PNGE 43 WVU Science Permeability Measurement Using Gas Laboratory Report

Laboratory Experiment No. 4Permeability Measurement Using Gas
Objective:
To determine absolute permeability using air as a flowing medium.
Theory:
Porosity has been shown to be the capacity of a rock to hold a fluid. Once this has been
determined, it is desirable to know what capacity the rock has to transmit the fluid. This is the
permeability. Permeability is measured in Darcys after Henry D’Arcy, who in 1856 first studied
the flow of fluids through a porous media. His experiments were confined to sand packs which
were 100% saturated with water. The relationship to flow that he determined is given below:
Q = kA
h1 − h 2
µL
k=
or
Q µL
A (h1 − h 2 )
where:
Q = Volumetric flow rate in cc’s per sec.
k = Constant of proportionality, or permeability.
A = Cross sectional area in cm2.
L = Length of the porous medium in cm.
h1 – h2 = Difference in head between upstream and downstream points, or the pressure
differential.
μ = Viscosity of the fluid in centipoise.
A porous medium has a permeability of 1 Darcy when a pressure differential of one
atmosphere causes a flow of 1 cc/sec of fluid of 1 cp viscosity to move through a 1 cm cube of
porous material, the flow being viscous.
The above equation describes incompressible flow and is used when fluid is the flowing
medium. However, if gas is the flowing medium, the above equation can not be used as gas is
compressible. In this instance, Q must be measured at the average pressure existing during flow.
The gas volume measured at the existing pressure at the downstream side of the porous medium
must be corrected to the volume corresponding to the average pressure during flow by an
application of Boyle’s Law. If P1 and P2 represent pressures at the upstream and downstream
sides respectively, then
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Q 2 P2 = Q m
P1 + P2
2
where:
Qm = Volume rate of flow under average pressure.
Q2 = Volume measured at P2.
The permeability equation becomes:
k=
Q m µL
A (h1 − h 2 )
This equation may be used for linear flow of either compressible or non-compressible
fluids moving through the porous medium.
For radial flow, the equation can be shown to take the following form for noncompressible fluids:
 R2 

 R1 
k=
2h(h1 − h2 )
µQ ln
and for compressible fluids
 R2 

 R1 
k=
2h(h1 − h2 )
µQm ln
where:
R1 = inside diameter of a hollow cylinder in cm.
R2 = outside radius of cylinder in cm.
h = length of cylinder in cm.
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All other symbols are as previously defined. The difference, h1 – h2, is also the pressure
differential dP and can be substituted into the formulas.
These equations can be used to describe the radial flow of fluids into a well bore from the
surrounding reservoir. For this, R1 represents the radius of the well bore and R2 represents the
radius of the reservoir. The formation (net pay) thickness is h.
Gas Permeameter
The gas permeameter is an instrument used to measure the permeability of consolidated
core sections. This is achieved by forcing a gas or liquid of known viscosity through a core of
known cross sectional area and length. Pressure and temperature are measured at different flow
rates and Darcy’s law is then used to determine K.
Use core samples that were cut during the first week of the semester for this experiment.
Be sure that the core was cut parallel to the bedding since this is the direction of maximum flow
in a reservoir rock.
The permeability of the core is calculated with the following formula:
kg

