Fractured reservoirs 

It contains a workflow extending from basic data-gathering to the incorporation of fracture networks in reservoir simulation models.

Underlying this work is the perception that carbonate reservoir rocks are far more susceptible to fracturing, because dolomite and limestone are more brittle than sandstones.

Workflow for characterising rock fractures in hydrocarbon fields. The upper panel shows various data sources, while the flow chart below depicts their incorporation in conceptual models (left) and their subsequent, upscaled transformation into a form suitable for incorporation in reservoir simulators (right).

In many cases, natural fractures not only have a significant impact on the production of carbonate reservoirs but reservoir deliverability may also depend entirely on the presence of open inter-communicating fracture networks.

The material is based on direct experience with three giant oil fields in Iran and lesser studies carried out in Algeria and Mexico. StatoilHydro’s latest research results are also included.
 
Besides educational and analytical topics, the document includes a workflow showing how fractures can be incorporated in reservoir descriptions and simulations of their performance.

One of the most important considerations is whether a single- or dual-porosity formulation is most appropriate.
 
If the fractures are short and unconnected, the values for matrix porosity and permeability can be increased in the direction of the fractures, as in a single porosity formulation.

If the fracture networks are interconnected, a dual porosity approach may be more suitable.

For dual porosity systems, models of fracture networks are built using specialist software programmes, which provide upscaled values of fracture porosity and permeability and the sizes of fracture-bounded matrix blocks.

This information is then entered into coarse reservoir simulation grids and married with matrix properties to predict the combined effect of fractures and matrix on production when subject to a variety of recovery scenarios.