Contents lists available at ScienceDirect Journal of Natural Gas Science and Engineering journal homepage: www.elsevier.com/locate/jngse Sensitivity of unsteady-state gas-water relative permeability to experimental artefacts and interpretation techniques; case study from a gas reservoir in south Iran Milad Farahani a , Hamed Aghaei b,∗ , Syed Reza Asadolahpour c a Special Core Analysis Lab, PetroMakhzan Kav (PMK), Tehran, Iran b Digital Rock Physics Research Group, School of Chemical, Petroleum and Gas Engineering, Shiraz University, Fars, Iran c Reservoir Engineering System, Petroleum Engineering Dept., National Iranian South Oil Company (NISOC), Ahvaz, Iran ARTICLE INFO Keywords: Gas relative permeability End-efect Gas slippage Anhydrite dissolution History matching ABSTRACT Relative permeability is a key determinant of how a reservoir simulation model performs. Although laboratory measurement techniques for two-phase relative permeability are well-established, but still the difculties and uncertainties persist. The present study aims to investigate the efect of several parameters, including pressure diference, end-efects, interpretation techniques (analytical and numerical), and mineral dissolution, on the unsteady-state gas-water relative permeability curves. For this purpose, a number of precisely planned experi- ments were carried out on two subsurface anhydrite dolomite samples collected from a well-known gas reservoir in south Iran. In summary, the measured gas relative permeability (k rg ) was underestimated at low injection pressures mainly due to the capillary end-efect and a signifcantly low volume of brine production in compare to the considerable volume that was retained at the end face. Moreover, despite the end-efect was overcame at high injection pressures, but signifcantly high k rg values were recorded at water saturations close to the residual value and the measurements were corrected considering a gas slippage factor at the endpoint. Furthermore, greater wetting phase relative permeability (k rw ) values were driven from numerical (history matching) interpretation of the results compared with the analytical approach that could be due to the automatic end-efect correction by the numerical techniques where the capillary pressure is accounted for. Finally, to investigate the efect of anhydride dissolution on overestimation of the measured gas relative permeability, the interaction of NaCl brine with anhydride was reviewed using the Computed Tomography (CT) images taken before and after the test. 1. Introduction Relative permeability, being one of the most crucial properties of a reservoir rock-fuid unit, is defned as the measurement of ability of a medium to conduct one fuid in the presence of others, and can be determined through the interplays of porosity, permeability, wett- ability, heterogeneity, fuid saturation, and saturation history. Today, several well-established laboratory techniques exist for two- phase relative permeability measurement mainly based on the fow experiments. However, the steady-state (SS) and unsteady-state (USS) relative permeability measurements are by far the two most common approaches compared with the other reported methods, such as, cen- trifuge and Digital Rock Physics (DRP). The USS technique is primarily based on interpretation of the data obtained during an immiscible displacement process (Dandekar, 2013) where the displacing phase (typically water or gas) is injected, either at constant pressure or constant rate, up to a predefned cessation cri- terion. Next, it will be following by measurement of the fnal end-point efective permeability to the displacing phase at residual saturation of the displaced phase. Generally, a constant pressure is preferred in the gas-liquid systems as it provides the chance for a precise control over the pressure. In a gas-water mode, the gas (N 2 ) is pressed through a water-satu- rated core sample and the gas-water relative permeability will be esti- mated based on volumes of the injected gas and the produced water. The Buckley-Leverett equation, as modifed by Welge, provides the foundation for calculation of the unsteady-state relative permeability where the gas relative permeability can be determined from the https://doi.org/10.1016/j.jngse.2019.102998 Received 8 February 2019; Received in revised form 6 August 2019; Accepted 5 September 2019 ∗ Corresponding author. E-mail address: aghaei.hamed@gmail.com (H. Aghaei). Journal of Natural Gas Science and Engineering 71 (2019) 102998 Available online 09 September 2019 1875-5100/ © 2019 Elsevier B.V. All rights reserved. T