Leached and Broken: Static and Dynamic Reservoir Modelling of Tight and Strikeslip Faulted Carbonates (Lower Cretaceous, Venezuela)

M. Pöppelreiter¹, M. Balzarini¹, B. Hansen², P. Koci¹, R. Nelson¹, 
with contributions by P. De Sousa³, S. Engel³, M. Galarraga³, X. Marquez³, R. Pimentel³, A. Rinaldi³, F. Rodriguez³

¹Shell, Global Solutions, 3737 Bellaire Blvd., Houston, Texas, 77025, U.S.A.,
E-mail: [email protected]
²Eriksfijord AS, Chausee de Fleurus 6040 Jumet Belgium
E-mail: [email protected]
³Shell Venezuela S.A. Torre Empresarial Claret, Av. 3E entre Calles 78 Y 79 Piso 14, Sector Valle Frio Maracaibo, Zulia, Venezuela

Cretaceous carbonates in the Maracaibo Basin of Venezuela produce 50.000 bpd light oil at a depth of up to 16,000 ft. Hydrocarbons are stored in a complex pore system with 1 to 15 % porosity resulting from early diagenesis, burial diagenesis and natural fracturing associated with strike-slip faulting. Planning of economically successful wells in this reservoir requires fluid flow models based on robust geological realisations.

The objective of the present paper is to give an outline of the workflow chosen to derive sound geological realisations for this complex reservoir including a novel approach to fault-related fracture modeling.

The reservoir consists of 1,000m of tight, sparsely fractured limestones with three important layers with matrix porosity each just 20 to 40 ft thick with minor shales and dolomites. The thin layers with matrix porosity are linked to skeletal-oyster packstones and skeletal grainstones containing 3 % to 15 % solution enlarged vuggy pores, preserved biomoldic and some interparticle pores. Biomoldic pores are preserved in early charged paleo-closures. In contrast vuggy porosity is developed in these three layers along specific faults due to fault-related leaching fluids.

Faults are not only important conduits for leaching fluids during the geological history, but also permeable fracture corridors linking patches of matrix porosity during hydrocarbon production. Three geological facies models were built in a geocellular model (Petrel) and stacked to derive a combined porosity model:

• Conceptual early diagenetic porosity model
• Conceptual late diagenetic porosity model
• Conceptual fracture model

Different realisations of each porosity type were put into the geocellular model. A layering scheme was developed that honours time-stratigraphy, matrix porosity and elastic properties. Special attention was paid to fracture modelling. Fractures associated with faults were modelled as fault corridors. These corridors were differentiated according to their geological history and their orientation with respect to present day maximum horizontal stress. Subsequently a geomechanical model was constructed based on borehole image observations, laboratory tests, sonic logs, fracture closure stress, pore pressure distribution and fault orientation. The model outputs normal and shear stress for every fault surface element, which can be semi-quantitatively scaled to poroperm properties. The full field geocellular model was up-scaled for input into a dynamic simulator where pressure and production data were history matched by globally modifying the various matrix and fault-related properties and other reservoir parameters. Pervasive joints of hydraulic origin were represented as permeability anisotropy. This approach was chosen to arrive at a predictive full field model without excessive processing time. The history-matched dynamic model was used for planning well trajectories and for determining the optimum drilling strategy. The integrated modelling approach provides a superior tool for well planning in the geologically challenging carbonate reservoirs. Application of this process resulted in the first history-matched and predictive dynamic model for these reservoirs.