Drilling cost and risk is the greatest impediment to global geothermal development. In the early 1990s, the use of lower cost slim holes was introduced for geothermal exploration. Although the industry was slow to adopt this method, slim holes are now commonly drilled and tested to evaluate geothermal resource potential. With the advancement of novel drilling techniques and miniaturized instrumentation, microbore exploration wells can reduce drilling cost and risk in EGS and conventional geothermal development.
Of critical importance in the use of a surrogate slim hole or microbore to assess resource capability is the assumption that test results can be accurately scaled to larger, more expensive production bores to be completed after successful discovery of a resource. The accuracy of this scaling varies with test bore diameter, resource conditions and the degree of scale-up to larger bores. Geothermal exploration wells are typically evaluated by discharging the well to surface equipment at atmospheric pressure to measure flow rate, enthalpy, and fluid composition. Reservoir characteristics are further evaluated by conducting injection tests, step-rate production tests, and pressure recovery measurements. However, low temperature resources or small diameter bores are often incapable of continuous, unassisted flow. In such cases, flow to the surface can sometimes be induced, or temporarily maintained, by air- or nitrogen-lift, or pumping, but these methods add significantly to the cost and complexity of the test operation. In addition, atmospheric flow tests require relatively large liquid storage facilities (sumps or tanks) or a nearby injection well, and test duration may be limited due to steam and gas emission considerations, hazardous liquid composition, or water disposal restrictions.
Using innovative test methods, slim hole and microbore resource evaluation can be completed using a drill stem test. This method eliminates errors associated with surface flow tests, and requires substantially less infrastructure and reduce the time required for resource evaluation.