Journal of Canadian Petroleum Technology 10 Summary Modern production-decline analysis is a robust technique for anal- ysis of production data from a well under variable operating con- ditions. It uses production rates and flowing pressures to provide reliable estimates of recoverable reserves and fluid in place. The mathematics behind this technique is similar to that of pressure- transient theory; however, the focus is different. It deals with long- term variable production data instead of short-term constant-rate transient data. Using modern decline analysis for two-phase-flow conditions (e.g., gas/condensate reservoirs) is under question because of the single-phase-flow assumption in the development of the “mate- rial-balance time” function. This is a time function that converts any decline (e.g., exponential decline) to harmonic decline to ac- count for variable operating conditions. The purpose of this work is to develop a model to use the concepts of modern techniques for analyzing production data of single-porosity gas/condensate reser- voirs. For this purpose, the governing flow equation is linearized, using appropriately defined pseudopressure and pseudotime func- tions. Then, the solution is obtained for constant-well-rate condi- tion. This is followed by employing the superposition theorem to account for variable well pressure/rate conditions, resulting in def- inition of two-phase material-balance pseudotime. The solution developed here is coupled with an appropriate material-balance equation and used to estimate the average reservoir pressure and original gas in place from analyzing production data. The depen- dency of relative permeability on capillary number and non-Darcy flow is included in the formulation. Verification of the proposed method is obtained with the anal- ysis of synthetic production data using a series of fine-grid com- positional numerical simulations over a typical range of gas/ condensate-reservoir parameters. Introduction Analysis of long-term production data helps reservoir engineers es- timate reservoir parameters (e.g., hydrocarbon in place, average res- ervoir pressure, and reservoir pore volume). For many years, analysis of production data was based on empirical correlations, while ana- lytical models were absent. Later, analytical/empirical type curves were developed. In these type curves, analysis of the transient pe- riod was based on analytical models and empirical correlations were used for the boundary-dominated-flow period. Modern methods, which emerged after 1980, apply analytical models in both tran- sient- and boundary-dominated-flow regimes in the same way as do pressure-transient-analysis (well-testing) techniques. Arps (1945) published three empirical decline-rate correlations that are exponential, hyperbolic, and harmonic. His methodology is simple and requires only well-rate data. Fetkovich (1980) developed a set of empirical/analytical type curves that extended the Arps type curves into the transient-flow region. He used analytical flow equa- tions to generate type curves for transient flow, and combined them with Arps empirical decline-curve equations. Blasingame and Lee (1986) introduced the concept of material-balance time for analysis of variable-well-rate data in boundary-dominated oil reservoirs. For dry-gas reservoirs, Al-Hussainy et al. (1966) defined pseudopres- sure and Fraim and Wattenbarger (1987) defined pseudotime to remove the nonlinearity of the gas-flow equation. For variable-gas- rate conditions, material-balance pseudotime was introduced by Palacio and Blasingame (1993). Agarwal et al. (1999) showed that material-balance time converts the constant-pressure solution into the widely used constant-rate solution. It should be mentioned that modern methods are applicable for single-phase volumetric res- ervoirs. In such methods, both well-rate and flowing-bottomhole- pressure data must be known. Special combined type curves and decline curves that employ the material-balance-time function were published. Some important type curves are Blasingame pressure- integral type curves (Blasingame et al. 1989), Blasingame rate- integral type curves (Doublet et al. 1994), and Agarwal-Gardner type curves (Agarwal et al. 1999). Also, type-curveless techniques [e.g., dynamic material-balance method (Mattar and Anderson 2003, 2005)] are reported in literature. There are numerous other studies in the field of production-data analysis (Mattar and McNeil 1998; Araya and Ozkan 2002; Li and Horne 2005; Blasingame and Rushing 2005; Rodriguez-Roman and Camacho-Velázquez 2005; Camacho-Velázquez et al. 2005; Amini et al. 2007; Heidari Suresh- jani and Gerami 2011). Recently, a paper has been published (Ilk et al. 2010) that addresses the challenges/pitfalls of production-data analysis and provides a state-of-the-art technology review of cur- rent diagnostic tools for analysis of production data. In gas/condensate reservoirs, when the wellbore pressure falls below dewpoint pressure, a condensate bank forms around the well and three regions are developed. In the first region, which is nearest the well, both phases are flowing, while in the second re- gion, only the gas phase is flowing. In the third region, only the gas phase is present. At sufficiently late times, the third region van- ishes because average reservoir pressure falls below dewpoint pres- sure. Dropout of condensate near the wellbore is the main source of well-productivity reduction in such reservoirs. Danesh et al. (1994) introduced a phenomenon known as the “positive coupling effect,” which describes the increase in relative permeability of gas/con- densate reservoirs caused by an increase in fluid velocities. Since then, numerous investigators have studied the dependency of gas/ condensate relative permeability on velocity (Bourbiaux and Lim- borg 1994; Boom et al. 1995; Chen et al. 1995; Fevang and Whitson 1996; Munkerud and Torsæter 1995; Henderson et al. 1996, 1997; Ali et al. 1997; Blom et al. 1997; Whitson et al. 1999; Mott et al. 2000; Henderson et al. 2001; Jamiolahmady et al. 2003, 2009). Inclusion of velocity dependence in relative permeability is per- formed through capillary number and non-Darcy-flow effects. In- crease in velocity and, as a result, increase in capillary number cause a positive coupling effect. On the other hand, as velocity in- creases, inertial forces associated with non-Darcy flow will de- crease the relative permeability. Because of the presence of two phases, gas/condensate reservoirs exhibit a complex flow behaviour that cannot be modelled as simply as that of dry-gas reservoirs. Using dry-gas techniques for analyzing production data of gas/condensate reservoirs introduces enormous A New Model for Modern Production-Decline Analysis of Gas/Condensate Reservoirs M. Heidari Sureshjani and S. Gerami, IOR Research Institute, NIOC Original manuscript received for review 11 January 2011. Revised manuscript received 5 April 2011. Paper (SPE 149709) peer approved 12 May 2011.