Dynamic analysis of heat extraction rate by supercritical carbon dioxide in fractured rock mass based on a thermal-hydraulic-mechanics coupled model

Heat production from geothermal reservoirs is a typical heat transfer process involving a cold working fluid contacting a hot rock formation.Compared to the thermal-physical characteristics of water,supercritical CO_(2)(scCO_(2))has a higher heat storage capacity over a wide temperature-pressure range and may be favored as a heat transfer fluid.Singularly characteristic of scCO_(2)-based heat extraction is that the hydraulic-thermal properties of the scCO_(2) vary dramatically and dynamically with the spatial pressure gradient during unsteady-state flow along fracture.This highly nonlinear behavior presents a challenge in the accurate estimation of heat extraction efficiency in scCO_(2)-based EGS.In this paper,a thermal-h ydraulic-mechanical(THM)coupled model is developed by considering deformation of the fractured reservoir,non-Darcy flow and the varying thermal-physical properties of scCO_(2).The proposed model is validated by matching the modeling temperature distribution with published data.The results show that during continuous injection of scCO_(2),the fracture first widens and then narrows,ultimately reopening over the long term.The sequential fracture deformation behaviors are in response to the combined impacts of mechanical compression and thermally-induced deformation.By controlling the injection parameters of the scCO_(2),it is found that the heat extraction rate is positively correlated to its pore pressure or mass flow rate.The heat extraction rate can be significantly enhanced,when the inlet temperature of scCO_(2) is below its critical temperature.As a result,the heat increment recovered per unit mass of scCO_(2) decreases as the hot rock is gradually cooled.Meanwhile,the heat increment recovered per unit mass of scCO_(2) decreases by increasing the inlet temperature of scCO_(2) or its mass flow rate,but increases as the outlet pressure rises.Furthermore,multi-linear regression indicates that controlling the inlet temperature of the scCO_(2) can significantly improve the thermodynamic efficiency of heat extraction.

A simple approach for the estimation of CO_2 penetration depth into a caprock layer

Caprock is a water-saturated formation with a sufficient entry capillary pressure to prevent the upward migration of a buoyant fluid. When the entry capillary pressure of caprock is smaller than the pressure exerted by the buoyant CO2plume, CO2gradually penetrates into the caprock. The CO2penetration depth into a caprock layer can be used to measure the caprock sealing efficiency and becomes the key issue to the assessment of caprock sealing efficiency. On the other hand, our numerical simulations on a caprock layer have revealed that a square root law for time and pore pressure exists for the CO2penetration into the caprock layer. Based on this finding, this study proposes a simple approach to estimate the CO2penetration depth into a caprock layer. This simple approach is initially developed to consider the speed of CO2invading front. It explicitly expresses the penetration depth with pressuring time, pressure difference and pressure magnitude. This simple approach is then used to fit three sets of experimental data and good fittings are observed regardless of pressures, strengths of porous media, and pore fluids(water,hydrochloric acid, and carbonic acid). Finally, theoretical analyses are conducted to explore those factors affecting CO2penetration depth. The effects of capillary pressure, gas sorption induced swelling, and fluid property are then included in this simple approach. These results show that this simple approach can predict the penetration depth into a caprock layer with sufficient accuracy, even if complicated interactions in penetration process are not explicitly expressed in this simple formula.

Investigation on microscopic invasion characteristics and retention mechanism of fracturing fluid in fractured porous media

Reservoir damage caused by guar gum fracturing fluid and slick water seriously affects the subsequent oil and gas production. However, the invasion characteristics and retention mechanisms of fracturing fluids in the fracture-matrix zone are still unclear. In this work, a microscopic model reflecting the characteristics of the fracture-matrix zone was designed. Based on the microfluidic experimental method, the process of fracturing fluid invasion, flowback and retention in the fracture-matrix zone was investigated visually and characterized quantitatively. The factors and mechanisms affecting fracturing fluid retention in the fracture-matrix zone were analyzed and clarified. The results indicated that in the invasion process, the frontal swept range of slick water was larger than that of the guar gum fracturing fluid, and the oil displacement efficiency and damage rate were lower than those of the guar gum fracturing fluid under the same invasion pressure. With the increase in invasion pressure, the damage rate of slick water increased from 61.09% to 82.77%, and that of the guar gum fracturing fluid decreased from 93.45% to83.36%. Before subsequent oil production, the invaded fracturing fluid was mainly concentrated in the medium-high permeability area of the fracture-matrix zone. The main resistance of slick water was capillary force, while that of the guar fracturing fluid was mainly viscous resistance. The fracturing fluid retention was most serious in the low permeability region and the region near the end of the fracture.The experimental and numerical simulation results showed that increasing the production pressure difference could improve the velocity field distribution of the fracture-matrix zone, increase the flowback swept range and finally reduce the retention rate of the fracture fluid. The retention mechanisms of slick water in the fracture-matrix zone include emulsion retention and flow field retention, while those of the guar gum fracturing fluid include viscous retention and flow field retention. Emulsion retention is caused by capillary force and flow interception effect. Viscous retention is caused by the viscous resistance of polymer, while flow-field retention is caused by uneven distribution of flowback velocity.