hi, I got interesting reseult seneario forecasting with fracture spacing(MINC) using Monte Carlo simulation.

As a first approximation, you could estimate/vary the fracture porosity instead of the fracture volume fraction. For example, whether you have a volume fraction (VF) of 2% combined with a porosity (within the fracture continuum) of 50% is (almost) the same as having VF=4% and a porosity of 25%. In both cases 1% of the total volume consists of fracture pore space. Please try this. If it does not satisfy you, it would be straight-forward for me to implement the estimation of volume fractions.