Effect of Injectant Composition and Pressure on Displacement of Oil by Enriched Hydrocarbon Gases
- J.J. Shyeh-Yung (Exxon Production Research Co.)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Engineering
- Publication Date
- May 1995
- Document Type
- Journal Paper
- 109 - 115
- 1995. Society of Petroleum Engineers
- 2 in the last 30 days
- 371 since 2007
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This paper presents results from laboratory tertiary gasflood and phase behavior studies investigating the effect of hydrocarbon gas injectant composition and pressure on local oil displacement efficiencies. In this study, injectant composition was varied by enriching dry gas with up to 60% liquefied petroleum gas (LPG). In addition, coreflood pressure was varied from 3800 to 3000 psig. Multiple-contact phase behavior experiments indicated that these injectants ranged from being close to first-contact-miscible to near-miscible with the oil.
Within the study conditions, gasflood residual oil saturation (Sorm) increased as pressure was reduced, or injectant enrichment level was lowered. At low pressures, Sorm was less sensitive to enrichment level than at high pressures. For lean injectants, Sorm was less sensitive to pressure than for rich injectants. Also, coreflood oil recoveries remained higher than 50% of waterflood residual oil (or higher than 15% of original oil in place) even for the dry gas injectant without any enrichment. Lower injectant mobilities were observed generally at lower enrichment levels and/or at lower pressures due to lower gas saturations. There may be a trade-off between local oil displacement efficiency and gas sweep efficiency in field near-miscible displacements.
We also found that low interfacial tension (IFT) existed for all injectant and crude oil mixtures. An equilibrium IFT of 1 dyne/cm, higher than required in typical surfactant flooding processes, appears to be sufficiently low to displace oil effectively. Stripping of light oil components is responsible for recovery of some of the oil. Miscible and near-miscible residual oils are essentially denuded of oil components lighter than C15.
A hydrocarbon gas injection project often involves blending LPG with a dry gas (mainly methane) stream to obtain an optimum injectant composition. A common practice in industry is to use an injectant composition at or above the minimum miscibility enrichment (MME) and maintain a pressure at or above the minimum miscibility pressure (MMP) required for developing multiple-contact miscibility. These minimum miscibility conditions are typically determined from slim-tube tests or equation-of-state calculations. A leaner injectant, one with less LPG enrichment in dry gas, is less expensive than a richer injectant because LPG is normally more valuable than dry gas. A lower-pressure process also has the potential to reduce injectant cost because injectant density is lower (less mass of gas is required for a given volume), and gas compression costs are reduced at lower pressures. A gas injection process using an injectant composition leaner than the MME or using a pressure lower than the MMP is called a near-miscible process here.
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