Comparison of Decline Curve Analysis Methods with Analytical Models in Unconventional Plays

Abstract

Unconventional plays have moved to the forefront of the energy industry in the U.S. over the last five years due to advancements in technology and the overall abundance of producible hydrocarbons discovered near existing infrastructure. In the present economic climate, there is an increased interest in liquid rich plays, and because of the relatively limited historical production data available for these resources, there is a lot of industry discussion regarding future decline performance and estimated ultimate recovery (EUR) per well. The objective of this paper is to discuss the uncertainty associated with estimating reserves in U.S. unconventional plays using common decline curve analysis (DCA) methods in comparison to analytical modeling. Broadly speaking, there are five common methods for estimating: use of analogs, volumetric analysis combined with an estimate of recovery efficiency, decline curve analysis (DCA), analytical models and numerical simulation. Among theaforementioneds, DCA is the simplest and often fastest way to estimate volumes. However, the theoretical basis for most DCA approaches does not apply to unconventional reservoirs, which introduces some uncertainty into estimation of volumes. Nevertheless, it is commonly applied because of its perceived simplicity. Different unconventional DCA methods were compared with results of an analytical model generated using commercial software: the power law model (PLE), the logistic growth model (LGM), and Duong's method. The analysis was performed on various unconventional plays based on reservoir type and well geometry. All historical production data is gathered from public documents. The application of the DCA methods was also extended to various fluid types to determine their suitability for application in oil as well as gas reservoirs. The results of the study show that comparing multiple DCA methods with an analytical model aids in the understanding of the range of uncertainty associated with the EUR of unconventional wells. The study also helps establish the most appropriate DCA methods for various reservoir types, well geometry, and fluid types. The results also suggest approaches for avoiding violating the SEC's guidelines for categorizing proven reserves.

Methods of Decline Curve Analysis for Shale Gas Reservoirs

“…Kanfar and Wattenbarger [12] showed that the Duong model is more accurate for linear flows and bilinear-linear flows. Meyet et al [28] showed that the EURs determined with PLE and Duong model vary the least with respect to the length of production history for all wells among all of the DCA methods in their study, and other DCA methods tend to converge towards the modified Duong model and PLE model. Furthermore, the Duong model tends to provide the most conservative results.…”

“…Zuo et al [35] discovered that, for shale gas wells in the Fayetteville formation, the DCA parameters are all within a certain small range. For example, for Fayetteville shale, parameter α defined in Equation (28) lies in the range from 0.65 to 0.75, which indicates that this parameter might have some relationship with the physical parameters. Zhang et al [40] showed that some recommended empirical coefficients can be determined for specific shale formations such as Eagle Ford, which may help, particularly in the early stages of development when little production data are available.…”

“…Kanfar and Wattenbarger [12] proved that the PLE model is reliable for linear flow, bilinear flow followed by linear flow, and linear flow followed by BDF, or bilinear flow followed by linear flow and finished with BDF flow. Meyet et al [28] illustrated that the PLE model varies the least with respect to length of production history for all wells, and the PLE and LGA models usually return similar results. Paryani et al [29] showed that the PLE model fits the historical production data 67% of the time, and provided the lowest forecasts among all the DCA models in their study and thus the most conservative forecast.…”

“…Meyet et al [28] pointed out that the selection of the decline rate D * can be empirically determined and has no physical basis. The authors also showed that the EUR forecasts by MHD are often greater than those of other DCA methods.…”

“…However, an upward inflection in the curve would occur if n > 1 [28]. The main assumption in this model is that the entire reservoir can be drained by a single well over a sufficiently long period [29].…”