In the Cantarell field in Mexico oil is produced primarily from naturally fractured carbonate reservoirs. These types of reservoirs are among the most difficult to develop given the uncertainties associated with detection and characterization, both locally in the well and regionally in the field, of the fracture networks that predominantly control fluid flow. Additional complexities result from difficulties understanding matrix-fracture interactions.In the last few years, the oil-water contact in the Cantarell field has been advancing, and this has considerably reduced the oil window zone. One likely explanation is that the natural fracture network (which provides most of the permeability in the field) favors production of water and gas over oil. This new challenge has forced Petróleos Mexicanos (PEMEX E&P) to make new efforts to characterize their naturally fractured carbonate reservoirs and improve their reservoir models. Using the critically-stressed-fault hypothesis, which assumes that the faults and fractures that are hydrologically conductive today are those that are critically stressed (active) in the current stress field, GeoMechanics International (GMI) analyzed image logs of over 20 wells in detail, and constructed a geomechanical model of the Cantarell field, to define the role of the fracture network in fluid flow. In a previous study, GMI used all available data, including seismic data, caliper logs, image logs, electric logs, drilling reports, sedimentological analyses of cores, and regional tectonic studies, to develop a comprehensive geomechanical model of the field. We then used and complement this model in conjunction with all available image logs to predict the orientations of the most and least critically stressed (hence permeable and impermeable) fractures. The results have been effectively used to improve well trajectories both to minimize costs and to optimize production throughout the life of the reservoir. The results have been particularly useful in horizontal wells, where preventing water production requires avoiding steeply dipping fractures that are critically stressed.

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This paper presents a multidisciplinary and integrated approach that can be used for understanding the connectivity and conductivity in fractured reservoirs. A case study from a carbonate reservoir in the Cantarell field in Mexico will be used to illustrate this approach. The fracture model is constructed by using seismic volumetric curvature attribute. The characterization of 3D fracture model is made with borehole images and fracture seismic classification analysis, which help to identify the two main scales and three sets of fractures in the reservoir. Fracture networks made by long size fracture corridors are assumed to be imposed on the small size diffuse fracture density fields. The 3D fracture model by integrating reservoir engineering data, including mud losses during drilling, flow meter data, transient well test analysis and water production data, resulted in the calibration of effective hydraulic conductivity for each fracture type. Based on the 3D fracture model, the hierarchical modeling concept is taken into account in effective permeability tensor computation. Matrix and fracture parameters including fracture density, length, aperture and orientation are calibrated and used to generate 3D fracture-permeability models for the reservoirs. An effective permeability model on a directional fracture-set base is developed to quantify the reservoir conductivity. In this paper, case studies are used to illustrate applications of these technologies and their efficiency in interpreting unexpected field production features such as early breakthroughs and better understanding the controls on the oil recovery.