--> Heavy Oil PVTX Characterization As A Prediction Tool To Maxinize Heavy Oil Recovery, by Gilles Levresse, Jordi Tritlla, Jacques Pironon, and Alejandro Carillo-Chavez; #90062 (2007)

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Heavy Oil PVTX Characterization as a Prediction Tool to Maximize Heavy Oil Recovery

Gilles Levresse1*, Jordi Tritlla1, Jacques Pironon2, and Alejandro Carillo-Chavez1
1 Programa de Geofluidos, Centro de Geociencias, Campus Juriquilla, UNAM, Carr. Qro-SLP km 15,5, Juriquilla, Queretaro 76230 Mexico *[email protected]
2 UMR G2R-CREGU, Vandoeuvre-lès-Nancy, France

Oil, considered as a resource, has two main stages regarding their P-T history. The first stage is characterized by the P-T reservoir conditions prior to anthropic disturbance, mainly by drilling (in Mexican mesozoic reservoirs around 150ºC and 400 bars). The second stage is controlled by the surficial P-T conditions prevailing during oil recovery (30ºC and 1 bar in Mexican Gulf platforms). As the surficial conditions do not experiment drastic variations, oil companies consider they in a nearsteady state as the oil characterization (APIº-GOR) is performed at 1 atm and 15ºC.

The P-T reservoir final (present day) conditions are the result of a series of geological events, that can be resumed in a P-T path, intimately linked with the geological (diagenetic and tectonic) history of the reservoir and the related fluids (brines+oil+gases).

Oil reservoir production implies a fast and drastic oil depressurization during its way to the surface. This oil depressurization induces both physical and chemical modifications, affecting the remaining hydrocarbons phase relations and compositions within reservoir as well as their related brines, the latter directly affecting the quality of the reservoir rock (changes in pH, fS and precipitation/dissolution of host rock). As a result, the oil remaining within the reservoir is increasingly enriched in the heaviest hydrocarbon components. This in-situ distillation affects all oil types, but it is specially important for heavier oils where the loss of volatiles provoke an increase in oil viscosity and density that can clog the reservoir porosity, drastically decreasing permeability, partially due to its preferential wetability on minerals. The main consequence of this distillation process is that the oil recovered and routinely characterized in surface does not represent the whole original composition of the oil contained within the reservoir. The characterization of the real composition of the hydrocarbons in the reservoir is, then, crucial to both maximize the recovery of this resource and minimize the degradation of the reservoir properties during the history of exploitation.

A factor usually underestimated is the water proportion (as emulsion) within the production oil. This water is generally the main vector of the salinity in the fluids, precipitating minerals or corroding the tubing and in addition controlling the amount of “salts” in production oil.

Even though oil can be formed in a same basin by similar processes and from comparable source rocks, the flow and diagenetic histories can be rather different and lead to different kinds of oils with contrasted compositions and PVT properties. Before production, the oil trapped within a reservoir was subjected to PVT changes due to their geological history, including: (1) the removal of part of the sedimentary column, which induces both decompression and a decrease in temperature as well as oil demixing and/or exsolution of a gas phase, specially affecting condensate-bearing oils; (2) biodegradation of oils due to their closeness to or connection and the surface, with the inflow of oxidant meteoric waters, with the subsequent increase in oil density; and (3) the burial of the reservoir, with subsequent P and/or T changes, inducing an in situ cracking, which generates pyrobitumen that can obstruct the reservoir porosity.

An oil PVT characterization at reservoir conditions must include both a careful diagenetic and a 1 precise fluid inclusions study, before decompression and well testing if possible. A careful core sampling is crucial to choose the most representative, untouched samples that can preserve fluid volumetric information. Diagenetic studies are necessary to define the chemical changes that occurred within the reservoir and their precise sequence, providing a fluid inclusion trapping chronology relative to the oil and brine migration. Moreover, this first studies are critical to reveal possible traces of oil degradation, as the presence of solids, like pyrobitumen, within the oil bearing fluid inclusions.

Minimum P-T trapping conditions can be estimated by classical microthermometric studies on both brine-bearing and hydrocarbon-bearing fluid inclusions. Historically, molecular composition has been tentatively obtained using several destructive techniques including decrepitation, crushing and leachate analyses (George et al., 1998). The main limitation of these techniques is cross-contamination during the extraction and the mixing of several generations of oil in a single analysis, which leads to unrealistic results.

During the last years, new analytical methods have been successfully applied to analyze single fluid inclusions, of a few microns in size, in diagenetic minerals. Confocal Laser Scanning Microscopy (CLSM) is presently applied to reconstruct both 3D shape and volumetry of the very same hydrocarbon-bearing fluid inclusions that will be subsequently analyzed by micro Fourier Transform Molecular Infrared Microspectrometry (FTIR) and Raman micropobe. Advances in FTIR permit to perform quantitative, non destructive molecular analyses, obtaining the density and composition of a single fluid inclusion and allowing their individual PVT modelling. Raman spectroscopy permit to quantitatively analyze light organic components (CO2-CH4-C3H8) in non or weak fluorescent hydrocarbon-bearing fluid inclusions. The CH4 content in aqueous fluid inclusions is used to ascertain the trapping pressure and temperature in gas-rich reservoirs, with good reported results in the North Sea siliciclastic reservoirs (Dubessy et al., 2001).

