Internal Flows in Geothermal Wells: Their Identification and Effect on the Wellbore Temperature and Pressure Profiles
- Malcom A. Grant (New Zealand Dept. of Scientific and Industrial Research) | Paul F. Bixley (New Zealand Ministry of Works and Development) | Ian G. Donaldson (New Zealand Dept. of Scientific and Industrial Research)
- Document ID
- Society of Petroleum Engineers
- Society of Petroleum Engineers Journal
- Publication Date
- February 1983
- Document Type
- Journal Paper
- 168 - 176
- 1983. Society of Petroleum Engineers
- 5.9.2 Geothermal Resources, 4.1.5 Processing Equipment, 5.3.2 Multiphase Flow, 1.6 Drilling Operations, 4.1.2 Separation and Treating
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Geothermal wells exhibit a variety of internal flow effects caused by the flow of water, steam, or both between distinct permeable zones tapped by the well. These internal flow effects are described and it is shown how they may be recognized from downhole pressure and temperature profiles.
Pressure transients measured at depths other than that of the well's primary permeable zone can be corrupted by such flows. The effects of such flows on injection and discharge transients are discussed.
Two types of flow can occur in wells in geothermal reservoirs: (1) interzonal flow, in which fluid enters the well at one depth, flows up or down the wellbore, and exits at a second depth, and (2) internal convection in which fluid circulates within the wellbore. The first is more common.
A geothermal wellbore is a long, vertical or nearvertical pipe penetrating a reservoir of heated fluid. Most geothermal reservoirs consist of fractured rocks, and a well draws its fluid supply from one or a few fractures (also called "permeable zones," "aquifers," "productive horizons," or "feedpoints"). Normally no attempt is made to isolate individual feedpoints from one another, so multiple feeds are often exposed to the well over a vertical distance of 3,000 to 6,000 ft (1000 to 2000 m).
Over the depth of open hole the well is exposed to the reservoir, which may contain waters of different temperatures or steam/water mixtures. In addition, the reservoir pressure distribution is not static because the natural throughflow of the reservoir causes a nonstatic distribution of fluid. In Wairakei, New Zealand, the preexploitation vertical pressure gradient was, for example, about 7% above hydrostatic (for the temperatures involved).
The very high permeabilities encountered in good geothermal wells [permeability-thickness of the order 3 to 300 darcy-ft (1 to 100 darcy.M)] mean that comparatively small pressure differences from buoyancy fects or from the nonstatic reservoir profile may cause substantial flows within a wellbore. If a well has more than one significant feedpoint, it is impossible for it to attain both thermal and pressure equilibrium with reservoir, and fluid will flow between the permeable zones. Such flow up or down the wellbore distorts temperature profiles, so measured downhole data reflect not reservoir temperatures but the physics of heat and mass transfer within the wellbore. Pressure profiles measured downhole likewise reflect not the reservoir pressure profile but the fluid column occupying wellbore.
In the absence of these strong interzonal flow effects, the fluid-filled wellbore still represents an effect means of vertical heat transport by convective circulation within it.
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