Decline Curve Analysis for Tight Gas and Gas Condensate Reservoirs
Authors: Sarisittitham Sawatdiwong , Jamiolahmady Mahmoud 
Published: May 2016

Abstract
To understand the reservoir flow behavior and estimate its parameters (e.g., permeability, skin), either transient pressure or rate data are usually used. However, in tight reservoirs, due to the economic and technical difficulties of transient pressure well test operations and data analysis, working with transient rate data, i.e., decline curve analysis, sometimes, is considered to be more attractive.
This study focuses on the application of the widely accepted Fetkovich type curves for tight gas and gas condensate reservoirs. Initially, a synthetic reservoir model was constructed to replicate one of the case histories of a tight oil reservoir studied by Fetkovich et al. (1987). Various sensitivities on permeability, skin, reservoir radius, and fluid type were performed to ensure the validity and generality of the model. The application of Fetkovich type curves was then investigated for three gas condensate fluids with various richness levels. Here, implications and limitations of this extension are highlighted when various reservoir parameters (i.e., skin and reservoir radius) are varied.
Our results demonstrate that Fetkovich type curves can be used to derive reservoir parameters for tight reservoirs, but caution needs to be taken for different fluid types and production constraints. For dry gas, the Fetkovich method can directly be applied. In gas condensate systems, this method together with the gas equivalent concept gives reliable results if bottomhole pressure is above the dewpoint. Extension of this approach when bottomhole pressure is below the dewpoint, leads to erroneous results. If the preferred two-phase pseudo-pressure approach is considered in the interpretation; the results are more accurate but still not fully acceptable. The findings of this study allow better evaluation of production potentials and improved management of unconventional gas reservoirs.

Introduction
Holditch and Madani (2010) highlighted that worldwide gas resources from conventional reservoirs are diminishing with unconventional reservoirs playing more important roles in sustaining production to satisfy energy consumption demand. They classified unconventional gas resources into the three most common types: tight sands, coalbed methane, and shale gas. It was discussed in their work that tight gas is expected to be the greatest contributor in terms of fulfilling the production capacity in the near future.
Generally, there is no specific definition for a tight gas reservoir. Naik (2013) gave examples of authors that referred to tight gas with different cutoffs. Holditch (2006) proposed a definition as a reservoir that cannot deliver natural gas at economic rate nor reach the ultimate economic recovery efficiency unless the productivity is enhanced by the use of reservoir stimulation or adapting the concept of horizontal or multilateral wellbores. It was discussed in his work that higher investment cost and more complicated technology are required to develop tight gas reservoirs. Hence, to optimize the development, a proper knowledge of the reservoir parameters (permeability, skin, etc.) is one of the keys for success. This knowledge could be acquired from either conventional well testing, which is based on the analysis of transient pressure data, or decline curve analysis, which is focused on the transient rate data.