Reserve Analysis for Multilayered Tight Gas Reservoirs

Abstract

Abstract This paper presents an extension of the previous work by the authors1 to examine common reserve analysis techniques for non-communicating multilayered tight gas reservoirs. Tight gas reservoirs present unique challenges to the practicing reservoir engineer. One significant problem is estimating ultimate recoverable reserves for a tight gas reservoir. The extended time required to establish boundary dominated flow in the tight gas systems conflicts with the need to accurately estimate performance-based recoverable reserves early in the life of the well. One other complicating factor is the application of single layer solution techniques to the multilayered commingled completion. Conceptual simulation cases are presented as a control set to test the accuracy of each analytical method. The simulation results are also used to test various methods of predicting the time required to establish boundary dominated flow in multi-layered, commingled completions. The two simulation cases under consideration are a uniform drainage area and a channel system. Well performance analysis is evaluated during both early time transient and late time boundary dominated flow. A field example is included comparing the results of each technique through time. Introduction For the purpose of this paper, gas reservoirs with a permeability of 0.5 md or less are considered "tight gas". Furthermore, there is no cross flow between layers. The only communication within the multilayered reservoir system takes place in the wellbore where the layers are commingled. In a tight gas environment, it is common practice to commingle a number of productive reservoirs in an attempt to establish economic production from the well. The simulated production presented in this paper represent production from non-communicating layers producing into a common wellbore. An accurate estimate of recoverable reserves can only be performed after the well's pressure transient is affected by all boundaries of the reservoir. The time required to satisfy this condition can take years even in a single layer system. The commingling of multiple layers further complicates reserve estimates for these completions because the time required to detect the boundaries is significantly increased. The paper provides a critical review of the application of rate-time decline curve analysis, material balance and production analysis techniques for reserve estimates in multilayered commingled tight gas reservoirs. Discussion topics are supplemented with conceptual simulation work and one field example. Background Theory The following is a summary of the reserve estimating techniques commonly used to estimate recoverable reserves for tight gas reservoirs. This discussion will focus on each technique's application to the non-communicating multilayered system. Rate-Time Decline Curve Analysis Rate-time decline curve analysis is based on the empirical equations of Arps2. This technique is based on the assumption that past performance trends can be characterized mathematically and used to predict future performance. For a non-communicating, multilayered system, past performance may not be representative of future performance due to variations in permeability, fluid properties and in-place volumes. The following is a list of inherent assumptions implied when performing rate-time decline curve analysis:The well is produced at or near capacity.The well's drainage area remains constant.The well is produced at a constant bottom hole pressure