Nonstatic Reservoirs: The Natural State of the Geothermal Reservoir

Abstract

Summary Above-hydrostatic-reservoir-pressure gradients are found in many geothermal fields before exploitation. In higher-temperature fields, steam and water also coexist in the upper levels. These factors indicate natural upflow in these reservoirs-i.e. the dynamic state of the fluid. Analysis of flow and pressure data under these dynamic conditions can lead to relevant large-scale reservoir information. Introduction It has been suggested that the existence of a natural surface discharge from an oil reservoir must imply a nonstatic pressure distribution down through the fluid in that reservoir. To date, however, no such nonstatic pressure distribution has been identified in any oil reservoir. In contrast, most liquid-dominated geothermal reservoirs -- i.e., both hot water and two-phase -- have significant natural throughflow, and hence the nonstatic pressure distribution has been sought with interest. Until recently, the data supporting the enhanced pressure gradient have been sparse because wells commonly have been discharged, and the reservoir disturbed, before a suitable spectrum of data has been obtained. The mere presence of a significant length of open hole, as is commonly the case in geothermal exploitation, also distorts the pressure pattern. Study and reinterpretation of early (1950–56) Wairakei data have led to a much better picture of the natural pressure/depth profile for that field. In this paper we use this new information with other information about the reservoir in its natural state to determine an average (field) vertical permeability and the depth to which boiling may have occurred. Because the horizontal permeability of the flow strata is usually at least an order of magnitude higher than the mean vertical component (which includes the effects of any lower-permeability cross strata), it is horizontal permeability that controls the pressure transients of pump and injection tests. The vertical component is, however, important once water begins to drain down to the well feed level as the field pressure declines, and it has a strong influence over long-term behavior. Although we now can distinguish between liquid/water and steam/water (two-phase) conditions around a well, we are not yet able to pick up the phase-change boundary unless it is close. Even in long-term field tests, the horizontal/vertical permeability contrast may result in the pressure transients sensing the side boundaries before they show any effect of this horizontal one. Since extraction and injection from and into a two-phase section of a reservoir result in different effects from extraction and injection from and into a liquid-charged section, a pre knowledge of the depth of the two-phase zone could result in some savings in both drilling effort and costs. The Geothermal Reservoir It first was suggested at the beginning of the century that hot spring water might be meteoric water that entered the ground some distance away and circulated deeply enough to pick up the required heat. JPT P. 189^