Optimization of well configuration for a sedimentary enhanced geothermal reservoir.

摘要

The extraction of geothermal energy in the form of hot water from sedimentary rock formations could expand the current geothermal energy resources toward new regions. In sedimentary geothermal reservoirs with low permeability, enhancement techniques (e.g., well hydraulic stimulation) are needed to improve the hydraulic connection between the wells to achieve commercial production/injection rates (Cho et al., 2015). The design of the well configuration was investigated in this research work. The performance of the sedimentary geothermal reservoir was evaluated using numerical reservoir modeling, which allowed studying different well configurations and productivity enhancement techniques. This research develops a methodology to maximize thermal recovery from the geothermal system. The objective is to maximize the well productivity/injectivity index by systematically changing the value of influencing variables of the system, subject to a constraint of thermal breakthrough time greater than or equal to 30 years (i.e., time required by a commercial successful project).;The reservoir simulation cases are classified into four main models based on well configuration, shown in Figure 3.3: one vertical well doublet system with hydraulic fractures, and three horizontal well configurations with open-hole completion, longitudinal fractures and transverse fractures, respectively. Since permeability is the fundamental property which decides the necessity of enhancement techniques, the proper permeability estimation is selected first.;Based on that and sensitivity analysis of the variables, including well type (vertical or horizontal), well spacing, length of horizontal section, hydraulic fracture direction, fracture half-length, fracture spacing, dimensionless fracture conductivity and ratio of reservoir vertical permeability to horizontal permeability, the most efficient well configuration and the corresponding influential variables are determined. The application of this methodology provides the procedure to identify the required hydraulic fracture parameters for each well configuration that maximizes the geothermal energy recovery.;The final step is to identify the optimization of well configuration. Surface response models based on the multiple linear regression method are built to analyze the change of reservoir thermal evolution and flow behavior in terms of influential parameters in well configuration. A horizontal well system with longitudinal fractures is the most favorable in pursuing a long thermal breakthrough time. A horizontal well system with multi-stage transverse fractures is most efficient to increase the productivity index and injectivity index. For optimal well configuration designs in both well configurations, the well spacing is assigned to the minimum value, and the well horizontal length can be chosen as long as reservoir and technique permits. This design achieves minimum the thermal breakthrough time requirement and maximum productivity and injectivity indices.