Modeling of heat flow and properties related to geothermal exploration often overlook the geological heterogeneity of the target units and therefore under or overestimate the quality of the reservoir for such purpose(s). In this project, we perform a detailed petrophysical analysis of 25 wells in the Texas Gulf Coast to obtain properties such as porosity, permeability and water saturation. Based on these rock and fluid properties, we selected vertical stratigraphic target zones that were upscaled for modelling using sequential gaussian simulation. Particular detail was paid in the upscaling integration to represent the vertical and spatial heterogeneity of the reservoir zones. The output models were used as input for the heat flow simulations and enthalpy production evaluation, which resulted in more realistic scenarios compared to the "layer cake" models commonly used in geothermal simulations.
The reservoir was built in a grid that covers an area of 113.8 sq-km (1.225 E6 sq-ft) and a depth range of 730 ft. Initial work included calculation of water saturation and permeability using the suite of logs available (either triple or quad combo, ie. Gamma Ray (GR), Resistivity (RDEEP), Photoelectric factor (PE), Bulk Density (RHOB), Neutron Porosity (NPHI) and Compressional Monopole Sonic (DT)), as well as 2d correlations (fence diagrams). To better represent the petrophysical properties, we used a relatively fine grid that covered the area previously described with a total of 242176 cells (44(i) x 43(j) x 128(k)). For petrophysical property models, we focused on the 4 subintervals of the Frio Formation that were selected based on their optimal reservoir quality (porosity, permeability, and water saturation). The connecting zones between intervals of interest were modeled as well (in less detail) to assess the heat flow vertically among targets. The properties were initially upscaled using arithmetic averaging and then modeled in 3 dimensions using sequential gaussian simulation.
The 3D porosity and permeability models provided an understanding of the reservoir quality along the area of interest. Analogously to oil and gas reservoir characterization, porosity and permeability are key controls on geothermal exploration; and an accurate representation is required to understand not only target zones but potential risks. According to a sensitivity analysis, permeability and porosity affect the heating extraction rate. The site and configuration of the wells also impact energy production. Finally, we conclude that heat extraction for power generating projects in the studied area is technically viable.
Exploring geothermal resources in sedimentary basins opens a new opportunity to expand geothermal energy production to regions that currently do not have access to that renewable energy. Sedimentary environments benefit from the enormous amount of existing data and the significantly decreased exploration costs compared with the cost of drilling and evaluating traditional igneous and metamorphic-originated geothermal environments. This project shows the importance of conducting an integrated assessment of both engineering and geological properties to identify the quality of the reservoirs from the geothermal standpoint.
