Enhanced gas condensate recovery by CO2 injection
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Perhaps no other single theme offers such potential for the petroleum industry and yet is never fully embraced as enhanced hydrocarbon recovery. Thomas et al. (2009, p. 1) concluded their review article with “it appears that gas condensate reservoirs are becoming more important throughout the world. Many international petroleum societies are beginning to have conferences specifically oriented to gas condensate reservoirs and discussing all parameters germane to such systems.” Gas condensate reservoirs however, usually experience retrograde thermodynamic conditions when the pressure falls below the dewpoint pressure. Condensate liquid saturation builds up near the wellbore first and then propagates radially away along with the pressure drop. This liquid saturation throttles the flow of gas and thus reduces the productivity of a well by a factor of two to four (Afidick et al., 1994; Barnum et al., 1995; Smits et al., 2001; Ayyalasomayajulla et al., 2005). The severity of this decline is to a large extent related to fluid phase behaviour, flow regime (Darcy or non-Darcy), interfacial forces between fluids, capillary number, basic rock and fluid properties, wettability, gravitational forces as well as well type (well inclination, fractured or non-fractured).Thomas et al. (2009, p. 4) added “... for gas condensate systems which exhibit high interfacial tensions where the pore throats are very small, which may correspond either to low permeability rocks or high permeability rocks but with very large coordination number, the success of flowing the liquid from the rock, once it has condensed, will be limited. In such cases, vaporisation (lean gas cycling) or injection of interfacial tension reducing agents (CO2) may be the only option to enhance the performance.” In their comparison of several EOR mechanisms, Ollivier and Magot (2005, p. 217) reported “since large changes in viscous forces are only possible for the recovery of heavy oil, the reduction (or entire elimination) of interfacial forces by solvents such as injection gases seems to be a practical way to achieve large changes in capillary number.” While the majority of the state of the art publications cover sensational aspects of gas condensate reservoirs such as phase couplings and mass transfer between original reservoir components, very little has been reported on fluid dynamics and interfacial interactions of CO2 injection into such systems. This, along with the conceptual frameworks discussed above, serves as the motive for this research work.High pressure high temperature experimental laboratories that simulate reservoir static and thermodynamic conditions have been established to evaluate the: (1) effectiveness of CO2 injection into gas condensate reservoirs through interfacial tension (IFT) and spreading coefficients measurements at various reservoir conditions, (2) efficiency of the process through recovery performance and mobility ratio measurements; with special emphasis on the rate-dependent, IFT-dependent, and injection gas composition-dependant relative permeabilities, and (3) the behaviour of CO2 injection into gas condensate reservoirs on a field scale through numerical simulations in heterogeneous, anisotropic, fractured and faulted systems. The study also investigates the performance of various reservoir fluid thermodynamic conditions, injection design variables, and economic recovery factors associated with CO2 injection.Condensate recovery was found to be a strong function of CO2 injection pressure (and thus IFT), displacement flow rate, injection gas composition as well as phase behaviour and fluid properties. These parameters control the orientation and continuity of the fluid phases, solubility, gravity segregation, mobility ratio, and the ultimate recovery efficiency. Simulation analysis also suggests that developments of fractured gas condensate reservoirs depend to a large extent on initial reservoir thermodynamic conditions (initial pore pressure and fluid composition) as well as on production operations (natural depletion, waterflooding, continuous CO2 injection, gas injection after waterflooding GAW, or water alternating gas WAG).Much like the interrelation between accuracy and precision in science and engineering statistics, this research work draws a link between the effectiveness (quality metric through IFT measurements) and the efficiency (productivity metric through coreflooding experiments) of CO2 injection into gas condensate reservoirs. The data reported in this research work should help reservoir engineers better characterise gas condensate systems. The results can also aid the engineering design of CO2-EOR and CO2 sequestration projects.
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