August Newsletter. Note from the Director

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1 August Newsletter Issue 2, August 2014 Fifth Annual Berg-Hughes Symposium The Berg-Hughes Center for Petroleum and Sedimentary Systems with participation from the Crisman Institute for Petroleum Research invites you to the Fifth Annual Bergh-Hughes Symposium from 8:00 a.m. to 5:00 p.m. on Friday, September 26, In this issue: Fifth Annual Berg- Hughes Symposium 1 The symposium will be held in the Annenberg Presidential Conference Center, which sits next to the George Bush Presidential Library on the West Campus of Texas A&M. The keynote speaker will be Dr. Kevin M. Bohacs of the ExxonMobil Upstream Research Company. Note from the Director 1 Nano-Scale Modeling of Unconventional Gas and Liquids-Rich Reservoir Systems 2 Highlights of the program include: Basin Modeling, Carbonate Reservoirs, Industry- Related Geophysics, and Unconventional Resources. If you are interested in attending, please RSVP to Dawn Spencer (dspencer@geos.tamu.edu) by September 19, Nanopore Confinement Effects on Multi- Component Phase Behavior in Wet-Gas Shale Reservoir System Upcoming Crisman Meeting 6 8 Note from the Director Feedback on the Fall 2014 Crisman proposals was due back to me by August 13. We are now in the process of sorting through the responses received and ranking them in order of preference. I expect we will be able to fund 6 to 8 proposals (of the 18 submitted), depending on the project length and the number of students required for each top proposal selected. The new projects will be announced as soon as possible. Thank you again to all who submitted proposals, provided feedback, and participated in this evaluation process. Bob 1

2 Nano -Scale M odeling of Unconventional Gas and Liquids -Rich Reservoir Systems Nano-Scale Modeling of Unconventional Gas and Liquids -Rich Reservoir Systems Advisors Thomas Blasingame t-blasingame@tamu.edu George Moridis george.moridis@pe.tamu.edu Student Termpan Pitabunkate Introduction Shale reservoirs are unconventional reservoirs that play an important role as resources of future energy in the U.S. Some studies have been done in the oil and gas industry to describe how hydrocarbon is stored and flows through ultra-small pores in the shale reservoirs. Most of these studies were derived by borrowing and modifying techniques used for sand or conventional reservoirs. The common pore size distribution of the shale reservoirs is approximately 1-20 nm (Clarkson et al. 2011). In such a confined space, interaction between the wall of the container (i.e., shale and kerogen) and the contained fluid (i.e., hydrocarbon) becomes significant to the fluid s behavior. Orientation and distribution of fluid molecules in the confined space are different from those of the bulk fluid. This can cause the change of fluid properties such as critical temperature and pressure, diffusion coefficient and viscosity. This study will provide a detailed study of the changes of PVT properties and flow behavior of fluid in the shale reservoirs. Mechanistic studies and development of model-based forecasting and EUR prediction for liquids-rich shale system are also included in this project. Objectives Characterize/model hydrocarbon phase behavior at the nano-/pico-volume scales. Characterize/model multiphase flow (gas, condensate/oil, and water) at the nano-/ pico-volume scales. Provide illustrative (model-based) mechanistic studies for gas and liquids-rich shale systems. Develop and demonstrate model-based forecasting and EUR prediction for liquids-rich shale system. Approach Grand Canonical Monte Carlo (GCMC) and Molecular Dynamics (MD) simulations are used to study molecular fluid behavior and thermodynamic properties of hydrocarbon in confined spaces such as pores in shale reservoirs. GCMC simulation is mainly used to determine the number of molecules accommodating in the system whose chemical potential equals that of the given bulk system s at constant pressure and temperature. The primary outcomes from the GCMC simulations are isotherms of pure components and mixtures of hydrocarbons in confined space. As a result, the fluid thermodynamic properties such as vapor pressure, critical pressure and temperature and phase diagram can be yielded. The MD simulation is based on the integration of classical mechanics equations (Newton s second law) for a system where molecules interact according to interatomic potentials that are functions of their relative positions. The outcome of the simulation is a trajectory of molecular configurations (positions and velocities of each molecule). From such trajectory, using statistical thermodynamics, all the thermodynamic properties and some transport properties can be evaluated. The main purposes of employing the MD simulation are to help understand how fluid molecules move in this system and to derive fluid dynamics properties (i.e., diffusion (Continued on page 3) 2

