Stratigraphic Well Correlations

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Stratigraphic well correlations for 3-D static modeling of carbonate reservoirsJean R. F. Borgomano, Francois Fournier, Sophie Viseur, and Lex Rijkels

AUTHORS Jean R. F. Borgomano Laboratoire de Geologie des Systemes et des Reservoirs Car ` bonates, case 67, Universite de Provence, 3, Place Victor Hugo, F-13331 Marseille Cedex 03, France; jean.borgomano@univ-provence.fr Jean Borgomano obtained a Ph.D. in carbonate geology in 1987 from the University of Provence in Marseilles, France. In 1988 2003, he worked at Shell as a senior carbonate geologist in various exploration and production Shell companies. He is currently a professor at the University of Provence and the director of the Geology of Carbonate Systems and Reservoirs Laboratory. His research focuses on the geological characterization and numerical modeling of carbonate reservoir architecture and properties. Francois Fournier Laboratoire de Geo logie des Systemes et des Reservoirs Carbo` nates, case 67, Universite de Provence, 3, Place Victor Hugo, F-13331 Marseille Cedex 03, France; Francois.Fournier@univ-provence.fr Francois Fournier received his M.Sc. degrees from the Nancy School of Mines, France, and from the Institut Francais du Petrole and a Ph.D. in carbonate sedimentology from the University of Provence in Marseilles, France. After a short experience in oil companies as an exploration geologist in France and Angola, he joined the Geology of Carbonate Systems Laboratory, Marseilles, France, as a lecturer in 2005. His research focuses on the relationship between sedimentology, diagenesis, and seismic reflections in carbonate reservoirs. Sophie Viseur Laboratoire de Geologie des Systemes et des Reservoirs Carbonates, ` case 67, Universite de Provence, 3, Place Victor Hugo, F-13331 Marseille Cedex 03, France; Sophie.Viseur@univ-provence.fr Sophie Viseur is a numerical geologist working as a researcher at the University of Provence. She received her Ph.D. from the Nancy School of Geology in 2001. Her primary interests are in geostatistics for channel simulations and their application to hydrocarbon exploration and production. In recent years, she has worked in developing methods for the integration of outcrop data and geological concepts into 3-D carbonate architecture models.

ABSTRACT The principles and purposes of stratigraphic well correlation in carbonate sedimentary systems are defined and discussed within the context of static reservoir modeling. The challenge of well correlations is to relate the heterogeneities measured at core and well scales to the spatial heterogeneities at reservoir and flow unit scales. The introduction of a priori knowledge in the process of stratigraphic well correlation is critical to support the stratigraphic rules and to establish a coherent geological and petrophysical concept. The links between well correlation and geostatistics are discussed with regard to the stationarity hypothesis and property trend analysis. We stress that wells are incomplete and biased samples of the geological reality, which is not dependent, unlike the dynamic reservoir behavior, on the well numbers, location, and spacing. Stratigraphic rules are applied as a function of the well spacing relative to the geological reality. A simple trigonometric method, combining angle of base profile, paleobathymetry, and well spacing, is introduced to check the validity of the well correlation in carbonate ramp-like systems. Two models, based respectively on outcrop and subsurface with seismic data, are discussed in detail to show the combined influence of the data set, sedimentary systems, and diagenetic transformations on stratigraphic well correlations.

INTRODUCTION This article discusses the principles of stratigraphic well correlations that form the foundation of most carbonate reservoir models used in hydrocarbon flow simulations. The poor

Copyright #2008. The American Association of Petroleum Geologists. All rights reserved. Manuscript received July 13, 2007; provisional acceptance November 7, 2007; revised manuscript received January 14, 2008; final acceptance February 21, 2008. DOI:10.1306/02210807078

AAPG Bulletin, v. 92, no. 6 (June 2008), pp. 789 824

789

Lex Rijkels Mrsk Olie og Gas AS, Esplanaden 50, DK-1263 Kbenhavn K, Denmark; rij@maerskoil.com Lex Rijkels is a reservoir engineer with Maersk Oil in Copenhagen. He is interested in linking field development decisions to the resolution of geostatistical and production data. He has worked on fractured and carbonate reservoirs, faulted fluvial sandstones, tight gas-condensate chalks, and a range of primary to tertiary recovery developments. He started his career at Shell on the Carbonate Team and on a business unit in Damascus.

