attention towards geothermal energy has been intermittent during the last 40 years as a function of the crisis of oil price. However, geothermal reinjection and clogging has been a challenge limiting geothermal development and utilization. In China, widely distributed sandstone geothermal reservoirs have reduced production due to technical constraints such as excessive reinjection pressure and blockage. In this paper, we took the Binzhou sandstone geothermal field in North China as an example and conducted displacement experiments under different temperature and flow rate conditions by collecting in situ geothermal fluid and core rock to obtain changes in sandstone permeability. By comparing the variation in geochemical and mineral composition of geothermal fluids and cores before and after the experiments, combined with a water–rock interaction simulation, we investigated the reasons for the changes in permeability and porosity. The results show that high temperature and low flow rate have relatively minimal displacement pressure, and a flow rate of 1.0 mL/min at 45 °C shows a minimal effect on permeability, while 1.0 mL/min at 55 °C and 0.5 mL/min at 45 °C show a minimal effect on porosity. Flow rate is the main factor controlling permeability, while temperature demonstrated a relatively minor effect. The shift in permeability and porosity is mainly caused by the precipitation of quartz and the conversion of albite to montmorillonite. The injection of fluids at 55 °C may have dissolved additional minerals with a minimal change in porosity. However, the permeability reduction at 55 °C is greater than that at 45 °C, indicating that the blockage, which led to the permeability reduction, contains multiple causes, such as chemical and physical blockages. From the laboratory studies, we recommended that reinjected geothermal water be cooled or kept below the reservoir temperature before reinjection and at moderate flow conditions.