Title:

Improved Test Method for Slim Hole and Microbore Exploration Drilling

Authors:

Dennis KASPEREIT, William L. OSBORN

Key Words:

microbore, drill stem test, slim hole, well test, flow capacity, productivity index, wellbore simulation

Conference:

Stanford Geothermal Workshop

Year:

2017

Session:

Emerging Technology

Language:

English

Paper Number:

Kaspereit

File Size:

1456 KB

View File:

Abstract:

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 miniaturized instruments and other components, microbore exploration wells can reduce drilling cost and risk. Of critical importance in the use of a surrogate slim hole or microbore 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 bore diameter, resource conditions and the amount of scale up to larger sizes desired. 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 may be limited due to steam and gas emission considerations, hazardous liquid composition, or water disposal restrictions. Using innovative test methods, microbore resource evaluation can be completed using injection, drill stem tests, and in situ chemical analysis. These methods require substantially less infrastructure and reduce the time required for resource evaluation.


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