3 coefficient and viscosity). In addition, it can be used to confirm structural properties of the PVT state points calculated from the GCMC simulations. A slit graphite pore is used to represent the pore inside kerogen in shale reservoirs as illustrated in Fig. 1. The separation between the two graphene sheets is varied to study the effect of confinement on the PVT properties of hydrocarbon mixtures. Accomplishments Fig. 1 (a) STM image of Haynesville shale (Curtis et al. 2010), (b) 3D molecular structure of kerogen (Vandenbroucke 2003) and (c) molecular model representing kerogen in shale reservoirs Tables 1 and 2 summarize the critical temperatures and pressures of confined methane and ethane in different pore sizes. It is noticeable that for the nano-scale pore size, the fluid critical properties change with the pore size and become different from the bulk properties. The smaller the pore is, the lower both critical temperature and pressure of the confined methane are. The effect of the interaction between the surface wall and the confined fluid becomes less significant when the pore size increases and the fluid critical properties approach the bulk properties. Fig. 2 Comparison of bulk methane-ethane mixture phase diagram (a) and that of the confined mixture (b) Figs 2a and 2b illustrate a bulk phase diagram of a bulk methane -ethane mixture derived from experimental data from Bloomer Remark: Phase transition cannot be observed when H>7.0 nm due to numerical problem (Continued on page 4) 3

4 et al. (1953) and the confined mixture in a slit graphite pore with 5.0 nm of separation, respectively. It can be observed that at the same temperature, the shapes and magnitudes of the bulk and confined phase envelopes change dramatically because of the restricted environment in kerogen pores of shale reservoirs. Furthermore, the subsequent shift of the critical locus curve of the mixture is in the direction of lower critical temperature and pressure, as similarly observed with the pure components. Fig. 3 Comparison of bulk and confined methane densities at the (a) minimum and (b) maximum Eagle Ford shale reservoir temperatures Figs. 3 and 4 are examples of comparisons of bulk and confined methane and ethane densities at the Eagle Ford shale reservoir conditions (Orangi et al. 2011) as summarized in Table 3. According to the results, it can be observed that the confined pure component of methane and ethane densities can deviate from their bulk properties up to 69.8% and 35.5%, respectively. As a result, significant errors may develop when the bulk properties are used for reserves estimation and production forecast for shale reservoirs. Collaborations This study was done in collaboration with Prof. Perla B. Balbuena, Department of Chemical Engineering at Texas A&M University. Significance Fig. 4 Comparison of bulk and confined ethane densities at the (a) minimum and (b) maximum Eagle Ford shale reservoir temperatures This research will help us better understand how fluids behave in shale reservoirs. Consequently, more accuracy of production (Continued on page 5) 4

5 forecast, EUR prediction, and hydrocarbon in-place can be yielded. Future Work Continue working to derive the critical properties of n-alkane hydrocarbon pure components. Continue working on finding phase envelops of hydrocarbon mixtures in different pore sizes. References and Related Publications Bloomer, O.T., Gami, D.C., and Parent, J.D Physical-Chemical Properties of Methane-Ethane Mixtures. Research Bulletin, Institute of Gas Technology 22. Curtis, M.E., Ambrose, R.J., Sondergeld, C.H., and Rai, C.S Structural Characterization of Gas Shales on Micro- and Nano-Scales. Paper SPE presented at the Canadian Unconventional Resources and International Petroleum Conference, Calgary, Alberta, Canada, October. Orangi, A., Nagarajan, N.R., Hanapour, M.M., and Rosenzweig, J Unconventional Shale Oil and Gas-Condensate Reservoir Production, Impact of Rock, Fluid, and Hydraulic Fractures. Paper SPE presented at the SPE Hydraulic Fracturing Technology Conference and Exhibition, The Woodlands, Texas, USA, January. Vandenbroucke, M Kerogen: from Types to Models of Chemical Structure. Oil & Gas Science Technology Rev. IFP 58 (2):

