| Title | Fracture Conductivity of a Bauxite-Propped Geothermal System |
|---|---|
| Authors | Stoddard, Trevor; McLennan, John; Moore, Joseph; Wagner, Dave |
| Year | 2010 |
| Conference | Geothermal Resources Council Transactions |
| Keywords | EGS; Proppant; Bauxite; Geothermal; Fractures |
| Abstract | Fracture conductivity is one of the most important, and possibly least appreciated, properties of geothermal systems. One of the areas of research in enhanced geothermal systems 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 the 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 was to investigate the effect of proppant on fracture stability and the proppant-fluid interactions at conditions seen within EGS reservoirs. Baseline tests at ambient temperature were completed for comparison to all subsequent results. Future and ongoing testing is being conducted at 200°C, to simulate conditions at in-situ conditions representative of a moderate temperature scenario. In addition to chemical interaction (dissolution and precipitation) between the proppant and geothermal fluid, it is necessary to evaluate how the proppant performed in the fracture in terms of embedment and crushing. Proppant was placed in a surrogate fracture, temperature applied and hydraulic confining pressure was also applied to provide a normal closing 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. 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. Under realistic in-situ effective confining pressure, some limited embedment and proppant breakage has been observed during post-mortem examinations with an optical microscope. Differential pressures through the fractures were measured at various flow rates and conductivity and permeability were calculated. Proppant permeability and fracture conductivity were then compared to literature values for the proppant used (30/60 commercial bauxite proppant). The completed base-line tests gave a good indication as to how the bauxite proppant will perform under stress. While the bauxite generally maintained permeability and conductivity through the fracture with limited crushing, tests at temperature are being carried out to fully assess the performance of the proppant under more severe service conditions. |