Analysis and Correlation of Nitrogen and Lean-Gas Miscibility Pressure(includes associated paper 16463 )

Abstract

Summary The vaporizing gas drive (VGD) process was modeled with the Peng-Robinson equation of state (PR-EOS) and a compositional simulator. The comparison of numerical results with available experimental data has shown that the PR-EOS overestimates the minimum miscibility pressure (MMP), but it correctly shows that the length required to achieve miscibility is different for N2 and methane. Experimental data and some simulation runs have been used to develop a simple and reliable correlation for the prediction of MMP. Introduction For some high-pressure oil reservoirs, N2or a lean hydrocarbon gas may be suitable for achieving miscibility conditions. These gases are particularly attractive because of the ease with which they can be handled and the potential they offer for establishing gravity-stabilized displacement in thick oil columns. Two field projects involving N2 and lean-gas injection are discussed. In the Devonian reservoir of Block 31 field in west Texas,1–4 the world's first large-scale high-pressure gas injection project has been under way since 1949. The reservoir rock consists of about 65 % tripolitic chert and 20% fine crystalline, sucrosic limestone. The remainder is variable amounts of lime mud, skeletal material, pellets, and quartz silt. The porosity is intercrystalline and averages 15%. The permeability averages 1 md, but is about 10 times this amount in the hairline fractures that are present. Lean-gas injection in Block 31 field started 3 1/2 years after field discovery, and the reservoir pressure was raised to the miscibility pressure in 1952. Since 1966, flue gas (88% N2 and 12% CO2) has been injected in one part of the field and N2-contaminated lean hydrocarbon gas has been reinjected in the rest of the field. Because of the high solubility of CO2 in the interstitial water, the displacing fluid was essentially N2. Even though the miscible wne seems to have difficulty in maintaining its integrity because of reservoir stratification, fractures, and an unfavorable (10:1)gas-to-oil mobility ratio, the ultimate recovery is expected to be greater than 65 % of the original oil in place (OOIP). A large part of Algeria's Hassi Messaoud field5is undergoing lean-gas miscible drive. The reservoir is made up of a highly siliceous, cemented quartzitic sandstone and is highly heterogeneous. Water is also injected in some parts of the field, and in other parts, water is alternated with gas. Ultimate recovery is expected to be about 50% of OOIP in the miscible gas injection area, about 33% of OOIP in the water injection area, and about 11% for naturally depleting areas. Stalkup6 provides details of several other VGD field projects. On the whole, the reservoirs undergoing high-pressure miscible drive have been rated successful;the recoveries usually exceed 50% of OOIP. In deep, volatile oil reservoirs, N2 and lean-gas miscible drive have the potential to recover oil that is unrecoverable by water injection alone. The most important parameter required for the design and evaluation of N2 or lean-gas miscible drive is the MMP. The literature does not contain any general correlation that can be used to provide an estimate of MMP for N2 or lean gases. In this paper we explore the potential of the PR-EOS to predict the MMP for the VGD process, discuss the effect of N2and lean gases on the MMP, and present a simple correlation for the estimation of VGD MMP. Review of Experimental Data Experimental data reported in the literature for VGD MMP are rather limited. Information on only eight reservoir fluids with known compositions was located. These data are briefly reviewed below.