SPE 160889 Transient Pressure Analysis of Gas Wells in Unconventional Reservoirs

Unconventional gas reservoirs have shown the potential to be the next world primary energy resource and have received a lot of attention in the past few years in the world. Even with the significant progress that has been made world-wide towards commercial development of natural gas from these unconventional resources, understandings or modeling tools of gas flow in these extremely low-permeability reservoirs are far behind the industry needs. In this paper, we present a numerical model for modeling transient gas flow behavior and its application to well testing analysis in unconventional gas reservoirs. Specifically, the numerical model simulates realistic reservoir gas flow processes, including Klinkenberg effect, non-Darcy flow behavior, and adsorption under reservoir flow condition, in addition to wellbore storage and skin effect. We use the numerical model to generate various type-curves for nonlinear gas flow for well testing analysis in unconventional porous and fractured reservoirs and demonstrate the conventional well testing analysis approach may still be applicable for analysis of unconventional gas reservoirs. Introduction This paper presents our continual effort in developing simulation models and tools for quantitative studies of unconventional gas reservoirs (Wu and Fackahroenphol, 2011; Wu et al. 2012). Specifically, we explore the possibility of performing well testing analysis using a numerical modeling approach. The numerical model simulates realistic unconventional reservoir gas flow processes, including Klinkenberg effect, non-Darcy flow behavior, and adsorption under single-phase and two phase flow condition. In addition, we also include wellbore storage and skin effect in gas production wells in the pressure transient analysis. We use the numerical model to generate various type-curves for nonlinear gas flow for well testing analysis in unconventional porous and fractured reservoirs. Even though significant progresses has been made towards commercial development of gas and oil from these unconventional resources (Denny, 2008; Bybee, 2008; King, 2010) in the past few years, characterization of gas flow in unconventional gas reservoirs, such as flow in shale gas or tight gas formations, remain a challenge. This is primarily because there exist so many co-existing nonlinear processes, Klinkenberg effect (Klinkenberg, 1941), nonlinear adsorption/desorption (Silin and Kneafsey, 2011), strong interactions between fluid (gas and water) molecules and solid materials within tiny pores as well as severe heterogeneity on any scales of unconventional formations, such as various scaled fractures. As a result, gas flow through unconventional reservoirs is more nonlinear, compared to that in conventional gas reservoirs, even under singlephase flow condition. Therefore well testing analysis for gas wells in unconventional gas reservoirs cannot rely entirely on traditional, analytical based analysis approaches and may have to be carried out by a numerical approach in general. This paper describes application of a two-phase flow model for simulating both gas and water flow in shale gas and tight gas reservoirs (Wu et al. 2012), in which single-phase gas flow is handled as a special case and used for simulation of transient flow and well testing analysis in this work. The unconventional gas flow model incorporates many of the important nonlinear gas physical processes in unconventional reservoirs, such as non-Darcy flow, Klinkenberg effect, and nonlinear sorption. The main outcome of the work is to provide a series of type curves for transient pressure responses in gas production wells for reservoir engineers to use in well testing analysis in unconventional reservoirs. The results can be used for quantitative studies of transient gas flow behavior and effects of non-linear flow or gas production processes.