The Effects of Water Injection on Miscible Flooding Methods Using Hydrocarbons and Carbon Dioxide

Abstract

Abstract The effects of mobile water saturations on oil recovery and solvent requirements were studied in miscible displacement tests on sandstone cores. it was found thatoil, if trapped by mobile water, cannot be easily contacted by solvent, and the amount of oil is directly related to measurable relative-permeability characteristics;miscible displacement Performances for secondary and tertiary conditions are equivalent;long-core tests describe the movement of fluid banks that would occur in field floods; andflooding response for solvent developed from multiple contact of crude oil with carbon dioxide or rich gas in long cores is the same as that for liquid solvents with first-contact miscibility. Introduction Miscible flooding is receiving increasing interest as a means of recovering tertiary oil left after waterflooding. Mobile water is a factor in tertiary flooding, and can also be a factor in secondary operations where alternate water and solvent injection is used to improve the low sweep efficiency of miscible flooding with hydrocarbon and acid gases. Several publications have reported a reduction in displacement efficiency when mobile water is present at the displacement front. Stalkup present at the displacement front. Stalkup summarized this information and also reported increased mixing caused by the mobile water. However, more information is needed to implement recovery operations where mobile water conditions can occur. The purpose of this paper is to provide information about the displacement behavior in those portions of a reservoir that may contain a high water saturation and that are contacted by a solvent. Factors examined arethe relationship of oil trapping by water to relative permeability and wettability,the development and growth of fluid banks,a comparison of first-contact and multiple-contact miscible displacement, before and after waterflooding,the effect of flow rate and system length on multiple-contact miscible displacement, andthe displacement of oil by the simultaneous injection of solvent and water. The experiments performed were in laboratory cores and are not scaled to field conditions in some respects. The study provides insight into some of the pertinent mechanisms of the displacement process rather than data that is directly applicable to a field situation. MATERIALS AND PROCEDURE OIL-TRAPPING TESTS Drainage and imbibition water-oil relative-permeability data were obtained on a water-wet Berea sandstone core using the steady-state test procedure. The dimensions of the Lucite-encased Berea core are given in Table 1. Two series of first-contact miscible-displacement test, one series involving the displacement of water and the other involving the displacement of oil, were performed at various levels of oil and water saturation on the same core. Saturations during the relative-permeability tests and miscible - displacement tests were determined using an X-ray absorption technique. The recovery performance was calculated by refractive index analyses of the produced fluids. To provide for these analyses, two bones and two refined oils were prepared for the tests. TABLE 1 - TRAPPING-ENVELOPE MISCIBLE-DISPLACEMENT TESTS, 2.1-IN.-DIAMETER BY 5.1-IN.-LONG BEREA SANDSTONE CORE Residual In-Place Saturation Solvent Liquid Test Flowing percent PV Flow Rate Saturation Number WOR Oil Water (PV/hour) (percent PV) ------ ------- --- ----- --------- ------------- Drainage Tests - 1.0-cp Nal brine displace by 0.95-cp brine D-1 oo 0 100 2.84 0 D-2 2 29 71 1.79 0 D-3 0.1 45 55 0.284 0.5 Imbibition Tests - 1.48-cp oil displaced by 1.42-cp oil containing iodobenzene I-1 0 74 26 2.85 0 I-2 0.2 43 57 2.28 7 I-3 1 34 66 0.78 13 I-4 5 32 68 2.72 17 I-5 1 35 65 1.21 10 SPEJ P. 217