Dual Porosity
Naturally fractured reservoirs often have 2 porosities, a matrix porosity and a fracture porosity. This is what is known as a dual porosity reservoir. The fracture network (left side of Fig. 1) usually consists of shear and extensional fractures and needs to be simplified in the model as a series of matrix blocks surrounded by fractures (right hand side). As the model looks like a stacked series of sugar cubes it is also called a sugar cube model. Geology does sometimes conform to this model (Fig. 2).
Fig. 1. Warren and Root (1963) model of fractured reservoirs as a dual porosity system.
Fig. 2. Outcrop example of matrix blocks surrounded by fractures. Lithology is Caithness flagstones (Devonian age) on North Coast of Scotland.
Often the volume of hydrocarbons stored in the fractures is lower than in the matrix (porosity) and the fracture permeability is higher although there is a range of possibilities presented by Ron Nelson (Fig. 4). In general, hydrocarbon production will occur firstly from the fractures then matrix will recharge the fracture system providing flow to the well (Figure 3).
Fig. 3. Matrix blocks surrounded by fractures (in the Hopeman Sandstone, Morayshire, UK) and illustration of a production well accessing fracture permeability. There is a first stage of rapid production from the fractures then recharge into the fractures from the matrix.
Nelson’s classification
This is very valuable way to describe your reservoir as it may dictate the economic decisions required to best dest develop your reservoir (Fig. 4). However, it may not be immediately obvious which of the types you are dealing with. Also, more than 1 type may be present within the same field i.e., well data may suggest some spatial and vertical variation.
Type I: Fractures provide all porosity and permeability.
Type II: Fractures provide essential permeability and matrix provides porosity
Type III: Fractures provide permeability assist in already (matrix) producible reservoir. Matrix provides most storage.
Type IV: Fractures provides no additional permeability and porosity and are often flow barriers.
Fig. 4. Nelson’s classification of naturally fractured reservoirs based upon matrix and fracture contribution to permeability and porosity.