Gas recovery potential of sandstones from tight gas reservoirs Z. Duan a,b,c , C.A. Davy a,b,c,n , F. Agostini a,b,c , L. Jeannin d , D. Troadec e , F. Skoczylas a,b,c a Université Lille Nord de France, F-59000 Lille, France b LML, CNRS, UMR 8170, F-59650 Villeneuve d’Ascq, France c Ecole Centrale de Lille, CS20048, F-59651 Villeneuve d'Ascq cedex, France d GDF/Suez E&P International SA, 1 place Samuel de Champlain, 92930 Paris La Défense cedex, France e IEMN, UMR CNRS 8520, Avenue Poincaré, BP60069, 59652 Villeneuve d’Ascq Cedex, France article info Article history: Received 26 July 2012 Received in revised form 20 September 2013 Accepted 24 November 2013 Available online 25 December 2013 Keywords: Sandstones Poro-mechanics Pore volume variation Partial saturation Gas permeability Microstructure abstract The aim of our experimental study is to characterize experimentally the petro-physical properties of a set of sandstones originating from different depths from a single tight gas field, in order to improve our knowledge on their gas recovery potential. The initial characterization of these sandstones is performed in the dry state, and in the “as received” states: porosity, initial water saturation level, and gas permeability at a confining pressure of 5 MPa. For two different samples under increasing confining pressure, we have evaluated the water saturation threshold, above which no more gas passes through the porous network, owing to hydraulic cut-off, to be about 63–68%. Then, at intermediate saturation level (on the order of 40%), two different sample types were identified, depending on whether their relative gas permeability is affected, or not, by confining pressure. For one sample of each type, poro-elastic experiments show that the variation in normalized matrix bulk modulus (due to confining pressure increase) may be attributed to the closure of portions of the connected pore network. Finally, to ascertain the above, a dedicated test was designed to measure the pore volume changes under confinement, simultaneously to volumetric strains. Whenever pore volume variation is significantly higher than volumetric strains, pore trapping is identified; otherwise, microstructure changes are mainly attributed to crack closure. & 2013 Elsevier Ltd. All rights reserved. 1. Introduction So-called tight gas reservoirs are constituted of low permeability sandstones, whose petro-physical properties may hinder proper gas recovery [1]. They have a low absolute permeability (below 0.1 mD i.e. 10 16 m 2 under ambient conditions), a connected poro- sity lower than 10%, and a strong sensitivity to in situ stresses compared to conventional reservoirs [2–8]. Moreover, a large transition zone related to capillary pressure effects is observed in situ. This zone, where partial water saturation is greater than irreducible water saturation, may extend over several hundreds of metres above the free water table. Inside this zone, the effective gas permeability is strongly reduced compared to absolute gas permeability and neither gas nor water may be sufficiently mobile for industrial extraction; it is the permeability jail [9], which is also called hydraulic cut-off. Our contribution is focused on sandstones cored at different depths from a single well of a tight gas field in Algeria [10]. Our aim is to determine whether they are prone to the permeability jail. First, we characterize the initial petro-physical properties of the different available sandstones: these are mainly their initial water saturation level, water porosity, and dry and relative gas permeability at low confinement [11]. The initial microstructure is assessed by Scanning Electron Microscopy (SEM), Energy Disper- sive X-ray Spectroscopy (EDS) analysis and FIB/SEM (Focused Ion Beam/SEM) imaging [14,15]. Second, at low confining pressure (on the order of 5 MPa), an experimental study is performed, in order to identify the actual water saturation level corresponding to the permeability jail, i.e. when gas no longer significantly crosses the pore network. Further, we investigate the gas transport sensitivity of these sandstones to confining pressure (P c ) changes at an intermediate water saturation level (35–46%), by measuring the evolution of gas relative permeability with P c . This sensitivity to external stress is interpreted as being due to changes in sandstone microstructure, i.e. either micro-crack closure or pore trapping. It is also investi- gated by using poro-mechanical experiments [12,13]. These pro- vide insights into the relationship between external confining stress and (1) drained bulk modulus K b and (2) normalized solid matrix bulk modulus K s . Finally, a dedicated test, which is similar to a gas pycnometric experiment [19], is performed to measure directly the change in Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/ijrmms International Journal of Rock Mechanics & Mining Sciences 1365-1609/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijrmms.2013.11.011 n Corresponding author. E-mail address: catherine.davy@ec-lille.fr (C.A. Davy). International Journal of Rock Mechanics & Mining Sciences 65 (2014) 75–85