| Abstract |
In Enhanced Geothermal Systems (EGS), hydraulic stimulation is used to improve well productivity. EGS is typically performed in a nearly vertical well, in one stage, with no proppant. In the past several years, the oil and gas industry has achieved radical improvements in stimulation performance by using multiple stages, proppant, and horizontal (or deviated) wells. For the most part, these technologies have not been adopted in EGS. The EGS community is focused on the concept of "shear stimulation," injecting water to induced slip on self-propping natural fractures. As a result, proppant is viewed as being unnecessary or ineffective. The use of packers to enable multiple stages is considered technically infeasible because EGS wells are completed openhole (to maximize connectivity to natural fractures) and reliable openhole packers are not available at high temperature. In this paper, we discuss an EGS design that relies on creating new fractures, rather than stimulating natural fractures. In this design, a horizontal (or deviated) well is drilled and completed with cemented casing. Cased hole packers or bridge plugs are used for zonal isolation, allowing multiple stage fracture treatments to be pumped through perforations in the casing. Proppant is injected, possibly along with a viscosifying agent. We performed simple calculations to estimate the potential effect of multiple stages and proppant on the flow rate that can sustained through an EGS doublet. These calculations were intended to give rough estimates and provide a sensitivity analysis, not provide detailed analysis. We found that an EGS design with multiple stages and proppant should give dramatically improved economic performance relative to current designs. With enough stages, flow rate will be limited more by pressure loss in the wellbores than in the reservoir. We did not perform calculations on thermal breakthrough, but we expect that the use of multiple stages would help improve reservoir contact and prevent premature thermal breakthrough. We reviewed the literature in order to assess the technical viability of our proposed design. It was found that cased hole packers rated to geothermal temperatures are available with current technology. A review of EGS field experience shows that in the rare cases when proppants have been used, they have consistently improved well productivity, even in granite. There is some laboratory evidence to suggest proppants may chemically degrade over time at high temperature, but there is also evidence that certain coated proppants are resistant to degradation. The proposed design would increase cost but deliver radical improvements in flow rate (and revenue) per well. |