Theoretical model of geothermal tail water reinjection based on an equivalent flow channel model: A case study in Xianxian, North China Plain

Abstract

Based on tracer tests and the equivalent flow channel model, this study proposes a theoretical modelling framework to back-calculate the unknown parameters of geothermal reservoirs and predict the thermal breakthrough in production wells. The inverse modelling module combines the analytical solution of the tracer transport equation with the method of moving asymptotes, and then an analytical model that considers heat advection and transfer was used to predict the thermal breakthrough in a production well. Based on the results of tracer tests in the double geothermal wells, Xianxian geothermal field, China, two equivalent flow channels were determined, and their average flow velocity, dispersion coefficient, section area, and channel length were estimated using this theoretical framework. The possible thermal breakthrough in the production well and its main influencing factors are also discussed. The theoretical model was applied to the engineering case, and the tracer test data were used to carry out the parameter back analysis and obtain the goodness of fit (R 2)¼0.7907 > 0.6, indicating that the optimization result strongly correlated with the measured value and the fit was good. The values of the thermal reservoir parameters from the tracer test's back analysis were used to predict the thermal breakthrough for the geothermal well system. The results indicated that due to the

Numerical Simulation and Design Optimization of Large-Scale Geothermal Production Based on a Multiwell Layout in Xianxian Geothermal Field

“…Among them, the exploitation well P48 and the corresponding reinjection well R49 are located 543 m away from each other, and the temperature of well P48 decreased by 1.2 °C after 100 years of operation. This indicates that it is feasible to set the well spacing at greater than 500 m under the current operation condition, which is consistent with the geothermal doublet system of Liu et al [30].…”

“…The first term on the right-hand side of equation ( 5) is used to describe the energy loss caused by the friction between the fluids and the well wall, whereby the Darcy friction factor f D is related to Re and the ratio of e to d i [30]:…”

Thermo-Hydro-Mechanical Coupling Processes in Carbonate Geothermal Reservoirs and Their Numerical Simulation

“…The fluid flow in fracture and cavity media can be described using four conceptual models up to now, namely, dual (multiple) continuous media model, equivalent continuous media model, discrete fracture network model, and the coupling model of discrete and continuous media. Since karst has not developed but fractures have developed in the geothermal reservoirs in the study area [20], the carbonate geothermal reservoirs are regarded as the dual media composed of a discrete fracture network and the porous media of the rock matrix.…”

“…It has affected the strata deposition in the area, resulting in the absence of the Guantao Formation and Paleogene sediments in the Xianxian Salient-a fourth-order tectonic unit. The burial depth of the bedrock in the Xianxian geothermal field is about 1300−1500 m, while that in the Raoyang Sag-a fourth-order tectonic unit in the Jizhong Depression-reaches more than 4000 m [10,20,37]. There are two types of geothermal reservoirs, namely, pore type and fissure type.…”

“…Deep geothermal reservoirs tend to be deeply buried and occur in a complex high-temperature and high-pressure environment, which makes it difficult to directly obtain their physical parameters. Meanwhile, the thermal breakthrough of the reservoirs can be not accurately predicted without ascertaining the hydraulic connection between production and reinjection wells as well as the seepage properties of the reservoirs [20,21]. Tracer tests are mainly aimed at determining the hydraulic connection and dominant flow direction of groundwater and measuring hydrogeological parameters such as groundwater flow velocity and permeability coefficient, and they are widely applied in geological and hydrogeological research fields [22][23][24][25][26][27].…”