Decline-curve analysis is one of the most commonly used techniques to estimate reserves from production data. In tight formations that have been stimulatedespecially when there are multiple layers that communicate only at the wellborethe uncertainty in reserves estimates from this technique is quite large because forecasting future performance is quite difficult. This uncertainty can affect the classification of reserves, and could limit what we should call "proved developed reserves." In this paper, we present new procedures to mitigate the complexity of decline-curve analysis in multilayer tight gas wells. Using synthetic and field examples, we demonstrate how reserves can be estimated more reliably.
For tight and multilayer gas wells, it is not uncommon that decline-curve analysis yields an Arps decline-curve parameter b greater than unity (Arps 1945). Single-layer hydraulically fractured tight gas wells also appear to have b-values greater than unity. Different practices are used to handle such complexity. For forecasts, some analysts simply use the b-value obtained from matching production data, while others force the b value to be unity. Still others use the hyperbolic decline and the matched value of b, but, when the decline rate reaches a predetermined limit, they switch to exponential decline for the remainder of the forecast. Thus, forecasted performance differs significantly when different analysts analyze the data. Consequently, the reserves estimate has large uncertainty, which can, in turn, affect its classification.
In this paper, we first present an in-depth investigation of decline behavior of tight, single-layer and multilayer gas wells by analyzing depletion characteristics using simulated data sets. We illustrate the long-duration transient effects present in single-layer stimulated tight gas wells and the complex flow regimes present when wells in layered reservoirs are produced commingled. Our work indicates that, as observed in field data, transient effects and coexistence of different flow regimes between layers lead to abnormal decline behavior (b < 1.0) in multilayer tight gas wells, which leads to errors in production forecasts. Our new procedure provides a method to minimize these errors.