Improved Energy Recovery From Geothermal Reservoirs
Authors: Bodvarsson Gudmundur S. , Pruess Karsten , Lippmann Marcelo , Bjornsson Sveinbjorn 
Published: December 2007

Numerical stimulation methods are used to study how the exploitation of different horizons affects the behavior of a liquid-dominated geothermal reservoir. Our reservoir model is a schematic representation of the Olkaria field in Kenya. The model consists of a two-phase vapor-dominated zone overlying the main liquid-dominated reservoir. Four different cases were studied, with fluid produced from:the vapor zone only.the liquid zone only,both zones, andboth zones but with lower values for vertical permeability and porosity assumed.

The results indicate that production from the shallow two-phase zone, although resulting in higher enthalpy fluids, may not be advantageous in the long run. Shallow production gives rise to a rather localized depletion of the reservoir, whereas production from deeper horizons may yield a more uniform depletion process if vertical permeability is sufficiently large. The exploitation from deeper zones causes boiling and subsequent upflow of steam that condenses at shallow depths. This tends to make temperatures and pressures more uniform throughout the reservoir, resulting in maximum energy recovery.Brief consideration is given to the possibility of achieving similar improvements in energy recovery from vapor-dominated reservoirs through production from deeper horizons. This appears unlikely but cannot be completely ruled out because of uncertainties about the characteristics of vapor-dominated systems at depth.

Introduction

In the development of a geothermal resource, an appropriate production strategy must be selected. This ineludes deciding on the optimal well spacing and depths of completion. In many cases, these decisions are based solely on achievable levels of power production and well completion costs. without due consideration being given to ultimate energy recovery. Thus. the production wells often are located very close to each other, and drilling is terminated as soon as sufficiently hot and productive horizons are penetrated. Completing production wells near the top of the reservoir may lead to a more rapid decline and a low recovery ratio for the geothermal resource, Selection of an exploitation strategy should be based on appropriate reservoir engineering calculations that will result in an optimal balance between energy recovery and investment costs. Calculations aiming at an optimization of field development were carried out by Morris and Campbell for the East Mesa geothermal field in the Imperial Valley, CA.The economic value of a geothermal well depends not only on its deliverability, as in he case of an oil well, but also on the enthalpy of the produced fluids. If there are two-phase or vapor zones in the field (e.g., Baca, U.S.; Olkaria, Kenya; Broadlands, New Zealand), there is an incentive to produce from them rather than from deeper liquid reservoirs, because fluids of higher enthalpy can be obtained. The short-term benefits are obvious, but in the long run a lower-energy recovery ratio from the field may result.In this work, the behavior of a liquid-dominated geothermal reservoir in response to production from different horizons is studied by use of numerical simulation methods. We use the Olkaria geothermal field in Kenya as an example of a two-phase vapor-dominated zone overlying the main liquid-dominated reservoir. One of the important questions in the Olkaria field development is which zone it is most beneficial to produce. This paper is the first attempt to answer that question. We also briefly consider the possibility of improving energy recovery from vapor-dominated reservoirs by tapping deeper horizons.

JPT

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