Abstract
Thousands of horizontal wells are drilled into the shale formations across the U.S. and natural gas production has substantially increased during past years. This fact is accredited to the fast progresses obtained in hydraulic fracturing and pad drilling technologies. The contribution of shale rock surface desorption to production is widely accepted and confirmed by the industry experts often observed through laboratory and field evidences. Nevertheless, the subsequent changes in porosity and permeability due to methane desorption combined with hydraulic fracture closures due increased net effective rock stress state, have not been captured in current shale gas modeling and simulation. Hence, it is very critical to investigate the effects of induced permeability, porosity, and stress by methane desorption on gas production.
We have developed a numerical model to study the effect of changes in porosity, permeability and compaction on four major U.S. shale formations considering their Langmuir isotherm desorption behavior. These resources include; Marcellus, New Albany, Barnett and Haynesville shales. First, we introduced a model that is a physical transport of single-phase gas flow in shale porous rock. Later, the governing equations are implemented into a one-dimensional numerical model and solved using a fully implicit solution method. It is found that the natural gas production is substantially affected by desorption-induced porosity\permeability changes and geomechanics. This paper provides valuable insights into accurate modeling of unconventional reservoirs and consequently leads to a significant change in future production predictions which might enormously contribute to the U.S. economy.
1. INTRODUCTION
Recoverable reserves of shale gas in the U.S. are estimated to be 862 Tcf [1]. Although challenges associated to exploration and management of shale assets are yet to be resolved, decreased evaluated risk promises a secure gas supply for next decades. The large accumulation of gas shale formations might serve as both a hydrocarbon source and a productive reservoir. Most of the gas is stored in organic-rich rock while a minor fraction of gas in place is in pore spaces [2]. Extremely low matrix permeability as well as highly complex network of natural fractures are challenging characteristics of shale formations. Permeability of shale rocks is estimated to be between 50 nD (nano-Darcy) to 150 nD [3]. Recent advances and innovations in hydraulic fracturing are key success of shale gas economic production as a viable global energy supply. Nevertheless, complexities associated with flow mechanisms and existence of many pressure dependent phenomena, such as combined hydraulic and natural fracture conductivity losses, Klinkenberg gas slippage effect, desorption/adsorption and Darcy/non-Darcy flow, are not yet completely understood and need more extensive studies and modeling in order to meet our industry needs. In this study, desorption-induced porosity and permeability changes of shale matrix as well as closure effect of hydraulic fractures are focused in detail to evaluate their impact on production form four very productive U.S. shale resources
