| Abstract |
Hydrothermal wells could greatly benefit from stimulation treatments that can be directed. This might allow them to target specific areas of a reservoir (such as known highly-permeable fracture zones), avoid faults, or prevent overlap with other wells – maximizing reservoir usage. In this work it is shown how stimulation treatments could be directed by modifying the reservoir stresses. The proposed methodology has been investigated numerically, using a 2-D, plane strain, sequentially coupled fully implicit flow and linear elastic mechanical simulator. The methodology is based around the idea that stresses in the subsurface can be altered through fluid production/injection activities (e.g. Segall 1989). To begin, one well is stimulated as usual - in this case in a reverse faulting stress regime. Then, the second well is drilled into the stimulated region of the first well. At this stage, both the first and second wells are produced. The advantage of this is that it alters the stresses outside of the stimulated region of these two wells. Specifically, the horizontal stress will decrease and the vertical stress will slightly increase in regions at the same depth but outside of the already stimulated area. As this is a reverse faulting stress regime, this makes shear failure less likely. Next, a third well is stimulated in this stress-altered zone. This stimulation treatment will be influenced by the altered stress field such that it is directed away from the first two wells. The stress changes caused by the preconditioning production phase are relatively small (on the order of 0.5 MPa in the region between the wells) and therefore do not affect the shear failure occurring near this third well during stimulation. However, these stress changes become significant at distances farther away from the stimulating well as the pore pressure changes at larger distances from the stimulating well are smaller. This then inhibits shear failure in the regions closest to produced area, resulting in a stimulation treatment which preferentially stimulates areas away from the first two wells. While the results presented here were for reverse faulting stress regimes, the methodology works similarly for strike-slip faulting stress regimes. One potential disadvantage of the extra production phase is that it induces stress changes above and below the producing wells. Specifically, it induces compressive horizontal stress changes and tensile vertical stress changes. As this is a reverse faulting stress regime, these stress changes can lead to shear failure (similar to the production-induced seismicity reported by Segall 1989). However, it should be noted that the production rates used to produce these wells during the preconditioning phase are significantly less than rates which are considered economical for, for example, EGS wells (Ziagos et al., 2013). This implies that, were this shear failure to be seen during the preconditioning phase, it also would have been seen during the normal field life of the producing well of the doublet pair under normal circumstances. REFERENCES Segall, P.: Earthquakes triggered by fluid extraction, Geology, 17, (1989), 942-946. Ziagos, J., Phillips, B., Boyd, L., Jelacic, A., Stillman, G., and Hass, E.: A technology roadmap for strategic development of enhanced geothermal systems, Proceedings, Thirty-Eighth Workshop on Geothermal Reservoir Engineering, Stanford University, Stanford, CA (2013). |