Rock deformability can greatly affect the response of naturally fractured reservoirs when subjected to changes in fluid pressure. This paper describes two numerical models that were developed to understand some of the fluid-rock interactions that can take place in this type of reservoir. Both models were used to study the response of a small 'sugar block' reservoir dissected by a large number of fractures and subjected to a number of field conditions and production rates. Results obtained during this preliminary phase indicate that fracture deformability plays a fundamental role in reservoir performance, and the associated compaction can represent an additional drive mechanism, even if significant reductions in fracture permeability are expected.
The newly developed models are useful reservoir management tools which allow to investigate the effect of matrix porosity and permeability, fracture aperture, mechanical properties, anisotropy and production rates on reservoir performance.
The reservoir sensitivity to the mechanical behavior of fractures subjected to pressure depletion has been indicated as an important factor in the analysis of the response of naturally fractured reservoirs in the La Luna/Cogollo Group Formations located in Blocks IX and XIV of the Lake of Maracaibo in western Venezuela (Fig 1). Geological, geomechanical and Reservoir Engineering studies indicated the existence of highly conductive fractures with different degrees of interconnectivity and morphological characteristics. Results of triaxial tests conducted on reservoir rock samples indicated that:
Pressure drop from the original value (0.55 psi/ft @ 13000') to the abandonment pressure of the test of approximately 0.29 psi/ft (bubble point) could lead to complete closure of the longest, very narrow fractures.
Heterogeneity in fracture aperture does not necessarily lead to a major reduction in conductivity of individual fractures, but can lead to a significant reduction in the overall fracture permeability, and therefore, to an unknown reduction of reservoir productivity.