| Title | Fracture Conductivity of a Bauxite-Propped Geothermal System at In-Situ Conditions |
|---|---|
| Authors | Trevor Stoddard, John McLennan, Joseph Moore |
| Year | 2011 |
| Conference | Stanford Geothermal Workshop |
| Keywords | EGS, proppant, Bauxite, geothermal, fractures |
| Abstract | Fracture conductivity is one of the most important properties of geothermal systems - conventional or enhanced. One of the areas of research in enhanced geothermal systems (EGS) involves using proppant to increase retained fracture conductivity - retained and maintained after hydraulic fracturing. Proppant is natural or manmade particulates that are injected concurrently with hydraulic fracturing fluids while creating an EGS fracture system. After fracturing operations the proppant remains in the fractures and “props” them open to ensure retained conductive pathways that would otherwise close under prevailing in-situ stresses. The primary purpose of this study is to investigate the effect of proppant on fracture stability and the effect of temperature on fracture conductivity in both a saw cut (nominally smooth, low friction) and wedge-split (higher friction, fractured) case. Proppant was placed in a surrogate fracture, temperature applied and hydraulic confining pressure was also applied to provide a normal stress acting to close the fracture. Pressure drop was measured for flow of water through this stressed and propped fracture allowing calculation of conductivity with time and inference of degradation via mechanical affects (such as embedment or chemomechanical alteration of the fracture surface). In addition to baseline tests that were run at ambient temperature, a range of tests at temperatures of 90°C, 150°C and 200°C have been run to determine the effect of increase in temperature on fracture conductivity and permeability. Even at a moderate temperature of 90°C, the conductivity is greatly reduced by this increase in temperature from ambient (~22°C). Baseline tests at ambient temperature were completed for comparison to all subsequent results and to determine the effect of increasing temperature on fracture conductivity. Future and ongoing testing is being conducted at a temperature of 200°C to simulate in-situ conditions representative of a moderate temperature scenario within a geothermal reservoir. To determine the effect of fracture roughness on conductivity, testing was done on both a nominally smooth, saw-cut fracture in a granitic sample and a rough-faced sample that was fractured with a mechanical wedge. Results have shown that temperature may have an impact on lowering the permeability and conductivity through the proppant pack. It was also found that surface asperities have little to do when there is a maximum concentration of proppant filling the fracture. |