6 Nanopore Confinement Effects on Multi - Component Phase Behavior in Wet -Gas Shale Reservoir System Nanopore Confinement Effects on Multi-Component Phase Behavior in Wet-Gas Shale Reservoir System Advisors Yucel Akkutlu akkutlu@pe.tamu.edu William McCain bill.mccain@pe.tamu.edu Student Behnaz Rahmani Didar Introduction Previously, we studied the behavior of single component gas molecules in slitpores of various size, temperature and pressure. We have now expanded our work into multicomponent fluids and studied binary mixtures of methane/n-butane in nanopores. Our newest simulations show that at temperature and pressures that we deal with, the binary mixture of methane/n-butane, when confined to a certain pore size and smaller, will not contain any gas for the study of phase changes. However, in larger pores (>4nm) we observe phase diagram shifts from the bulk. There exists an upper limit to the pore size, larger from which phase diagram does not shift significantly. We will report these phase diagrams and this upper limit. Objectives In this work, with the help of molecular simulations, we focus on multi-component fluids with various compositions and pore sizes, and study their phase behavior to find their critical parameters in each condition. The idea is to correlate these critical parameters to those of their constituent single components and therefore develop a mixing rule for these mixtures in terms of composition, temperature, pressure and pore size. Fig. 1 Phase diagram of a mixture of 50% methane and 50% n-butane when mixture is in bulk (filled circles) and when confined to 4nm pore (crosses). Approach These simulations are carried out in the NPT-Gibbs ensemble of Monte Carlo, in which two computational boxes are set up; box 1 contains bulk gas, box 2 is the slitpore (i.e., the box is restricted by two parallel graphite walls). Simulations are set to run until they reach equilibrium. Total number of molecules (N), system pressure (P) and the temperature (T) are kept fixed throughout the simulation. In such mixtures, the heavy component, here n-butane, tends to settle in the area right by the pore walls, allowing the lighter component, methane, to occupy the central area and also dominate box 1. Accomplishments If the pore is large enough, inner molecules in the pore will be less affected by the wall, and will make up a volume which, at the end of the simulation, will be in equilibrium with the aforementioned box 1. In other words, if box 1 is in gas phase, this inner volume is also in gas phase. As the pressure increases, box 1, as well as the inner space of the pore in equilibrium with it, will shift to a two-phase vapor-liquid and finally into a single-phase liquid. It is by this methodology that we keep track of phase changes inside the pore and plot phase diagrams. Based on this approach, we have generated the phase diagram of a binary mixture of 50% methane and 50% n-butane in a 4nm pore (Fig. 1). (Continued on page 7) 6

7 Fig. 2 Vapor-liquid coexistence envelopes of methane (in circles) and n-butane (in diamonds) when in bulk (filled symbols) and in 4nm pore (empty symbols). Significance It is found that in a binary mixture of 50% methane and 50% n- butane in a 3nm pore, the critical temperature and pressure changes significantly compared to the same fluid in otherwise unconfined conditions (Fig. 2 and Table 1). In fact, the strong effect of pore walls on the fluid in nanopores dictate such a behavior and question the practice of using bulk state critical parameters for reservoir characterization estimations in shale. Furthermore, pure and mixture fluid critical parameters being affected in nanopores, would generate two schools of thought in our dealings with mixture (multicomponent) fluid in nanopores; one in which, similar to Kay s mixing rule, corrected critical parameters of pure fluids are summed with respect to their composition in the mixture, and the other in which the mixture itself in the pore, with any given composition, is allowed to evolve leading to its specific phase diagram and critical parameters, both of which only molecular simulations can help yield. Critical parameters that we are primarily interested in are directly related to gas properties such as z-factor, formation volume, factor and viscosity. The bulk values of critical parameters that we currently use in our industry and reservoir simulators to arrive at shale gas properties are quite different from their actual values Table 1 Critical parameters of a confined fluid mixture calculated from if we carefully consider the effect of various methods nanopores. This research aims to introduce a new class of mixing rules and parameters that are more appropriate for shale gas in nanopores. Also, we intend to study the nature of binary and ternary fluids in nanopores of shale and how this would affect production. 7 Future Work We will continue to work on generating phase diagrams of multicomponent gases in various pore sizes leading to their critical parameters. References and Related Publications Singh, S.K., Sinha, A., Deo, G., Singh, J.K Vapor-Liquid Phase Coexistence, Critical Properties, and Surface Tension of Confined Alkanes. J. Phys. Chem. C 113 (17): Rahmani Didar, B., Akkutlu, I.Y Pore-Size Dependence of Fluid Phase Behavior and the Impact on Shale Gas Reserves. Paper SPE presented at the Unconventional Resources Technology Conference, Denver, Colorado, USA, August.

8 Upcoming Crisman Meeting The Fall Crisman Meeting will take place December in the Richardson Building, room 910. The agenda for the two-day event will be ed and posted as soon as it is available. Information on the dinner will also be released soon, via . If you know of someone in your company who would be interested in receiving these s, please forward the information to Kathy Beladi at If you would like to have your removed from the mailing list, let her know that as well. Printed versions of the 2013 Crisman Annual Report will be available at the meeting. Online versions are available here. For more information on other research projects, please visit the Crisman website. New sletter Information Robert Lane, Director Nancy H. Luedke, Editor Kathy Beladi, Editor Harold Vance Department of Petroleum Engineering 3116 TAMU College Station TX Harold Vance Department of Petroleum Engineering at Texas A&M University. All rights reserved. 8