Model 720
A
properly equipped centrifuge has many applications in studies of rock-fluid
properties of hydrocarbon reservoirs. Processing the centrifuge core test results
yields relative permeabilities and capillary pressure data applicable to
reservoir production performance calculations. Because enormous forces on the
pore fluids are easily generated in the centrifuge, experiments modeling
gravity drainage production processes are possible. Rates and end points are
key concerns in gravity drainage that can be derived form the measured
centrifuge core test data.
The
Model 720 Automated Rock-Fluid Centrifuge was specially designed for
rock testing to be capable of smooth operation from 200 to 3,600 RPM. The
maximum rotational speed translates to a capillary pressure in excess of 70 psi
depending upon the arm length of the sample holder. Unlike conventional laboratory centrifuges designed to operate at
one speed using a lightweight rotor mounted on a gimble, the Automated
Rock-Fluid Centrifuge uses a massive rotor mounted on a rigid shaft. The mass of the rotor is large compared to
the test samples and the center of mass movements that occur during a core plug
test. The result is a centrifuge that will operate smoothly throughout the
speed range. The resonance speeds that
plague conventional centrifuges are above the maximum operating speed of the Automated
Rock-Fluid Centrifuge.
Core
holders are heated directly on the centrifuge rotor while spinning, and the
surface temperature of the holder is measured and controlled with an infrared
sensor. The centrifuge tub (rotor and holder enclosure) is operated under
vacuum which thermally insulates the core holders from the rest of the lab
room. In the tub, convection heat losses from the sample holders are negligible
and radiation heat losses are small. Slip rings, mounted on the rotor shaft,
conduct low voltage to heaters mounted in the core holders. Small temperature
differences between holder surface and the core plug sample area have been
measured and the correction values are supplied. A magnetic coupling isolates
the rotating shaft for a leak proof vacuum seal.
A
variety of core holders are offered; sizes from 1.0 inch diameter X 1.0 inch
long to 1.5 inch X 3 inch long, overburden pressured, back pressured, made of
aluminum, titanium, or stainless steel, and receivers made of quartz or
plastic. Core holders and replacement parts are competitively priced. The
centrifuge is supplied with one set (4) of drainage and one set of imbibition
room temperature and atmospheric 1.5-inch diameter X 3-inch core holders with
polycarbonate receivers.
The
nominal maximum temperature of the test is 250 F. Licensed core holders are available up to 500 F.
Unattended
operation is possible by preprogramming the test parameters. Rotor speeds, running time at each speed
value, and core holder temperature can be set using the menu driven
program. Fluid production data versus
time can be plotted on the monitor in real time as the test continues.
Production
volumes are measured using a CCD line video camera. A synchronous strobe light is employed to illuminate each receiver
vial. Position of the fluid interface
enhancer (plastic washer) is recorded digitally. The data rate is one per revolution up to about 700 RPM and then
10 per second up to the maximum rotor speed.
The method is modeled after the published work by Shell Development
Company. The position of two fluid
interfaces are measured and reported.
The position of the first fluid interface is measured and reported as
the number of camera pixels from the bottom of the vial view slot to the
interface. If a second fluid interface
is present (i.e., water-crude oil), the number of pixels from the top of the
first interface to the bottom of the second is reported. If a second fluid interface is not present
(i.e., water-gas), the number of pixels from the first fluid interface to the
top of the vial view slot is reported.
Production
during the rotor speed ramp up can be measured and this transient data used to
calculated relative permeabilities.
Rotor speed can be increased in a single step or in multiple steps over
any time period. The camera data is
recorded at intervals spaced in a logarithmic fashion. The data rate can be selected from 5 to 50
values per time decade. A 486 VGA PC
controls the camera and centrifuge and is used to display and reduce the data. The centrifuge and camera operating program
are menu driven and require only basic computer knowledge.
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