Tadesse Weldu Teklu

Phase Behavior and Minimum Miscibility Pressure in Nanopores

Numerous studies indicate that the pressure/volume/temperature (PVT) phase behavior of fluids in large pores (designated “unconfined” space) deviates from phase behavior in nanopores (designated “confined” space). The deviation in confined space has been attributed to the increase in capillary force, electrostatic interactions, van der Waals forces, and fluid structural changes. In this paper, conventional vapor/liquid equilibrium (VLE) calculations are modified to account for the capillary pressure and the critical-pressure and -temperature shifts in nanopores. The modified VLE is used to study the phase behavior of reservoir fluids in unconventional reservoirs. The multiple-mixing-cell (MMC) algorithm and the modified VLE procedure were used to determine the minimal miscibility pressure (MMP) of a synthetic oil and Bakken oil with carbon dioxide (CO2) and mixtures of CO2 and methane gas. We show that the bubblepoint pressure, gas/oil interfacial tension (IFT), and MMP are decreased with confinement (nanopores), whereas the upper dewpoint pressure increases and the lower dewpoint pressure decreases.

Low-salinity Water-alternate-surfactant in Low-permeability Carbonate Reservoirs

Low-salinity water injected into carbonate cores, which have undergone sea-water injection, can produce additional oil more economically if a low-concentration non-ionic surfactant is added to the low-salinity water and injected as chase fluid. One major reason for the additional oil recovery is that low-concentration surfactant effectiveness favors the low-salinity environment. Several coreflooding, contact angle, and IFT experiments were performed to assess the proposed process. The core flooding sequence includes seawater, low-salinity water, and low-concentration non-ionic surfactant. However, for field application, we proposed low-salinity water-alternate-surfactant injection. The surfactant concentration in low-salinity water was 1,000 and 5,000 ppm. Phase behavior and cloud point measurements were conducted prior to surfactant injection. The core permeability is 0.5 to 1.5 md, and porosity ranges from 0.18 to 0.25. Cores were aged for eight weeks at reservoir pressure and temperature. The pendant drop oil-brine IFT and captive oil-droplet contact angle measurements were performed at variable brine salinity in the presence of surfactant. Seawater and low-salinity waterflooding corefloods yielded ultimate oil recoveries of up to 57 percent. Up to 10 percent additional oil recoveries was obtained from low-concentration non-ionic surfactant in low-salinity waterflood. With decreasing salinity, in presence of 1,000-ppm surfactant, favorable wettability alteration from intermediate-wet to water-wet was observed by contact angle measurements. Moreover, addition of small concentration of surfactant decreased the IFT and altered the wettability of several one-inch diameter, crude-aged, discs to water wet.

Matrix-Fracture Interface Cleanup Protocol for Tight Sandstone and Carbonate Reservoirs

In tight sandstone and carbonate reservoirs, the hydrocarbon recovery is mainly governed by the matrix- fracture interface mass transfer efficiency. To improve the matrix-fracture interface mass transfer in tight sandstone and carbonate reservoirs, we propose the following protocol to clean up the matrix-fracture interface and improve hydrocarbon production: (1) Inject one of the following – CO2 followed by anionic or non-ionic surfactant, Anionic or non-ionic surfactant followed by CO2, miscible or near miscible CO2 gas, low concentration anionic or non-ionic surfactant. (2) Repeat the process when another cycle of treatment is needed. The same procedure can be applied into injection well to improve injectivity. The proposed cleanup protocol improves production because – wettability alteration, reduction of IFT, swelling of the oil leading to lower viscosity, and molecular diffusion among other thermodynamic effects– all acting at the matrix-fracture interface. To assess the proposed matrix-facture interface clean up protocol, IFT and contact angle measurements that mimic the proposed processes were conducted. Favorable wettability alteration of tight carbonate and sandstone crude-aged cores and oil-brine IFT reduction were observed.