The present paper is an account of the continuation of the work which has beenreported by Kantzas et al. (1,2). Chatzis et al. (3) and Dullien et al.(4)
After being coated with epoxy resin the cores were flushed with carbon dioxideto replace the air originally present. Carbon dioxide was miscibly displacedwith several pore volumes of brine. The cores were then positioned verticallyand flooded with several pore volumes of Soltrol oil injected at the top. Awaterflood was carried out with brine injected at the bottom of the cores.Mixed-wet semipermeable membranes were placed against the bottom face of thecores and gravity drainage was started while injecting nitrogen under lowpressure (3 psig) at the top. The schematic diagram of the set-up is shown in Figure 1.
The function of the semipermeable membrane is to eliminate capillary endeffects in cores undergoing gravity drainage. It permits the passage of oil andbrine at the producing end of the core, white preventing gas breakthrough underthe excess nitrogen pressure applied. The membrane consists of a mixed-wetpaste supported by a stainless steel porous plate. The mixed wet paste is amixture of calcium carbonate (water-wet) and active carbon (oil-wet) powdersfinely crushed.
The size of the grains should be small enough so that the displacementpressures of the oil-gas and brine-gas interfaces in the pores between thegrains are larger than the pressure of gas injection. We used calcium carbonate Analar from BDH and active carbon Norit-A from Fisher Scientific company. Witha Herzog HSM50M crusher, our crushing times are 1 hr 30 min for the calciumcarbonate and 3 hrs for the active carbon. Grain size distributions measured bya scanning electron microscope are as follows: between 0.1 and 1 microns forcalcium carbonate, and between 0.5 and 5 microns for active carbon. Equalvolumes of the two powders are then mixed in a beaker. Oil and brine are addedlittle by little in equal volumes. They are mixed with the powder mixture, until the resulting paste has the consistency of soft peanut butter. The pasteis then spread onto a porous stainless-steel plate (Mott Metallurgical Corporation) of large permeability on the order of ha~ a Darcy, as shown in Figure 2. The porous plate acts as a support for the mixed-wet paste. After thepaste has been compressed between the porous plate and the bottom face of thecore undergoing gravity drainage, it acts as a mixed-wet semipermeablemembrane. Its measured effective permeability is approximately the same to oiland to brine, equalling 0.35 md. In our experiments, the thickness of the pasteafter compression was 2.5 mm, and its diameter close to 50 mm. The maximum gaspressure applicable without gas breakthrough was 4.3 psi.
Results of tests on two Berea sandstone cores of different lengths are shown in Table L The production history in the long core is shown on Fig. 3.
