Abstract
Gas transport in unconventional shale strata is a multi-mechanism-coupling process that is different from the process observed in conventional reservoirs. In micro fractures which are inborn or induced by hydraulic stimulation, viscous flow dominates. And gas surface diffusion and gas desorption should be further considered in organic nano pores. Also, the Klinkenberg effect should be considered when dealing with the gas transport problem. In addition, following two factors can play significant roles under certain circumstances but have not received enough attention in previous models. During pressure depletion, gas viscosity will change with Knudsen number; and pore radius will increase when the adsorption gas desorbs from the pore wall. In this paper, a comprehensive mathematical model that incorporates all known mechanisms for simulating gas flow in shale strata is presented. The objective of this study was to provide a more accurate reservoir model for simulation based on the flow mechanisms in the pore scale and formation geometry. Complex mechanisms, including viscous flow, Knudsen diffusion, slip flow, and desorption, are optionally integrated into different continua in the model. Sensitivity analysis was conducted to evaluate the effect of different mechanisms on the gas production. The results showed that adsorption and gas viscosity change will have a great impact on gas production. Ignoring one of following scenarios, such as adsorption, gas permeability change, gas viscosity change, or pore radius change, will underestimate gas production.
Highlights
Due to the increasing energy shortage in recent years, gas production from shale strata has played an increasingly important role in the volatile energy industry of North America and is gradually becoming a key component in the world’s energy supply [1, 2]
Firstly we investigated the impact of parameters such as the initial reservoir pressure, matrix permeability, fracture permeability, matrix porosity, and fracture porosity on shale gas production
This paper presents a theoretical model and mechanism study for shale gas production
Summary
Due to the increasing energy shortage in recent years, gas production from shale strata has played an increasingly important role in the volatile energy industry of North America and is gradually becoming a key component in the world’s energy supply [1, 2]. Shale strata or shale gas reservoirs (SGR) are typical unconventional resources with a critically low transport capability in matrix and numerous “natural” fractures. Core experiments have shown that 90% of shale bedrock permeability are less than 150 nd, and diameters of the pore throat are PLOS ONE | DOI:10.1371/journal.pone.0143649. Modeling of Gas Production from Shale Reservoirs authors' adherence to all the PLOS ONE policies on sharing data and materials, as detailed online in the guide for authors. Natural gas has been stored in SGRs in three forms [2]: (a) as free gas in the micro fractures and nano pores; (b) as dissolved gas in the kerogen [4]; and (c) as adsorption gas in the surface of the bedrock. About 20%~85% of gas in SGRs is stored in form [5]
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