The properties of reservoir fluids axe critical for the successful exploitation of petroleum oil and gas reservoirs. Integrated PVT sampling and testing programs are key to successful reservoir development applications. Recently developed laboratory measurements and advanced computational tools have enhanced knowledge of phase behavior and physical properties of reservoir fluids. However, many aspects of phase behavior remain unresolved.

This poster session highlights the integrative aspects of PVT programs and provides insights and solutions for unresolved problems. Issues covered include the following:

RESERVOIR FLUIDS SAMPLING COMPLEXITIES

  1. Sampling Under Dynamic Reservoir Conditions: Petroleum reservoirs are continually in a dynamic state. Thus, obtaining representative samples during any stage of depletion or development presents its own unique challenges. Case histories are provided of the pros and cons of bottomhole vs. surface sampling.

  2. Sour Crude Oil System Sampling: Sampling and laboratory testing of systems which contain hydrogen sulfide (H2S) deserve special consideration as absorption of H2S on the surface of sampling vessels can occur. This is of particular importance in studies of minimum miscibility pressure (MMP) since H2S is beneficial in lowering MMP for miscible hydrocarbon gas displacements.

  3. Condensate Fluids Sampling: Well conditioning for sampling of condensate fluids are critical in ensuring that representative fluids are obtained. The difficulty in obtaining representative samples results from liquid dropout of the condensate in the near wellbore region.

  4. Compositional Variation Within a Reservoir, Oil Fingerprinting: Utilization of oil fingerprinting, a chromatographic technique, represents a significant stride for reservoir surveillance and reservoir management of fields which contain multiple crude oils. The technique can be used as a production allocation tool for reservoir surveillance and is much more accurate and cost effective over conventional production logging techniques.

The properties of reservoir fluids axe critical for the successful exploitation of petroleum oil and gas reservoirs. Integrated PVT sampling and testing programs are key to successful reservoir development applications. Recently developed laboratory measurements and advanced computational tools have enhanced knowledge of phase behavior and physical properties of reservoir fluids. However, many aspects of phase behavior remain unresolved.

This poster session highlights the integrative aspects of PVT programs and provides insights and solutions for unresolved problems. Issues covered include the following:

  1. Sampling Under Dynamic Reservoir Conditions: Petroleum reservoirs are continually in a dynamic state. Thus, obtaining representative samples during any stage of depletion or development presents its own unique challenges. Case histories are provided of the pros and cons of bottomhole vs. surface sampling.

  2. Sour Crude Oil System Sampling: Sampling and laboratory testing of systems which contain hydrogen sulfide (H2S) deserve special consideration as absorption of H2S on the surface of sampling vessels can occur. This is of particular importance in studies of minimum miscibility pressure (MMP) since H2S is beneficial in lowering MMP for miscible hydrocarbon gas displacements.

  3. Condensate Fluids Sampling: Well conditioning for sampling of condensate fluids are critical in ensuring that representative fluids are obtained. The difficulty in obtaining representative samples results from liquid dropout of the condensate in the near wellbore region.

  4. Compositional Variation Within a Reservoir, Oil Fingerprinting: Utilization of oil fingerprinting, a chromatographic technique, represents a significant stride for reservoir surveillance and reservoir management of fields which contain multiple crude oils. The technique can be used as a production allocation tool for reservoir surveillance and is much more accurate and cost effective over conventional production logging techniques.

  1. Near Critical Fluids: When oil and gas equilibrium compositions vary considerably with spatial position and time, the assumption of solubility of gas in oil being dependent on pressure alone is not valid. Therefore, reservoir problems requiring compositional treatment can be grouped into depletion/cycling of volatile oil and gas condensate reservoirs and miscible flooding with Multiple Contact Miscibility (MCM) generated in situ. The distinction is that depletion/cycling usually involves phase compositions removed from the critical point, while MCM generally requires calculation of phase compositions and properties converging at the critical point, hence the word "near critical fluids".

  2. Presence of Two Liquid Phases for Hydrocarbon Miscible EOR Systems: The presence of two liquid phases is a common phenomenon in "low temperature" reservoirs which are candidates for hydrocarbon gas miscible processes like CO2 flooding. Recent experimental investigations have focused on defining the fluid properties of each of the liquid phases.

  3. Prediction of Asphaltenes: The prediction of asphaltenes is critical for hydrocarbon miscible processes. Currently, investigators rely primarily on experimental investigations to determine its presence in a particular crude oil.

  1. Construction of Molecular EOS's: A difficulty in modelling the MCM process is achievement of consistent, stable convergence of gas- and oil-phase compositions, densities, and viscosities as the critical point is approached. The use of an equation of state offers the advantage of a single, consistent source of calculated K-values, phase densities and their convergence near the critical point. Therefore, an equation of state constructed for the individual molecules ("molecular EOS") is the most desirable.

  2. Miscibility Pressure Calculations: MMP calculations are important for developing reservoir development strategies with EOR like miscible hydrocarbon gas processes. Recent experience on utilizing correlations and the differences observed with laboratory investigations are explained.

  3. Validation of Original and Current Fluid Properties: In reservoirs with a long production or development history, it is critical to "match" original and current fluid

properties when available just as reservoir engineers match oil or water production and pressures in a history match. Obtaining a "match" on all available PVT data ensures that fluid properties are properly accounted for in integrated reservoir studies, especially ones utilizing reservoir simulation.

The concepts presented provide meaningful information to practicing production and reservoir engineers for reservoir appraisal and fluid sampling in both mature and new fields. The techniques are integral for the successful planning of EOR projects. The technical contributions include:

Integrated PVT Programs and Case Histories: Design details of integrated PVT programs with case histories of implementing such programs with real problems and solutions.

Laboratory Quality Assurance: Meaningful assessment of the quality of laboratory PVT data and design of quality assurance programs for reservoir engineering studies.

Role of PVT Correlations: Guidelines for selecting and validating correlations for reservoir fluid properties.

The concepts described in this poster session will be thought provoking in consideration of fluid properties for reservoir development applications and predicting future reservoir performance.

This paper was selected for presentation by an SPE Program Committee following review of information contained in an abstract submitted by the author(s). Contents of the paper, as presented, have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material, as presented, does not necessarily reflect any position of the Society of Petroleum Engineers, its officers, or members. Papers presented at SPE meetings are subject to publication review by Editorial Committees of the Society of Petroleum Engineers. Permission to copy is restricted to an abstract of not more than 300 words. Illustrations may not be copied. The abstract should contain conspicuous acknowledgment of where and by whom the paper is presented. Write Librarian, SPE, P.O. Box 833836, Richardson, TX 75083-3836, U.S.A. Telex, 163245 SPEUT.

The authors thank the management of Fina Oil and Chemical Company and BP (Exploration) Alaska Inc. for their support in dissemination of this technology.