Reaction Transport Modelling (rtm) in the Prediction of Dolomitisation Distribution in Carbonate Reservoirs.

Abstract

Abstract The application of Reaction Transport Modelling to cases of dolomitization allows testing the genetic interpretation and may help in predicting the lateral extension of the dolomite hydrocarbon reservoir in the subsurface. The results of numerical diagenetic modelling simulations of four different dolomitization processes are presented. In all the cases the simulations suggest that the hydrogeologic system is the most important driver for dolomitization: in fact the final geometry of dolomite bodies is greatly affected by the permeability field and presence of fractured zones. Intoduction and methods The dolomitization processes produce alterations in the pore network system of carbonate bodies from deposition to deep burial (Machel, 2004); therefore the comprehension and the distribution prediction of the processes and their product is of paramount importance in hydrocarbon research both in explorative and development phases. A well constrained diagenetic interpretation allows assumptions to be made about the distribution of the dolomitized bodies in the subsurface. The new approach in diagenetic studies is to integrate the classical investigative techniques with the reactive transport modelling (RTM) in order to better define the conceptual models in terms of mass balance, kinetic and thermodynamic constraints (Whitaker et al., 2004). The numerical modelling applied to carbonate diagenetic systems is a fast growing discipline in the petroleum industry for its help in prediction of the extension of the diagenetic processes and related changes in petrophysical properties. Four different dolomitization models have been simulated using a sensitivity analysis approach in order to strengthen the conceptual interpretation and to evaluate the main constraints of each dolomitization process. The RTM was performed using the TOUGHREACT code (Xu et al., 2004); in this code the geochemical reactive transport is coupled to the TOUGH2 framework of multi-phase fluid and heat flow. The code calculates rock/water system evolution through the simulation of brine flux in a porous media, transport of chemical species in water solution and mineral/fluid reactions. The simulator can be applied to 1-, 2-, or 3-dimensional unstructured domains with physical and chemical heterogeneities. The first parameter to be simulated in a dolomitization reconstruction is the fluid flow into the rock. In the cases of synsedimentary dolomitization the amount and rate of fluid flow is the result of the chemio-physical conditions, for example in the reflux dolomitization the fluid flow is the direct consequence of fluid density in the system; in the dolomitization cases in which the fluid flow is linked to processes external to the area under simulation, for example in compaction dolomitization model, the evaluation of fluid flow amount and rate is to be simulated using a basin simulator (SEBE, Eni proprietory coftware).