Multi-Phase Flow Properties of Fault Rocks: Implications Prediction of Across-Fault Flow During Production

Fisher, Quentin¹
 Al Hinai, Suleiman²
 Grattoni, Carlos A.³
 Guise, Phil³

Abundant data are now available on fault on the absolute permeability and thickness of fault rocks, which allow transmissibility multipliers to be calculated that can be applied to grid block faces adjacent to faults within simulation models to account for the effects of faults on fluid flow. In many cases, transmissibility multipliers calculated from absolute permeability values appear to overestimate fault transmissibility; potentially because the multi-phase flow properties (capillary pressure and relative permeability) of fault rocks have not been taken into account. Here we present results from the first ever systematic experimental study aimed at measuring the multi-phase flow properties of fault rocks. These new results suggest that failure to take into account the multiphase flow properties can result in an overestimation of the transmissibility of some cataclastic faults by several orders of magnitude. When incorporated into production simulation models these new results help explain the extent of fault compartmentalisation in reservoirs where cataclastic faults are abundant. In some circumstances it seems unnecessary to take into account multi-phase flow behaviour of fault rocks to achieve an history match of production data. This may reflect the lack of continuity of low permeability fault rocks or the fact that some poorly lithified fault rocks have low capillary entry pressures and high relative permeabilities under reservoir conditions.

Laboratory studies have also been undertaken to measure the stress sensitivity of the absolute and relative permeability of fault rocks and their analogues. The relative permeability of some samples appears highly stress dependent (i.e. reducing by a factor of around 20 as effective stress is increased by 2000 psi). These results suggest that accurately modeling across-fault flow in compartmentalized reservoirs may require the stress-dependence of the single and multi-phase flow properties of fault rocks to be incorporated into production simulation models. In some situations it may be relatively easy to estimate variations of reservoir stresses during production. In other situations, it may be necessary to couple production simulation models to geomechanical models to estimate the fluid behaviour of faults during production.