Impact of Diagenetic Alterations on the Petrophysical and Multiphase Flow Properties of Carbonate Rocks Using a Reactive Pore Network Modeling Approach

Sedimentary reservoir rocks generally have complex and heterogeneous pore networks that are related to the original depositional rock texture and subsequent diagenetic alterations. Such alterations are in part controlled by the original mineralogy and sedimentological facies, the compaction history, the involved fluids (and rock/fluid interactions), the flow history and the related physico-chemical conditions. During the diagenetic evolution (paragenesis), cycles of alternating dissolution (porosity enhancement) and precipitation (porosity destruction) caused by changes in chemical and thermodynamic conditions may lead to heterogeneous rock structure at both local and reservoir scale. In the absence of cored plugs to measure the petrophysical properties (i.e. porosity, permeability and formation factor) and multiphase flow properties (i.e. capillary pressure, relative permeability and resistivity index), a numerical tool that calculates these properties from pore structure data by predicting its evolution during the diagenetic cycle is of great interest for the petroleum industry and reservoir characterization studies. A Pore Network Model (PNM) provides opportunities to study transport phenomena in fundamental ways because detailed information is available at the pore scale. It has been used over the last decades to understand basic phenomena such as capillarity, multiphase flow or coupled phenomena. In particular, this modeling approach is appropriate to study the rock/fluid interactions since the mass exchange at surfaces can be modeled explicitly. It can provide quantitative information both on the effective transport property modifications due to the reactions and on the structure evolution resulting from dissolution/precipitation mechanisms. In the present paper, this approach is used to study the effect of the diagenetic cycle on the petrophysical properties of carbonate rocks. It involves three discrete steps. The first step consists of replacing the original complex pore structure of real porous media by a conceptual network. The second step consists of resolving the governing equations of the precipitation and dissolution phenomena (i.e. reactive convection diffusion equation) in the conceptual 3D pore network and deducing the local reactive fluxes and the motion of the fluid-solid interface. The third step consists of updating the new pore structure and calculating the new petrophysical properties of the modified porous media. Those steps are repeated in order to mimic a given diagenetic scenario. Finally, the multiphase flow properties of the current porous media are calculated. The impact of one diagenetic cycle of dissolution and precipitation on the pore networks’ heterogeneity and consequently on the petrophysical properties (i.e. porosity and permeability) and multiphase flow properties (i.e. relative permeability and capillary pressure) have been investigated. The permeability and porosity evolution during a given diagenetic cycle are calculated and analyzed as a function of the relevant dimensionless numbers (Peclet and Damköhler numbers) that characterize the flow and reaction regime. The correlation between these numbers and the dissolved/precipitated layer thickness distribution is investigated. This work contributes to improve the understanding of the impact of dissolution and precipitation on permeability and porosity modification. Using the PNM approach, multiphase flow properties and permeability-porosity relationship have been determined for different reactive flow regimes. These relationships are relevant input data to improve the quality of reservoir simulation predictions

Quantification and Prediction of the 3D Pore Network Evolution in Carbonate Reservoir Rocks

This study presents an integrated approach that allows the reconstruction and prediction of 3D pore structure modifications and porosity/permeability development throughout carbonate diagenesis. Reactive Pore Network Models (PNM-R) can predict changes in the transport properties of porous media, resulting from dissolution/cementation phenomena. The validity and predictability of these models however depend on the representativeness of the equivalent pore networks used and on the equations and parameters used to model the diagenetic events. The developed approach is applied to a real case of a dolostone rock of the Middle East Arab Formation. Standard 2D microscopy shows that the main process affecting the reservoir quality is dolomitisation, followed by porosity enhancement due to dolomite dissolution and secondary porosity destruction by cementation of late diagenetic anhydrite. X-ray μ-CT allows quantifying the 3D volume and distribution of the different sample constituents. Results are compared with lab measurements. Equivalent pore networks before dolomite dissolution and prior to late anhydrite precipitation are reconstructed and used to simulate the porosity, permeability characteristics at these diagenetic steps. Using these 3D pore structures, PNM-R can trace plausible porosity-permeability evolution paths between these steps. The flow conditions and reaction rates obtained by geochemical reaction path modeling can be used as reference to define PNM-R model parameters. The approach can be used in dynamic rock typing and the upscaling of petrophysical properties, necessary for reservoir modeling