CQorifice hw L
200 A
where
kg
C
Qorifice
hw
L
A
= Permeability to gas, millidarcies
= C gauge value
= Orifice constant, cc/sec
= Orifice manometer reading, mm
= Length of core, cm.
= Cross sectional area of the core, cm2
Repeat the experiment for at least 3 different gas pressures and find the absolute permeability using
Klinkenberg equation:
𝑏
𝑘𝑔 = 𝑘𝑎 (1 + )
𝑝̅
Precautions to be observed at all times:
1. KEEP Orifice Vent valve open except for readings
2. DO NOT pressure the Hassler core holder unless there is a core properly placed in it.
3. DO NOT open the Middle Water valve unless the Mercury Regulator is adjusted to a
“C” value greater than 700 on the Mercury Manometer.
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Procedure to prepare the core for testing:
1. Close “Hassler” and “Exhaust (Vent)” valves.
2. Open “Vacuum” valve.
3. Loosen the finger-wheel nuts and lower the yoke at the upstream end (bottom part) of
the core holder.
4. Insert the core sample into the sleeve and push gently followed by the upstream head
until the core is held against the downstream end.
5. Close “Vacuum” valve.
6. While holding the yoke, open the “Exhaust (Vent)” valve to permit the release of
vacuum and thus allowing the rubber gasket to grasp the core in place.
7. Press the yoke upward and hand tighten the finger wheel nut.
8. Close the “Exhaust (Vent)” valve.
9. Open the “Source” valve located on the left side panel.
10. SLOWLY open the “Hassler” valve and apply 200 psig or slightly higher pressure
against the sleeve.
11. Close the “Hassler” valve and make sure the pressure is not leaking.
12. Now the core is ready for the experiment.
Procedure to measure core permeability:
1. Make sure the “Source”, “Sample”, and “Middle Water Manometer” valves are closed.
2. Check to make sure the “Permeameter Regulator” is off by backing off (rotate counter
clockwise to turn it off.) If necessary adjust “Permeameter Regulator” by baking off
(rotate counter clockwise).
3. Check to make sure the “Mercury Regulator” is off by baking off (rotate counter
clockwise). If necessary adjust Mercury Regulator by baking off (rotate counter
clockwise)
4. Turn valve selection to the “High Pressure (Low Permeability)” position. (This closes
the system to “Low Pressure (High Permeability)”)
5. Open the “Orifice Vent” valve located at the right side of the panel.
6. Open the “Source” valve located on the left side panel.
7. Open the “Sample” valve.
8. Using the “Permeameter Regulator” adjust the “C” gauge to a reading of 10 (if flow is
too high for a reading with the water orifice manometer, reduce “C” gauge to 20 or 30).
9. Select an orifice with the proper Orifice Q value and attach it the Orifice Outlet located
at the right side of the panel with a short section of rubber tubing.
a.
Initially select an orifice with a high Orifice Q value. The high Orifice Q value
works for high permeability cores.
b.
If the core has a low permeability use an orifice with a small Orifice Q value.
c.
As a rule of thumb use the following criteria:
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i.
10.
11.
12.
13.
Use an Orifice Q value of 10 or higher for cores with permeability values
higher than 100 md
ii. Use an Orifice Q value between 0.1 and 10 for cores with permeability
values between 1 and 100 md
iii. Use an Orifice Q value of 0.1 or less for cores with permeability values
lower than 1 md
SLOWLY start closing the “Orifice Vent” valve while observing the water level in the
“Orifice Manometer.” If the gas flow rate is too high the water will be expelled from the
manometer. Make sure the water is not expelled from the manometer.
If the gas flow rate is too high proceed to next step (step 12) otherwise go to step 13 and
record data.
Based on the selected Orifice Q value adjust one of the following valves:
a.
If the Orifice Q is larger than 1 (This is for medium to high permeability cores)
i.
Turn selection valve to “High Permeability (Low Pressure)” position
(This automatically closes the system to “Low Permeability (High
Pressure)”)
ii. Adjust the “Mercury Regulator” to achieve a value of 60 on the
“Mercury Manometer.
iii. If the desired value of 60 is not obtained on the “Mercury Manometer”,
back out the “Mercury Regulator.” (Make sure the “C” value on the
“Mercury Manometer” is greater than 700).
(1) Open the “Middle Water” valve for the “Middle Water
Manometer.”
(2) Adjust the “Permeameter Regulator” and then the “Mercury
Regulator” to yield a 200 mm reading on the “Orifice
Manometer.”
(3) Read the “Middle Water Manometer” and convert it to “C” value
using the conversion table.
iv. Go to step 10.
b.
If the Orifice Q value is less than 1 (This is for low permeability cores)
i.
Turn selection valve to the “Low Permeability (High Pressure)” position
(This closes the system to “High Permeability (Low Pressure)”.)
ii. Go to step 10.
When the flow rate is stabilized (or a steady state is reached) as indicated by the steady
manometer readings, record the following:
a.
“C” value (or “Middle Water” column height in millimeters and then convert it
to C value using conversion table)
b.
Orifice Q value.
c.
Orifice Manometer reading, hw, in millimeters.
d.
Length of the core, L, in centimeters.
e.
Diameter of the core in centimeters.
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Procedure to remove the core from the core holder:
1. Terminate gas flow through the sample by backing off permeameter and mercury
regulators.
2. Close “Sample” valve located on the left side panel.
3. SLOWLY open “Exhaust (Vent)” valve (This bleeds of the sleeve pressure).
4. Close “Exhaust (Vent)” valve.
5. Open “Vacuum” valve.
6. While holding the yoke, loosen the finger wheels at the upstream end and slowly
remove the upstream head.
7. If the core is not released with the upstream head, open the downstream end (this is the
top of the core holder) and carefully push the core down using a wooden stick or a
similar tool. Make sure to catch the core at the upstream end.
Questions:
1.
2.
Explain the gas slippage effect and how does it affect the permeability measurement?
Name 10 factors affecting permeability.
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