Pore-Network Evolution Induced by Interaction Between Minerals and Migrating Fluids: Implications for Rock Diagenesis

The reservoir quality of sedimentary rocks is closely related to diagenesis, a process involving post-depositional alteration of freshly deposited sediments. Present-day rock properties such as porosity, permeability, pore-size distribution, and information on heterogeneity and spatial correlation of variables can be the products of diagenetic modification of original (paleo-) properties. Other than compaction, the most important property-modifying diagenetic processes are chemical reactions between minerals and migrating pore fluids. Fluid-mineral reaction is a dynamic process that involves many effects: complex fluid flow, pore space evolution, surface chemistry, and mineralogy. We have developed a pore network model to incorporate essential features necessary to describe the process. The general idea behind the model is to approximate the description of the pore space, surface reaction, diffusion, pore growth/shrinkage, and movement of fluids therein by a small set of parameters and simple rules governing the complex alteration of rocks in situ. A two-dimensional pore network consists of narrow 'pore throats', modeled as laminar-flow tube reactors, and wider 'pore bodies', modeled as stirred-tank reactors. The fluid flow field is controlled by interconnected pore throats. Pore bodies and pore throats either grow or shrink as chemical reaction proceeds at their surfaces. The rate of growth or shrinkage is governed by the transport from the fluid to the pore surface and reaction kinetics at the surface. One advantage of the network approach is that crucial issues such as the complexities of surface reaction and transport in the pore space, development of secondary porosity and evolution of permeability are included. Other advantages are that the relative importance of each variable can be evaluated quantitatively and factors that previously have not been thought to be important now become apparent. The fully developed network model also possesses a predictive capability beyond its explanatory function. The main practical application of all this is accurate prediction of secondary porosity type and its relation to permeability in potential hydrocarbon reservoirs for exploration and production. In this study the network model is employed to simulate diagenetic processes of two types of rocks: dolomitization from carbonate rocks and calcite cementation of sandstones. The results show the origin of dolomite, permeability reduction of sandstone by cementation, time evolution of pore growth/shrinkage patterns, nature of pore-size distribution, degree of heterogeneity and spatial correlation, secondary porosity and permeability development.