Geothermal wells need to undergo a test program before they are used. This is so that the baseline conditions of both the wells and the geothermal aquifers that they tap can be determined. This baseline data are critical because all future information is compared against them.
After drilling has been completed and before the initial discharge, the well downhole conditions are measured. The temperature and pressure are measured by using a clockwork Kuster gauge or electronic logging tools. Standard practice is to initially do an injection (or completion) test, that is, sometime at a series of flows, where the temperature, pressure, and possibly flows using a spinner tool are
measured at intervals down the open hole section of the well. The well is then shut in and allowed to fully heat up. This can take less than 1 day (e. g., for a steam well) to more than 6 months (e. g., for a deep well with poor permeability). The downhole conditions during heat-up are also measured at intervals. The well is shut in to minimize circulation so that conditions in the cased-off section can be measured and so that the gas column can depress the liquid in the well to the upper feed zone. The downhole measurements will reveal the production zones in a well because hot geothermal fluids circulating at the production zones will cause these areas to heat up faster. The pressure profile of a well can reveal the water level in the well. The water level can also be determined by lowering a float into the well.
Once the new well is fully heated, it needs to be discharged to rid it of drilling mud, fluid, and debris before the well can be put into service. The initial discharge is normally a vertical one due to the erosive nature of the materials. The well is then tested to determine its output characteristics, that is, mass flow rate and enthalpy as a function of wellhead pressure (Fig. 10). Chemical sampling is also done during this output test. These data are very important because they comprise the baseline information.
The well output can be measured by several methods. For a small well discharge, a simple calorimeter can be used. The well output is discharged into a large vessel containing a mass of cold water. A heat balance will determine the output.
A method unique to the geothermal well test is the James lip pressure method. This method makes use of discharging two-phase fluid at choked flow conditions through an open pipe. By measuring the static ‘‘lip’’ pressure at the open end of the pipe, the flow rate can be determined from a correlating equation. The equation now is in several forms. The 1200
original equation in imperial units is G = 11400 p0′/h1102,
where G = flow in pounds/(s feet2) of the lip pressure discharge pipe cross-section area, P = lip pressure in psia, and h = fluid enthalpy in Btu/pound.
In metric units, the equation is
G = 0.184 p096/h1102,
where G = tonnes/(scm2), P = bar absolute, and h = kJ/kg.
The lip pressure method requires knowledge of the fluid enthalpy. However, the equation is relatively insensitive to enthalpy, so it can be estimated if not known. From experience, it is known that the discharge shape of the fluid at the lip pressure pipe will give some indication of the enthalpy. An estimate can also be made if the downhole conditions are known. If the fluid is known to be saturated water at a certain pressure during drilling, the enthalpy is also known. If the discharge is made into a silencer and the water flow rate is measured by a weir plate, the total mass flow can be determined. A quick estimate of the heat discharge or power potential of a well can be made by the lip pressure method during the first well vertical discharge. This number is useful for planning of the surface equipment for further tests.
A well output is most accurately measured by the separator method. The well is discharged into a separator. The steam phase is measured in a steam pipe using an orifice plate or Annubar, and the water is measured by a weir plate in a weir box at the silencer. The water flow rate needs to be corrected for that flashed in the silencer. However, this method is expensive. Production well surface equipment is normally designed to allow regular output tests with minimum disruption.
There are other well tests, such as downhole well pressure and temperature measurements, that are normally done with the well on bleed. Once a well has been initially discharged, it should not be completely shut off because a column of cold water may form in the well, preventing the well from discharging again without stimulation.
The discharge of a well may interfere with the flow of a nearby well. Interference tests can be done by either measuring the downhole pressures or measuring the discharge from a well while a nearby well is discharging at different flow rates.
After enough wells have been tested to confirm the geothermal field’s capacity and suitability for the production of electricity, steamfield design needs to be carried out. It can be carried out in parallel with
further production well drilling. This is to determine the best method for collecting the geothermal fluids from the wells and processing them, especially when the geothermal fluids are two-phase ones. In general, a geothermal steamfield consists of the following major plants and equipment: the wells, the wellhead, the flasher/separator for a wet field, the silencer, fluid transmission pipelines, and a reinjection system.