Over the past decades, different researchers have proposed various decline relations attempting to model the rate/time behavior of unconventional reservoirs in order to estimate reserves and forecast future production performance. Reserves estimation is a process that is thoroughly renewed during the life of a reservoir. Its accuracy depends on the amount of available data and the method of forecast. Forecasting production and estimating ultimate recovery (EUR) in unconventional reservoirs have long been problematic.
Unconventional reservoirs exhibit fracture-dominated flow regimes and rarely reach boundary-dominated flow (BDF), even over several years of production. The reserves estimation in these reservoirs is not straight forward like in the conventional ones because of the variation of reservoir properties and completion type. Many challenges are facing the forecasting of such reservoirs due to
high initial rates,
sharp decline in the transient flow period, and
shallow decline resulting from BDF.
For that, it is difficult to match the whole production trend of an unconventional wells using a single decline curve relation.
Traditional decline curve such as Arps' method have been successfully used for forecasting production and estimating EUR of conventional reservoirs. However, using the traditional Arps' with low permeability reservoirs yields over-forecasted reserves. Arps' model can be adapted for estimating reliable reserves of unconventional reservoirs by using different values of b-factor based on the flow regime. Different relations and models have been presented to model the production behavior in shale gas reservoirs as alternatives for the traditional Arps' decline model, such as Modified Hyperbolic decline, Power Law Exponential Decline (PLE), Stretched Exponential Decline (SEPD), Logistic Growth Model (LGM), and Duong method. All of them are based on empirical observations of a particular scenario, and thus often provide different forecasts. The application of those methods is depending on the behavior of each relation.
This work presents a new look at decline curve methods for reserves estimation in unconventional gas reservoirs. Recommendations are also presented based on practical applications, which might help in understanding the behavior of such problematic calculations. Additionally, a recommended workflow is presented for better application of decline curves in estimating unconventional reserves using a short period of production.