Characterization of Geothermal Feedzones and Interwell Connectivity
Abstract/Contents
- Abstract
- Interwell connectivity could be characterized using models that include various data sets to constrain the value of relevant parameters. These data sets included tracer returns, thermal drawdown, borehole image logs, and geological information. Characterization of the interwell connectivity would aid in the prevention of premature thermal breakthrough from injection wells through proper reservoir management. Relevant parameters for interwell connectivity include: channel height, channel length, channel aperture, porosity, diffusivity coefficient, equivalent injection temperature, and fracture density. Scaling correlations determined from empirical observations were used to relate channel aperture to the channel length using power law scaling. These scaling correlations could be used as additional constraints in the model definitions. Furthermore, scaling correlations could be used to infer channel length from aperture values derived from borehole imaging data. Borehole imaging data provided the channel aperture of near-wellbore fracture planes. In addition, geomechanical models were used to identify optimally oriented fractures that influenced well productivity the most. Field studies showed that both high density and high aperture values could be used to identify permeable zones. Influence of lithology on permeable zone location had likewise been observed for shallower feedzones. Aside from geophysical measurements for fracture apertures and density, analytical models for tracer transport and temperature drawdown were used to determine relevant interwell model parameters. The availability of production well temperature data in recent long-term circulation tests in EGS reservoirs and other published data made it possible to create alternative analytical models. There were two analytical models included in this study. One is a single fracture connection model. The second model consists of a sheared fracture plane or a brecciated porous channel. For the first model, an analytical model was derived to relate both the thermal breakthrough and mean tracer arrival times to the effective fracture aperture. Calculated effective apertures from this single-fracture model ranged from 2.1 cm to 42.6 cm. The second model was a porous model that used a combination of thermal and tracer response analyses to determine unknown parameters using nonlinear least squares optimization. Five main parameters for the porous channel model were: channel half-aperture (b), channel height (H), porosity (ϕ), saturated pore diffusivity (D), and equivalent injection temperature (T_inj). There were several scenarios simulated for this model. The five parameter scenario did not take scaling correlations into account. In the six parameter scenario, the half-aperture (b) was correlated to the channel length (H) using the scaling coefficient (á) and the scaling exponent or the fractal dimension (â). For four parameter scenarios, both á and â were specified using data from literature. Three scaling correlations were available. The scaling correlations were:b=0.0004L^1; b=0.0039L^1; and b=0.1689L^0.4. Overall, the coupling of thermal and tracer response analyses resulted in the detailed characterization of interwell connectivity which could be used to predict future thermal response to injection. Cooling predictions were done for high and low injection cases. Models derived from tracer analysis alone consistently gave the most pessimistic results for both high and low injection cases. The most optimistic model was the configuration with the lowest aperture. For the low injection case, the different models gave a wide range of thermal drawdown forecasts. However, at high injection cases, all the porous channel models collapsed into a narrow temperature range of predictions. It was recommended that borehole image log analysis be included in the suite of standard geophysical analyses. Uncertainty analysis could be applied to borehole imaging data so that they could be used in analytical and simulation models describing interwell connections in geothermal reservoirs. Scaling relationships could be combined with borehole image logs to create better reservoir models. In addition, further assessment of combined data analysis on the different model configurations could be explored to characterize interwell connections more effectively. More accurate downhole temperature with higher frequency is recommended to have more reliable models. Lastly, downhole pressure should be included as an additional data constraint in characterizing interwell connections.
Description
| Type of resource | text |
|---|---|
| Date created | June 2012 |
Creators/Contributors
| Author | Co, Carla Kathryn Dee |
|---|---|
| Primary advisor | Horne, Roland N. |
| Degree granting institution | Stanford University, Department of Energy Resources Engineering |
Subjects
| Subject | School of Earth Energy & Environmental Sciences |
|---|---|
| Genre | Thesis |
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Preferred citation
- Preferred Citation
- Co, Carla Kathryn Dee. (2012). Characterization of Geothermal Feedzones and Interwell Connectivity. Stanford Digital Repository. Available at: https://purl.stanford.edu/yc472qr6472
Collection
Master's Theses, Doerr School of Sustainability
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