| Keywords |
fluid-induced, fluid injection, depletion, geothermal reservoir, pore pressure, effective stress, poroelasticity, stress tensor, stress path, stimulation, consolidation, hydraulic diffusivity |
| Abstract |
At the latest since Segall (1989), Teufel et al. (1991), poroelasticity can explain, so to say despite Terzaghi, how fluid depletion destabilizes reservoirs either at their flanks or at their top and bottom, depending on the (extensional or compressional) tectonic regime. By the same token, poroelasticity can alleviate some undesired effects of pore pressure buildup by fluid injection into deep reservoirs, yet it can also counteract desired stimulation effects for the latter. In geothermal reservoirs subject to balanced fluid turnover by simultaneous injection and production from multiple wells, things are less clear-cut (cf. Segall and Fitzgerald 1998). The joint outcome of poroelastic effects and of pore pressure directly on effective stresses will strongly depend on geometry details of multi-well configurations as well as on geological heterogeneity. A scoping simulation example of a hydrothermal reservoir operated by means of a well triplet (one producer, two re-injectors) is presented, to which by placing a fourth well (second producer) within a certain reservoir cone, self-stimulating effects will be induced, by virtue of competing pressure diffusion and poroelasticity, within a certain radius around each producer well; the stimulation radius appears to be larger at early operation stages, yet may persist at significant (few MPa) levels even after decades of fluid turnover. Such producer-centered stimulation effects are found to occur only for the quadruplet, and not for the triplet configuration. At the real-world site underlying this simulation example, adding a fourth well (second producer) is being endeavored in order to maximize the benefit from surprisingly high injectivity at the already existing two injectors, whereas the modest productivity of the existing producer acted as the turnover-limiting factor in the currently operating triplet. Up-sizing to a quadruplet configuration (two producers instead of one) might thus also, by virtue of competing pressure diffusion and poroelastic effects, improve the productivity of the first producer, so to say as an added bonus against the drilling costs of up-sizing. In the currently operating triplet regime, injectivity further appears to increase with operation time, i. e., with the cumulative volume of fluid turnover, this being attributed to thermal-hydrogeochemical rather than hydraulic-poroelastic effects. Within the ongoing study, hydraulic and poroelastic evaluations are to be complemented by a comparison of fluid residence times along with thermal lifetime expectations between triplet and quadruplet settings, revealing further benefits of the latter, which pay up for the additional drilling costs. |