Limitations for Compositional Modeling in Vaca Muerta Numerical Simulation

Abstract

This article, written by JPT Technology Editor Judy Feder, contains highlights of paper SPE 191822, “Vaca Muerta Numerical Simulation: A Warning in Compositional Modeling,” by Sergio Bosco, Mariano Suarez, SPE, and Elisabet Savoy, SPE, YPF, prepared for the 2018 SPE Argentina Exploration and Production of Unconventional Resources Symposium, Neuquén, Argentina, 14–16 August. The paper has not been peer reviewed. Vaca Muerta formation fluids run from medium black oils to very dry gas, according to source-rock maturity. High-shrinkage volatile oil and very rich gas condensate are found and compositional variation is areally continuous. Despite this huge difference in fluid nature, where saturation pressures are present, more than 200 kg/cm2 undersaturation is reported. Because of the complex dependence of fluid behavior with composition for near-critical gas condensate and volatile oil, the use of compositional modeling is mandatory. Cubic equations of state (CEOS) are the only types implemented until now in commercially available compositional numerical simulators. In this paper, the authors show the limitation of CEOS for modeling reservoir behavior of liquid-phase black and volatile oil in highly undersaturated reservoirs. Introduction The Vaca Muerta shale, located in the Neuquén Basin, Argentina, ranks among the top shale oil and gas resources in the world. It is characterized by its areal extent, high gross thickness, and high organic content. The first well was drilled and completed in 2010 and despite more than 650 wells to date, Vaca Muerta shale history is relatively recent compared with other known shale plays. Source-rock thermal maturity seems to be the main driver for fluid distribution, running from dry gas in very mature zones, progressing through gas condensate and volatile oil, to black oils of less than 100 m3/m3 gas/oil ratio (GOR) in immature zones. Production forecasts on new wells are based primarily on the history of geographically close wells, but forecasts for new zones involve numerical simulation of regionally distributed properties, including reservoir-fluid characteristics. The original aim of the work described in this paper was the equation of state (EOS) description of the pressure/volume/temperature (PVT) properties of fluids in known zones, to use in extrapolation to those zones where information is sparse. The models and work flows described in papers SPE 177058 and URTeC 2436107 were the starting point for this paper; in those earlier works, the authors aimed to extend the work flow into more complex fluids. An extensive EOS fitting program was launched. Surface and subsurface samples were taken for fluid characterization and PVT tests were performed for each. All presented, at least, 200 kg/cm2 of undersaturation (Fig. 1).