Modelling the permeability evolution of carbonate rocks: Diagenetic ‘back-stripping’

Diagenesis is a major control on the porosity and permeability characteristics of carbonate rocks, and therefore significantly impacts fluid flow in the subsurface. Diagenetically-modified carbonates often show highly heterogeneous and tortuous pore networks, such that conversion from porosity to permeability to multi-phase flow properties is far from straightforward. This means that there is often a mismatch between static and dynamic properties as insufficient emphasis is placed on the control of petrophysical properties that result from the diagenetic evolution of the rock. There is therefore a clear need to link both conceptual depositional and diagenetic models with the evolution of flow properties. While changes in porosity can be directly related to diagenetic petrographic characteristics such as cement distribution and dissolution features, quantifying how these textures relate to attendant changes in permeability is more challenging. Here, we demonstrate how pore-scale models, representing typical carbonate sediments and their diagenetic histories gained from study of paragenetic sequences, can be used to quantify the evolution of petrophysical properties in carbonate rocks. We generate 3D pore architecture models (i.e. the spatial distribution of solid and pores) from 2D binarized images, representing the typical textural changes of carbonate sediments following hypothetical diagenetic pathways. For each 3D rock model, we extract the pore system and convert this into a network representation that allows flow properties to be calculated. The resulting porosity and permeability evolution scenarios display several ‘diagenetic tipping points’ where the decrease in permeability is dramatically larger than expected for the associated decrease in porosity. The effects of diagenesis also alter the capillary entry pressures and relative permeabilities of the synthetic cases, providing trends that can be applied to real rocks. Indeed, values of porosity and absolute permeability derived from these synthetic 3D rock models are within the range of values measured from nature. Use of such process-based diagenetic pathway models and paragenetic sequences can be used to provide constraints on predicted multiphase flow behaviour during burial/uplift scenarios using ‘diagenetic back-stripping’ of real carbonate rocks. From this we can gain insight into intermediate stages during basin evolution and populate burial models with dynamic properties.