3D Hydrocarbon Migration in Alternate Sand- Shale Environment Through Percolation Technique

Paolo Ruffo, Anna Corradi, Antonio Corrao, and Claudio Visentin
ENI, Milan, Italy

       Modeling 3D migration of hydrocarbons in a system of interconnected channels is a complex task. The interpretation of each sedimentary body in the mixed sand-shale environment is challenging not only because it requires a significant amount of time, but also for the need of defining which seismic facies are distinguishable and can be safely interpreted through the entire volume. In addition to that, the resulting facies volume must be given in depth domain in order to model hydrocarbon migration, adding a further complexity with the definition of a robust seismic velocity volume.
       With reference to current approaches to hydrocarbon migration three different alternatives are possible: ray-tracing, Darcy flow, percolation flow.
       Ray-tracing is actually a 2½ D approach, as it requires horizon or fault surfaces as “guidelines” to be followed as gradient paths to the potential traps. This approach clearly is not adequate to model hydrocarbon migration in a mixed sand-shale environment, as the actual migration is a real 3D process where migration follows the systems of interconnected channels instead of the boundary between lithofacies.
       Darcy flow is a fully 3D approach to migration but memory and CPU time required to process a complex model, even taking advantage of parallelization, binds to oversimplify the model using a coarse scale at which actual channel connections are heavily modified or even lost.
       As it is actually impossible to model the flow at the scale at which it happens, the real matter is to use the technique that best approximate the flow, at the scale at which flow modeling is affordable in terms of model details and computational time.
       Percolation flow appears to be the right choice in the case just described, as, even if the dimensions of the cells of the computational mesh are still coarse (with respect to the microscopic scale of hydrocarbon flow), they are small enough to allow to maintain a sufficient detail of the channels shape and of their connections. Upscaling is therefore reduced to a minimum, resulting almost in a simple resampling from the seismic volume to the percolation mesh. In the presented example the migration (percolation) modeling is performed with a software called MPath (by The Permedia Research Group Inc) which is based on the invasion percolation technique.
       The workflow is composed by a few steps:
       a) after a regional identification of the main seismic sequence boundaries and systems tracts, the first step consists in a semi-automatic classification of the sand and shale prone seismic facies that are interpretable in the seismic volume. A proprietary software (based on supervised neural network techniques and textural seismic attributes) supports volumetric classification. Each class represents a different facies with specific petrophysical properties (threshold pressure, porosity, irreducible water saturation, etc.), assigned according to concurrent sedimentological studies. The result of this step is thus a lithofacies 3D Model.
       b) Generation and expulsion are computed on a regional 3D model, providing hydrocarbon quantities expelled from the source rock. Expelled masses maps (oil and gas) are inserted in the lithofacies 3D Model and hydrocarbons find their way from the source through the channels according to the threshold pressures of the different facies.
       This workflow has been applied in a deep water setting, characterized by an interconnected channel system, where a number of prospects had to be evaluated. A well was drilled in the area prior to this study, resulting dry in spite of being located in an area where the source rock is present and is deemed to have expelled interesting amounts of hydrocarbons. This study has shown that hydrocarbons expelled from the source appear to be deviated laterally by the presence of a local seal and therefore do not reach the channel system in the well drilled location.

AAPG Search and Discover Article #90066©2007 AAPG Hedberg Conference, The Hague, The Netherlands