Modelling the Interfering Effects of Gas Condensate and Geological Heterogeneities on Transient Pressure Response

Abstract

Numerous publications have investigated the effect of gas condensate fluid on the transient pressure well-test (WT) response. However, to the best of authors' knowledge, its combined effect with geology has not been studied. Our findings presented here demonstrate that geology can complicate the WT response and make it difficult for interpretation. In this study the impact of geological heterogeneities on the WT response of a commingled braided-fluvial gas/condensate reservoir has been investigated. Numerical well-test data were generated for a single-well model with a commercial compositional reservoir simulator. Several sensitivity simulations were performed to explore the effects of correlation length, vertical permeability, relative permeability, production rate and drawdown time, on the pseudo-pressure derivate curves. The WT weighting Kernel function and the calculated well-pressure sensitivity coefficients were implemented to demonstrate different trends of drawdown and build-up responses encountered in this study.The results clarified that some geological heterogeneities and production parameters can alter pressure distribution and condensate saturation, and mask the native model WT signatures. In this exercise, it was demonstrated that ramp effect, a geologically complex phenomenon in high net:gross commingled reservoirs, is affected by the condensate formation. This interfering phenomenon is reflected on the derivative curves and is magnified in the presence of the shorter correlation lengths, the lower vertical communications and the higher production rates. We also examined the stepwise stripping of the reservoir heterogeneity, in order to show the impact of each facies on the build-up and drawdown transient pressure response.. The time dependent sensitivity coefficients were calculated to show that the drawdown test is very sensitive to effective permeability in near wellbore areas, where condensate is prone to build up with time. In the build-up, on the other hand, the condensate saturation is almost invariant with time and affects the early time region. This work leads toward better understanding of the influence of geology in gas/condensate well-test interpretation. IntroductionGas condensate reservoirs are those with their temperature lying between the critical temperature and cricondentherm of the system. In such systems, retrograde condensation occurs when the pressure falls below the initial dew point pressure (Muskat 1949). This causes the liquid phase to build up near the producing well, which in turn can dramatically reduce the well productivity even in lean gas condensate reservoirs in which the liquid dropout is as low as 1% (Afidick et al. 1994). It is now well documented that in the near wellbore area, simultaneous flow of gas and condensate is also affected by the combined effect of coupling (increase in k r by an increase in velocity or decrease in IFT) and inertia (a decrease in k r by an increase in velocity) (Danesh et al. 1994, Henderson et al. 1996, Whitson et al. 1999and Jamiola...

Application of Compositional Simulation in Seismic Modeling and Numerical Well Testing for Gas Condensate Reservoirs

“…Figure 2 illustrates the fact that a higher saturation change during the15 days of the drawdown period (Fig.2(A)) produces a stronger spatial 4D seismic signature than the 15 days of build-up (Figure 2(B)). The geological well-test response of the model shows a "ramp effect" phenomenon in which the pressure (or pseudo-pressure) derivative response increases over at least one log-cycle on the well test log-log plot (Corbett et al, 2011).This pseudo-pressure derivative rise is affected by the multi-phase flow in near well bore area which in turn causes the derivative response to deviate from that of the ramp effect (Hamdi et al, 2011). Figure 3 shows that the deviation is magnified in the build-up response when the compressible zone is passing over the already accumulated condensate dropout region.…”

“…We show that the response strength of well testing and the 4D seismic are complementary in each particular flow period (drawdown and build-up). Drawdown well testing is not usually able to show the effect of condensate saturation changes within the test while build-up response can illuminate the near wellbore saturation accumulation (Hamdi et al, 2011). Unlike the well testing behaviour, the condensate effect can be monitored during the drawdown period by the 4D seismic signature.…”