The evaluation of Expected Ultimate Recovery (EUR) for tight gas wells has generally relied upon the Arps equation for Decline Curve Analysis (DCA) as a popular approach. However, it is typical in tight gas reservoirs to have limited production history that has yet to reach boundary-dominated flow due to the low permeability of such systems. Commingled production makes the situation even more complicated with multi-boundary behavior. When suitable analogues are not available, Rate Transient Analysis (RTA) can play an important role to justify DCA assumptions for production forecasting.

The Deep Basin East field has been developed with hydraulically fractured vertical wells through commingled production from multiple formations since 2002. To evaluate potential of this field, DCA type curves for various areas were established according to well performance and geological trending. Multiple segment DCA methodology demonstrated reasonable forecasts, in which one Arps equation is used to describe the rapidly decreasing transient period in early time, and another for boundary-dominated flow. However, a limitation of this approach is the uncertainty of the forecast in the absence of extended production data as the EUR can be sensitive to adjustments in some assumed DCA parameters of the second segment. In this paper, we used RTA to assess reservoir and fracture properties in multiple layers and built RTA type well models around which uncertainty analyses were performed. The distributions of the model properties were then used in Monte Carlo analysis to forecast production and define uncertainty ranges for EUR and DCA parameters. The resulting forecasts and EUR distribution from RTA modeling generally support the DCA assumptions used for the type curves for corresponding areas of the field. The study also showed how the contribution from the various commingled layers changes with time. The proposed workflow provides a fit-for-purpose way to quantify uncertainties in tight gas production forecasting especially in cases when production history is limited and field-level numerical simulation is not practicable.

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