Decline curve analysis using the Arps rate-time equations has historically been the primary tool used for reserve evaluations in tight gas sands. However, characteristic tight gas sand properties often preclude accurate assessments using only or primarily decline curve analysis — especially early in the productive life. Moreover, the Arps models are essentially used as curve-fitting devices with (very) limited theoretical basis. As such, this paper presents a reserves appraisal process that complements traditional decline curve analyses with theoretically-based production analysis techniques.
Specifically, we incorporate multiple reserves estimation techniques ranging from simple extrapolation techniques to rigorous model-based analysis/interpretation methods, including:
Arps exponential and hyperbolic relations — rate-time and rate-cumulative plots, as well as other auxiliary plots,
Semi-analytical rate-cumulative techniques for boundary-dominated flow,
The "flowing material balance" approach in its various formulations, and
Model-based analysis ("rate-transient" or "model-matching" approaches).
We provide both separate and combined analysis, including "diagnostic" plots which guide the evaluation process and help prevent erroneous reserve estimates resulting from use of single methods. Implementation of the proposed workflow will prevent unrealistic (either too low or high) reserve bookings by integrating multiple production analysis techniques. We demonstrate where and how each component technique should be used as well as their limitations. We also demonstrate how each technique can be used to reinforce/validate results from other techniques (i.e., multiple redundancies of methods).
The primary objectives of this work are to:
Provide a consistent workflow for integration of multiple reserve estimating techniques for tight gas sands,
Provide an assessment of strengths and weaknesses of individual reserves estimation techniques, and.
Provide demonstrative examples for implementing this workflow (including diagnostics).