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.