Modeling Reservoir Connectivity and Tar Mat Using Gravity-Induced Asphaltene Compositional Grading Sai R. Panuganti, † Francisco M. Vargas, ‡ and Walter G. Chapman* ,† † Department of Chemical and Biomolecular Engineering, Rice University, Houston, Texas 77005, United States ‡ Department of Chemical Engineering, The Petroleum Institute, Abu Dhabi, United Arab Emirates ABSTRACT: Reservoir compartmentalization is one of the major issues on both on- and offshore reservoirs. High capital costs are involved especially in deepwater exploration and production, making it essential to assess prior to production the extent of compartmentalization within a reservoir. Within a continuous reservoir, fluid properties vary with depth because of compositional grading. Considerable fluid flow is required to attain thermodynamic equilibrium yielding compositional gradients, suggesting connectivity. In this work, an algorithm that makes use of the perturbed-chain statistical associating fluid theory (PC-SAFT) equation of state (EoS) is proposed to address the isothermal asphaltene compositional grading in a uniform gravitational field. The model is validated against well log and production data. The results are compared to field data to evaluate the reservoir compartmentalization. An approximate analytical solution for asphaltene compositional grading, derived from solution thermodynamics, is also presented. Asphaltene compositional grading under extreme cases can lead to tar-mat formation. The PC-SAFT asphaltene compositional grading introduced in this study is extended to model the possibility of a tar-mat formation because of gravitational segregation of asphaltene. 1. INTRODUCTION Asphaltene are defined as the fraction of a crude oil that dissolves in toluene and is insoluble in n-heptane or n-pentane. Modeling asphaltene thermodynamic behavior has a potential application in understanding reservoir connectivity. Compart- mentalization arising because of different factors, such as impermeable layers and faults, leads to the lack of reservoir connectivity. This work proposes a combined solution to the individually ill-posed problems of reservoir compartmentaliza- tion and tar-mat occurrence. Tar mat predominantly represents a non-producible oil in place and low-permeability intra- reservoir flow barrier. Compartmentalization is one of the major problems in both on- and offshore reservoirs. It makes recovery more difficult because of poor drainage for a given number of wells. High capital costs are involved, especially in deepwater, making it essential to assess prior to production the extent of compartmentalization within a reservoir. Seismic data and discontinuous variation of fluid properties (e.g., chemistry, density, and viscosity) are commonly used in an attempt to assess the level of compartmentalization. The varying fluid properties along the depth of a reservoir are due to compositional grading. 1 In oil columns, considerable fluid flow is required to reach thermodynamic equilibrium, yielding such compositional gradients, and, thus, suggests reservoir connectivity better than pressure communication where little fluid flow is required. 2 In particular, reservoir connectivity can be best understood on the basis of continuous characteristics of the asphaltene compositional gradient, because to equilibrate asphaltene, the heaviest component of crude oil with by far the least mobility necessitates substantial permeability. 3 The continuous variations in the gas/oil ratio (GOR) and composition have been reported by industries for continuous reservoirs through significant ranges in depth. 4 The continuous variation is accounted for thermodynamic equilibrium and, thus, connectivity of a reservoir. 5 In general, there is a greater GOR during production from the upper parts of a formation. The analysis by Hoier and Whitson 6 accounts for the GOR and composition of equilibrium fluids as measured by the downhole fluid analysis (DFA). DFA also measures crude oil optical density (OD) using infrared light. This OD is directly proportional to the asphaltene content. 7 However, Whitson or others did not analyze compositional grading related to asphaltene because of limited knowledge of asphaltene prior to 2000. In this work, an algorithm that makes use of the perturbed-chain statistical associating fluid theory (PC-SAFT) equation of state (EoS) is proposed to address the isothermal asphaltene compositional grading. The crude oils A and B studied here are characterized at downhole conditions using the PC-SAFT characterization methodology developed by Rice University and The Petroleum Institute. In their PC-SAFT characterization paper, it was showed that parameters estimated at a particular operating condition work well to predict the liquid-liquid/vapor equilibrium at other operating conditions, including a change of system composition. 8 PC-SAFT EoS, which has been successfully applied for asphaltene precipitation in flow assurance, is for the first time extended to model asphaltene distribution in oil columns. In the present work, the possible changes in asphaltene composition with depth, which could result from the attainment of thermodynamic equilibrium in a uniform gravitational field, are modeled by PC-SAFT and compared Special Issue: 12th InternationalConference on Petroleum Phase Behaviorand Fouling Received: August 24, 2011 Revised: November 24, 2011 Published: November 28, 2011 Article pubs.acs.org/EF © 2011 American Chemical Society 2548 dx.doi.org/10.1021/ef201280d | Energy Fuels 2012, 26, 2548-2557