Quantification and Prediction of the 3D Pore Network Evolution in Carbonate Reservoir Rocks Quantification et prédiction de l’évolution d’un réseau 3D de pores dans des roches réservoir de carbonates

This study presents an integrated approach that allows the reconstruction and prediction of 3D pore structure modifications and porosity/permeability development throughout carbonate diagenesis. Reactive Pore Network Models (PNM-R) can predict changes in the transport properties of porous media, resulting from dissolution/cementation phenomena. The validity and predictability of these models however depend on the representativeness of the equivalent pore networks used and on the equations and parameters used to model the diagenetic events. The developed approach is applied to a real case of a dolostone rock of the Middle East Arab Formation. Standard 2D microscopy shows that the main process affecting the reservoir quality is dolomitisation, followed by porosity enhancement due to dolomite dissolution and secondary porosity destruction by cementation of late diagenetic anhydrite. X-ray μ-CT allows quantifying the 3D volume and distribution of the different sample constituents. Results are compared with lab measurements. Equivalent pore networks before dolomite dissolution and prior to late anhydrite precipitation are reconstructed and used to simulate the porosity, permeability characteristics at these diagenetic steps. Using these 3D pore structures, PNM-R can trace plausible porosity-permeability evolution paths between these steps. The flow conditions and reaction rates obtained by geochemical reaction path modeling can be used as reference to define PNM-R model parameters. The approach can be used in dynamic rock typing and the upscaling of petrophysical properties, necessary for reservoir modeling. Cette étude présente une approche intégrée qui permet la reconstruction et la prédiction des modifications de structure 3D de pores ainsi que le développement de la porosité/perméabilité tout au long de la diagenèse des carbonates. Des modèles de réseau de pores réactifs peuvent prédire les changements en matière de propriétés de transport de milieux poreux, résultant des phénomènes de dissolution/cimentation. La validité et prédictibilité de ces modèles dépendent toutefois de la représentativité des réseaux de pores équivalents utilisés et des équations et paramètres utilisés pour modéliser les événements diagénétiques. L’approche développée est appliquée au cas réel d’une roche dolomitique de la formation arabe moyen orientale. La microscopie 2D standard montre que le processus principal affectant la qualité de réservoir consiste en la dolomitisation, suivie d’un renforcement de la porosité dû à une dissolution de la dolomie et à une destruction de la porosité secondaire par cimentation de l’anhydrite diagénétique tardive. La microtomographie par rayons X informatisée (X-rayμ-CT; X-ray computer (micro)tomography) permet de quantifier le volume et la distribution en 3D des différents constituants d’échantillon. Les résultats sont comparés avec les mesures de laboratoire. Des réseaux de pores équivalents avant la dissolution dolomitique et préalablement à la précipitation d’anhydrite tardive sont reconstruits et utilisés pour simuler les caractéristiques de porosité, de perméabilité lors de ces étapes diagénétiques. En utilisant ces structures 3D de pores, la PNM-R (Pore Network Modeling Reactive; modélisation réactive de réseau de pores) peut suivre les voies d’évolution plausible de porositéperméabilité entre ces étapes. Les conditions d’écoulement et les vitesses de réaction obtenues par modélisation des voies de réaction géochimiques peuvent être utilisées en tant que référence pour définir les paramètres de modèle de PNM-R. L’approche peut être utilisée pour un typage dynamique de roches et le passage à une échelle supérieure de propriétés pétrophysiques, nécessaires pour la modélisation de réservoir