FLOW TESTING
INITIATION OF A FLOW FROM HIGH TEMPERATURE WELLS
If water level reaches ground surface during recovery after drilling, production can be initiated by opening the well. This can also be done if the well can develop a geo/steam pressure on top.
-Keep the well closed for some days and open it when a well head pressure of 5 bar or more has developed. START PRODUCTION SLOWLY.
If water level doesn’t reach to the surface, and no pressure develops on top, the well has to be stimulated to flow.
- Swabbing (piston): Swabbing is a desperate method in which a drill rig operates a plunger to physically lift water out of the wellbore. If the method is successful in starting discharge, the swabbing equipment must be removed with the well under pressure, a particularly difficult and dangerous operation.
- Airlifting: As with any well containing water, flow can be obtained in geothermal by wells by airlifting. In this method tubing is inserted into the well to an appropriate depth and a gas lift is started by pumping air down the tubing. A rig normally is required to insert and then to remove the tubing once the well has commenced flowing. For shallow airlifts flexible rubber tubing may be used. This can be rapidly recovered from the well while the flow is small.
- Pressurizing with air: This method is used where the water surface can be depressed to a level such that the BPD curve from the depressed water level intersects the stable downhole temperature profile.
-Keep a critical pressure for some time to allow downhole temperatures to recover.
-Suddenly, open the wellhead control valves to permit discharge of compressed air, followed by boiling geothermal fluid.
Figure 1 shows the mechanism responsible.
ABC: Stable shut-in pressure with corresponding shut-in temperature.
XZ: Boiling point curve (BPD) corresponding to profile ABC.
DBC: Pressure profile after depression. Here the liquid column has been pushed down the well and out into the formation at the feed point.
B’C’: Created pressure profile when the air pressure is released instantaneously (B is reduced to atmospheric pressure).
X’Y’Z’: Boiling point curve corresponding to the pressure profile B’C’.
As it is seen, temperatures exceed boiling over the interval Y’Z’, therefore the fluid in this segment will boil. If the boiling segment long enough, the overlying fluid will be lifted and discharge will commence.
Figure 1. Profiles in a well pressurized with air.
Warning: The start of production from a high temperature well is followed by a heating period during which the temperature (and pressure) at the wellhead and along the length of the well will increase. If casings are heated suddenly, the sudden expansion of the steel can lead to casing collapses.
DISCHARGE MEASUREMENTS OF GEOTHERMAL WELLS
The type and size of equipment needed for a flow test depends on the type of well and reservoir being tested and on the expected maximum flow rate and discharge enthalpy.
Since the approach taken will depend on the fluid at wellhead, we will discuss them separately.
Low enthalpy (liquid only) wells
Some wells tapping low temperature geothermal reservoirs will produce and discharge liquid water. The flow characteristics of these wells can be simply deduced by recording the wellhead temperature and pressure, and the mass flow. The mass flow is most conveniently measured by flashing the water to atmospheric pressure and measuring the separated water flow over a sharp-edged weir (Figure 2).
Weirs are typically installed in open channels such as streams to determine discharge (flow rate). The basic principle is that discharge is directly related to the water depth above the crotch (bottom) of the V; this distance is called head (h). The V-notch design causes small changes in discharge to have a large change in depth allowing more accurate head measurement than with a rectangular weir.
V-notch weir equations have become somewhat standardized. ISO (1980), ASTM (1993), and USBR (1997) all suggest using the Kindsvater-Shen equation, which is presented below from USBR (1997) for Q in cfs and heights in ft units. All of the references show similar curves for C and k vs. angle, but none of them provide equations for the curves. To produce automated calculations, LMNO Engineering used a curve fitting program to obtain the equations which best fit the C and k curves. Our equations are shown below. The graph shown is from our fits. If you compare it to the graphs shown in the references, it looks nearly identical which implies that our fits are very good.
