Technologies and Exploitation Strategies Applied to the Development of a Naturally Fractured Carbonate Reservoir: The Cretaceous Cogollo Group in La Concepción Field, Maracaibo Basin, Venezuela
Authors: Marchal D. , Gonzalez I. , Benito J. , Balseiro P. 
Published: December 2007

Abstract
Complexity and heterogeneity inherent to naturally fractured reservoirs pose important technical, economic, and managerial challenges to operating companies. In order to meet these challenges, operators ought to apply innovative technologies and tailored exploitation strategies. This paper summarizes the reservoir surveillance and characterization approach, as well as the operational strategies used in the development of the Cretaceous fractured carbonate Cogollo Group in La Concepción Field, western Venezuela, with emphasis on the successful application of selective technologies and methodologies.
Discovered in 1948, the Cretaceous reservoir had produced over 90 MMSTB of 36°-API oil from 29 wells up to January 2005. Since 1998, Petrobras Energía Venezuela's operations have resulted in a significant increase of the total reservoir oil throughput from 2500 to a maximum of circa 17000 BOPD, with an associated cumulative production of 23 MMSTBO.
A key factor in the successful redevelopment of the mature Cretaceous reservoir was the timely recognition of the major features driving the reservoir productivity and performance, in addition to the opportune identification of the main operational risks and the suitable technology to mitigate them. Then, as we drilled and intensively acquired data, we refined the reservoir description, improved operational indicators and costs, and implemented new technology and risk management strategies to face the increasing challenge of the remaining reservoir targets. The integration of the new structural model (en echelon transpressional stepover zones), derived from the 3D seismic data, modern well-logs and dynamic (production logs and pressure tests) data acquired from re-entries and new wells, and rock description and analysis from 950’ of core, allowed us to define a clear relationship between fault-related fracture systems and production. In brief, the Cretaceous reservoir is classified as a type-II fault-related fractured reservoir containing a micro-fissured pseudomatrix.
Currently, based on our model, the main development strategy is to drill wells that cross "seismically visible" faults with the aim of intersecting their associated fracture networks. Discontinuity seismic attributes are used to delineate the fault zones and optimize well locations. Reprocessed seismic volumes are now used to diminish the uncertainty in the spatial geometry of the targeted faults. In addition, the structural interpretation is being tested and enhanced with the results from physical experiments (sandbox analog modeling).
Presence of discrete highly productive fractured zones, in conjunction with low reservoir pressures due to depletion, led to the adoption of under-balanced drilling (UBD) with Nitrogen injection as the drilling technique in the reservoir section to avoid severe circulation losses. During the UBD phase, a calibrated drilling dataset is used along with mudlogging data to check for geological events and monitor well productivity in real time. Furthermore, the complexity and uncertainty associated to the targets impose the use of strict directional control to conform to the designed complex well trajectories.
The presented field development strategy has proved to be successful since it has enabled to achieve a sustained economic oil production in the naturally fractured Cretaceous reservoir of the mature La Concepción field.