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Slim Hole Reservoir Characterization for Risk Reduction

PROCEEDINGS, 41st Workshop on Geothermal Reservoir Engineering Stanford University, Stanford, California, February 22-24, 2016 SGP-TR-209 1

Presented by DOSECC President Dennis Nielson, Ph.D., PG and DOSECC Associate Sabodh K. Garg, Ph.D.


High initial capital requirements and subsurface risk are commonly cited as significant impediments to more widespread development of hydrothermal resources. However, expensive production wells are often drilled at the early stages of development when risk is high. By drilling and testing slim holes at early stages, a developer can realize a significant reduction in risk at reasonable cost. In this paper, we will review different drilling approaches, their costs and impact on risk reduction. Slim holes have been promoted for reservoir characterization because of lower cost. However, they are also conducive to enhanced data collection that is critical for establishing conceptual models. Properly conducted injection tests provide reservoir engineering information that is equivalent to data collected from large well tests. When utilized in the reservoir testing phase, slim holes can provide greater volumetric sampling of a prospect than production-size wells at an equivalent cost. Therefore, significant risk reduction can be realized before the initiation of high-cost field development. Exploration is a knowledge-based activity and data collection and application to conceptual reservoir models is a requirement. We briefly discuss the data framework of the Play Fairway approach to demonstrate risk reduction.

slim hole drilling considerations

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Geothermal Play Fairway Analysis of the Snake River Plain, Idaho

Geothermal Play Fairway Analysis


Presented by Dennis L. Nielson, DOSECC President, at the Workshop on Geothermal Reservoir Engineering Stanford University, Stanford, California, January 26-28, 2015


Play Fairway Analysis is a systematic approach to exploration that integrates data at the regional or basin scale in order to define exploration targets (plays), and then interrogates these data to highlight plays that have the highest likelihood of success. Play Fairway Analysis provides greater technical rigor than traditional geothermal exploration approaches, and facilitates quantification of play risks even when data are sparse or incomplete. It is a mature practice in petroleum, but represents a new approach for geothermal that we believe will aid in the discovery of buried or blind systems. A key challenge will be adapting fairway analysis to geothermal exploration in a way that provides both meaningful results and measurable return on investment. In this project, we focus on the Snake River Plain where, during Project HOTSPOT, our team discovered a blind hydrothermal system at Mountain Home Air Force Base in Idaho. From that discovery we are able to define key parameters that characterize the elements necessary for a geothermal reservoir based on basaltic (plume-related) magmatism, fracturing that defines a reservoir volume, seals that are provided by lake beds, hyaloclastics, and highly altered clay-rich basalts, and fluid recharge that is controlled by faulting and the primary permeability of basalt flows. Project Hotspot identified three different play types in the SRP (a) high thermal gradients along the volcanic axis beneath the SRP aquifer, (b) extremely large low temperature systems, and (c) blind high-temperature systems like that discovered at Mountain Home. Phase 1 of this project will assess the distribution and viability of these plays throughout the SRP region; Phase 2 will focus on detailed analyses of specific plays as we move from a Regional/Basin focus to a Play/Prospect focus. Our approach is to analyze direct and indirect methodologies for identifying critical reservoir parameters: heat source, reservoir permeability, seal and recharge.

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