C = 0.607165052 - 0.000874466963 Ø + 6.10393334x10-6 Ø2
k (ft.) = 0.0144902648 - 0.00033955535 Ø + 3.29819003x10-6 Ø2 - 1.06215442x10-8 Ø3
where Ø is the notch angle in degrees
Figure 2. Schematic of a triangular V-notch weir
(source:
References
ASTM (1993). American Society for Testing and Materials. ASTM D5242. Standard method for open-channel flow measurement of water with thin-plate weirs. 1993. Available from Global Engineering Documents at
ISO (1980). International Organization of Standards. ISO 1438/1-1980(E). Water flow measurement in open channels using weirs and venturi flumes - Part 1: Thin plate weirs. 1980. Available from Global Engineering Documents at
USBR (1997).U.S. Department of the Interior, Bureau of Reclamation. Water Measurement Manual. 1997. 3ed. Available from
The discharge enthalpy is obtained from the wellhead pressure and temperature using steam tables. Since a well of this form obviously produces from a single-phase reservoir, the enthalpy will normally remain constant with varying flow rate. Therefore, although it may be possible to have liquid at the wellhead only if the well is throttled to low flow, the same flow enthalpy can be assumed at high flows where flashing may be occurring in the wellbore.
Medium enthalpy wells
This is the most common case. Under natural discharge, the well produces a steam-water mixture at wellhead, at all flow rates. Such discharges are produced from reservoirs containing high temperature liquid water or two-phase fluid.
Measuring the flow rate of a two-phase mixture in a pipe is a difficult problem. The most reliable method for measuring of a two-phase mixture is to use a separator, which separates the steam-water flow into a flow of water and a flow of steam at the pressure of the separator. Figure 3 shows a typical separator used in well testing.
Figure 3. Separator as used in flow testing.
The flow rate of each separated phase can be measured individually using an orifice plate and differential pressure sensor. An orifice meter is an instrument that records the flow rate of gas through a pipeline. The flow rate is calculated from the pressure differential created by the fluid passing through an orifice of a particular size and other parameters such as static pressure, temperature, density of the fluid and size of the pipe (Figure 4).
Figure 4. Orifice meter
The flow rate (kg/s) through an orifice is given by
Where C is the orifice constant, P the differential pressure, and v is the specific volume of the fluid.
Alternatively, the water phase can be flashed to atmospheric pressure and the water flow measured over a sharp-edged weir.
Lip Pressure Method: This method is based on an empirical equation developed by James (1970) and is considered to be the most versatile method for testing all medium enthalpy wells. The lip pressure method is not quite as accurate as the separator method but is desirable because a minimum of hardware and instrumentation is required to obtain good results. The largest producing can not be satisfactorily tested by any other method.
It has been found that if fairly large flow of steam (or steam/water mixture) is expanded along a pipe to the atmosphere, then there is at the extreme end of the pipe a positive pressure above atmospheric which is proportional to the mass flowing and enthalpy.
To use this method, the steam-water mixture is discharged through an appropriately sized pipe into a silencer or some other device to separate the steam and water phases at atmospheric pressure (Figure 5). The lip pressure is measured at the extreme end of the discharge pipe using a liquid-filled gauge to damp out pressure fluctuations. Water flow from the silencer is measured using a sharp-edged weir near the silencer outlet.
Figure 5. Flow measurement by lip pressure and silencer.
James’s formula, which was tested over an enthalpy range of 400 – 2800 kJ/kg, relates mass flow, discharge pipe area, enthalpy, and lip pressure:
Where Plip is the lip pressure measured in MPa, D is the total flow rate in kg/s, A is the lip pressure pipe area in cm2, and Ht is the enthalpy in kJ/kg.
The total mass flow rate obtained from Lip pressure measurement can be related to water flow rate measured at the silencer after flashing;
Where W is the water flow rate measured at the silencer after flashing in kg/s, Hs and Hw are the saturated steam and saturated water enthalpies at the pressure of silencer measurement in kJ/kg. If the water measurement is made at atmospheric pressure,
Hs = 2676 kJ/kg and HL = 420 kJ/kg, therefore,
If we combine the totla mass flow rate equations from lip pressure measurement and water mass flow rate, we obtain
The above equation indicates that, James’s method includes of measuring the lip pressure and water flow rate from silencer, finding the enthalpy.
An application of James’s method is given in the following example.
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