*Review* **Review of Formation and Gas Characteristics in Shale Gas Reservoirs**

#### **Boning Zhang 1, Baochao Shan 2,\*, Yulong Zhao 1 and Liehui Zhang 1**


Received: 3 June 2020; Accepted: 14 October 2020; Published: 17 October 2020

**Abstract:** An accurate understanding of formation and gas properties is crucial to the e fficient development of shale gas resources. As one kind of unconventional energy, shale gas shows significant di fferences from conventional energy ones in terms of gas accumulation processes, pore structure characteristics, gas storage forms, physical parameters, and reservoir production modes. Traditional experimental techniques could not satisfy the need to capture the microscopic characteristics of pores and throats in shale plays. In this review, the uniqueness of shale gas reservoirs is elaborated from the perspective of: (1) geological and pore structural characteristics, (2) adsorption/desorption laws, and (3) di fferences in properties between the adsorbed gas and free gas. As to the first aspect, the mineral composition and organic geochemical characteristics of shale samples from the Longmaxi Formation, Sichuan Basin, China were measured and analyzed based on the experimental results. Principles of di fferent methods to test pore size distribution in shale formations are introduced, after which the results of pore size distribution of samples from the Longmaxi shale are given. Based on the geological understanding of shale formations, three di fferent types of shale gas and respective modeling methods are reviewed. Afterwards, the conventional adsorption models, Gibbs excess adsorption behaviors, and supercritical adsorption characteristics, as well as their applicability to engineering problems, are introduced. Finally, six methods of calculating virtual saturated vapor pressure, seven methods of giving adsorbed gas density, and 12 methods of calculating gas viscosity in di fferent pressure and temperature conditions are collected and compared, with the recommended methods given after a comparison.

**Keywords:** shale gas reservoir; geology; Gibbs excess adsorption; supercritical adsorption; gas viscosity