The combination of microthermometry, CLSM, FTIR and Raman spectroscopy of single fluid inclusions in a well characterized FIA (Fluid Iinclusion Association), coupled with thermodinamic modelling using the Petroleum Inclusions Thermodinamics software (PIT), based upon a complete range of hydrocarbon compositions, permit the calculation of the real P-T fluid trapping conditions and the hydrocarbon composition modelling through the calculation of the α and β parameters (Thiery et al., 2000; Pironon et al., 2000; 2005). This methodology has been recently implemented at the Centro de Geociencias (UNAM, Mexico) and successfully applied to the hydrocarbon PVT and composition modelling of dolostone-bearing reservoirs in SE Mexico by the authors (Levresse et al., 2005a; 2005b; 2006; Tritlla et al., 2005; 2006a; 2006b).

This technique is a very powerful tool for the petroleum industry when facing the exploration and extraction for heavy and super heavy oils. Systematic fluid inclusion studies in cores recovered from heavy oil prospects allow to precisely determinate the PVTX (Pressure-Volume-Temperature- Composition) oil evolution during geological time and, also, the reservoir present day conditions. The determination of the oil P-T path geological evolution can give an insight into the formation of heavy and super-heavy oils as well as to evaluate a potential secondary gas migration.

The information obtained with this studies should be evaluated and compared with the recovered oil, gas and brine compositions, as they represent parts of the same original fluid. This allow a complete and more efficient modelling of the different degasification processes that take place during the oil extraction, to estimate the effects produced on the quality of the reservoir due to mineral and bitumen precipitation as well as to predict the present and future oil production.

References

Dubessy, J.; Buschaert, S.; Lamb, L.; Pironon, J.; Thiery, R. (2001). Methane- bearing aqueous fluid inclusions: Raman analysis, thermodynamic modeling and application to petroleum basins. Chemical Geology, 173, 193-215.

George, S.C.; Eadington, P.J.; Lisk, M.; Quezada, R. A. (1998). Constraining the oil charge history of the South Peper oil field from the analysis of oil bearing fluid inclusions. Organic Geochemistry, 29,631-648.

Levresse, G.; Tritlla, J.; Gonzalez-Partida, E.; Pironon, J.; Teinturier, S.; Priftulli, E.; Oviedo-Perez, A; Martinez-Kemp, H.; Gonzalez-Posadas, F. (2005a) Reconstruction of the P-T conditions during methanogenesis and oil-filling of a Mesozoic dolostone reservoir: the case of the Saramako oil field, SE Mexico. Proceedings of the XVIII ECROFI, Siena, Italia.

Levresse, G.; Gonzalez-Partida, E.; Pironon, J.; Tritlla, J.; Priftulli, E.; Sanchez-Trejo, A. (2005b) High pressure oil filling as recorded in fluid inclusions: the case of Pol oil field, Southern Mexico. Proceedings of the XVIII ECROFI, Siena, Italia.

Levresse, G. ; Tritlla, J.; Bourdet, J.; Pironon, J.; Carillo-Chavez, A. (2006). The influence of salt tectonics in the brine and hydrocarbon evolution of K-T-bearing oil fields, SE Mexico. 17th International Sedimentological Congress, 27th August -1st septembre 2006, Fukuoka, Japan.

Pironon, J.; Lhomme, T.; Bourdet, J.; Levresse, G ; Gonzalez-Partida, E. ; Tritlla, J. (2005). Study of petroleum and aqueous inclusions in carbonate reservoirs: a necessary adaptation. Proceedings of the XVIII ECROFI, Siena, Italia.

Pironon, J.; Thiery, R.; Teinturier, S.; Walgenwitz, F. (2000). Water in petroleum inclusions: evidence from Raman and FTIR measurements, PVT consequences. Journal of Geochemical Exploration, 69-70, 663-668.

Thiery, R.; Pironon, J.; Walgenwitz, F.; Montel, F. (2000)PIT (Petroleum inclusion Thermodynamic): a new modeling tool for the characterization of hydrocarbon fluid inclusions from volumetric and microthermometric measurements . Journal of Geochemical Exploration, 69-70, 701-704.

Tritlla, J.; Gonzalez-Partida, E.; Levresse, G.; Pironon, J.; Banks, D. (2005). Fluid origin and “in situ” O.M. maturation at the la Encantada-Buenavista fluorite deposits, Coahuila, Mexico”. Proceedings of the XVIII ECROFI, Siena, Italia.

Tritlla, J.; Levresse, G.; Carrillo-Chávez, A.; Gonzalez-Partida, E.; Pironon, J.; Teinturier, S.; Clara-Valdez, L.; Martinez-Kemp, H.L.; Gonzalez-Posadas, F. (2006a). Condiciones P-T durante los procesos de dolomitización y metanogénesis en dolomías mesozoicas: el caso del campo Saramako, SE de México. Simposium “Plays y yacimientos de Aceite y Gas en rocas carbonatadas: Prospectos”, plática 04. Asociación Mexicana de Geólogos Petroleros, Cd. del Carmen, Campeche, 15-17 de Marzo del 2006.

Tritlla, J.; Levresse, G.; Pironon, J.; Carillo-Chavez, A; Teinturier, S.; Oviedo-Perez, A.E.; Martinez-Kemp, H.L.; Gonzalez-Posadas, F. (2006b). Fluid P- T conditions during burial diagenesis, methanogenesis and oil filling in Mesozoic dolostones from the Saramako oil field, SE Mexico. 7th International Sedimentological Congress, 27th August -1st

 

AAPG Search and Discovery Article #90062©2006 AAPG Hedberg Research Conference, Veracruz, Mexico