energies Article An Extensive Study on Desorption Models Generated Based on Langmuir and Knudsen Diffusion Hamda Alkuwaiti * , Hadi Belhaj , Mohammed Aldhuhoori, Bisweswar Ghosh and Ryan Fernandes   Citation: Alkuwaiti, H.; Belhaj, H.; Aldhuhoori, M.; Ghosh, B.; Fernandes, R. An Extensive Study on Desorption Models Generated Based on Langmuir and Knudsen Diffusion. Energies 2021, 14, 6435. https:// doi.org/10.3390/en14196435 Academic Editor: Dameng Liu Received: 29 August 2021 Accepted: 25 September 2021 Published: 8 October 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Department of Petroleum Engineering, College of Engineering, Khalifa University of Science and Technology, Abu Dhabi, United Arab Emirates; hadi.belhaj@ku.ac.ae (H.B.); 100051985@ku.ac.ae (M.A.); bisweswar.ghosh@ku.ac.ae (B.G.); ryan.fernandes@ku.ac.ae (R.F.) * Correspondence: 100052986@ku.ac.ae Abstract: Although gas desorption is a known phenomenon, modeling fluid flow in tight gas reservoirs often ignores the governing desorption effect, assuming that viscous transport is the predominant controller, resulting in an erroneous prediction of mass transport and fluid flow cal- culations. Thus, developing a new model accommodating all the major contributing forces in such a medium is essential. This work introduces a new comprehensive flow model suitable for tight unconventional reservoirs, including viscous, inertia, diffusion, and sorption forces, to account for fluid transport. Based on Langmuir law and Knudsen diffusion effect, three models were generated and compared with different known models using synthetic data. The model was solved and ana- lyzed for different scenario cases, and parametric studies were conducted to evaluate the desorption effect on different reservoir types using MATLAB. Results show that the contribution of the sorption mechanism to the flow increases with the reducing permeability of the medium and lower viscosity of the flowing fluid and an additional pressure drop up to 10 psi was quantified. Keywords: unconventional reservoirs; desorption; modeling; fluid flow 1. Introduction Unconventional reservoirs embody the upcoming revolution in the oil and gas in- dustry due to the rapid growth of world energy demand anticipated by the United States Energy Information Association Outlook [1] and the continuous draining of the current conventional hydrocarbon resources. The increase in demand has led to an aggressive investigation of unconventional accumulations that were once shelved due to production complexities to be considered for exploring opportunities. Unconventional reservoirs are reservoirs with divergent geological characteristics [2], inconstant geochemical characteris- tics, intricate petrophysical properties, eccentricities in fluid phase behavior, and various governing flow mechanisms/forces [3]. Unconventional reservoirs include various types of reservoirs including gas hydrates, tight gas and oil, heavy oil, and shale oil and gas. In their natural form, gas hydrates consist of water molecules (ice molecules) that act as hosts [4]. In contrast, guest molecules are methane, propane, isobutene, and ethane, among other gases that are chemically bonded with van der Waals forces in low degrees of temperature and, at the same time, under high pressure. Natural hydrate gas is primarily white and has the same appearance as ice. Natural hydrates are commonly known to have methane, which can burn, thus having the name “fire ice”. Natural gas hydrates have long been deemed a nuisance for blocking transmission pipes, endangering the foundations of deep-water platforms and pipelines, and risking more disruption to the output of deep- water oil and gas [5], maybe a significant possible source of energy in the future. In the permafrost and beneath the ocean floor, huge deposits of natural gas hydrates are widely dispersed [6]. Geologically, shale is the source rock where oil was originated and trapped with part migration to the reservoir rock where migration path was available. Shale oil is Energies 2021, 14, 6435. https://doi.org/10.3390/en14196435 https://www.mdpi.com/journal/energies