ACKNOWLEDGEMENTS The idea of this article was initiated during a reservoir characterization project at Shell Research in 2000. It significantly benefited from stimulating discussions with many staff members on the Shell Carbonate Team and operational business units during the 20002001 period. We specially thank Cathy Hollis from the Shell Carbonate Team who gave constructive suggestions for the improvement of the manuscript. We thank Jean-Pierre and Mugget Masse from the University of Provence who contributed to some outcrop mapping and Ludovic Laugero who helped in finalizing the drawings.

reputation of carbonate reservoirs in term of flow predictability is directly proportional to the expectation that these reservoir units behave like flow units, as most siliciclastic reservoirs. However, in carbonate reservoirs, the spatial correlations between reservoir units and flow units (Amaefule et al., 1993) are commonly weaker than in siliciclastic reservoirs. Complex diagenetic transformations and capillary forces are possible causes of these weaker spatial correlations (Figure 1). In addition, the high degree of heterogeneity of carbonate rock properties at all scales, as measured on well logs and core samples, for example, leads generally to the realization of subsurface models characterized by a high-frequency variability that cannot be predicted nor correlated in the space between wells. The challenge in carbonate reservoir modeling is to relate the heterogeneities measured at core and well scales to the spatial heterogeneities at reservoir and flow unit scales. Well correlations and seismic data are the only possible links between these two scales. The relevance of chronostratigraphical well correlations for siliciclastic reservoir modeling purposes has already been discussed by Ainsworth et al. (1999). The deterministic method of correlating stratigraphic markers between production wells implies spatial correlations between well and seismic data and has an equal influence on the threedimensional (3-D) reservoir rock property model and the seismic and geostatistic modeling methods. The intrinsic complexity of the carbonate reservoir at all scales (from pore network to stratigraphic architectures) makes it necessary to concentrate stratigraphic well correlation efforts on the level of reservoir heterogeneity, which matters in terms of reservoir and flow units. The main objectives of this article are to (1) review the processes of stratigraphic well correlation in carbonate reservoirs, (2) discuss its impact on reservoir and flow unit modeling, and (3) present some recommendations adapted to specific stratigraphic systems and reservoirs.

WHY CORRELATE WELL DATA? Short-range variability has very limited influence on the overall flow pattern (Jennings, 2000). Fluid displacement fronts tend to follow the larger scale structures, such as strata and sedimentary bodies, whereas small-scale heterogeneity, such as pore networks, rock textures, and sedimentary structures, only smears the front of the flow. Such principles also hold when the short-range permeability variation is much larger than the changes of the average permeability of larger scale790 Well Correlations in Carbonates

Figure 1. Comparison between flow units and reservoir units in carbonate and siliclastic marine sequences. Siliciclastic sedimentary sequences are characterized by simple correlations between grain size, mineralogy, porosity, and permeability. Rock properties can be detected and correlated to stratigraphic sequences allowing the characterization of reservoir and flow units. These relationships are more complex or nonexistent in carbonate sequences characterized by a high level of rock property heterogeneity. OWC = oil-water contact.

structures. Jennings (2000) considerations were in line with Araktingi and Orr (1993), who also studied the effect of heterogeneity on flow, for different mobility ratios. They found that the difference in fluid mobility during water flooding may cause fingering at low heterogeneity, but that the viscous bypassing of oil becomes much less important for more heterogeneous samples. As a consequence, to understand the flow behavior at the interwell scale,

one needs to identify the structures that are correlatable between wells. Anything smaller than those structures is averaged out into the permeability of the correlatable units. This does not imply that one can ignore all the detail below this scale. One needs a reliable measure of the permeability statistics within each unit to identify the representative average properties. However, constructing a geological model at the smallest scale is not always necessary.Borgomano et al. 791

In principle, one should aim to construct the architecture of the model at the scale of the correlated structures between wells and to use the measurement statistics made on smaller scales (e.g., core plugs) for each structures. Very thin units (

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