Arps Decline Curve Analysis (DCA) has been the standard for evaluating expected ultimate recovery (EUR) in oil and gas wells since the 1950's. Although this empirical method has served the conventional petroleum industry well, its misapplication to wells in ultra-low permeability plays, most notably shale, often yields ambiguous results due to invalid assumptions. Depending on the application of DCA, these forecasts, with constant hyperbolic decline exponent (‘b-value’) assumptions, have proven to be either overly optimistic or overly pessimistic. The applicability of the Arps method is limited to wells exhibiting boundary dominated flow. However shale wells exhibit transient flow for years, making them unsuitable candidates for the conventional approach of DCA. With the industry now focused on shale, new tools are required to help reduce uncertainty in well forecasts.
This paper presents a two-part study on the forecasting of shale gas well production. Part one utilizes a recently developed trilinear flow model (Ozkan et al. 2009) to improve forecast reliability while preserving simplicity for practical application. It is designed to account for 1) considerations of multi-fractured horizontal wells (MFHW), 2) improved localized effective permeability within the stimulated reservoir volume (SRV), 3) regional permeability contributions to the SRV and, 4) potential contributions of adsorbed gas to expected ultimate recovery. A workflow using this model is proposed and exemplified using production data from two gas wells from the Marcellus Shale (North-East Pennsylvania).
Although analytical models provide a deeper understanding of shale gas reservoirs as well as more reliable forecasting capability, Arps decline curves are, and will continue to be, the language of the reserves evaluation. That is why it is useful to understand the dynamic behavior of the decline exponent resulting from flow regime transitions during the transient period. Part two of this paper highlights an important consideration in the application of DCA to shale: the effect of desorption on the b-value. It will be shown that when desorption is invoked, the b-value will increase. This difference has been quantified for a MFHW in the Marcellus using a Langmuir adsorption isotherm representative of the region.