Integrating Permeability Trends in Reservoir Characterization Using Diagenesis Modeling
Authors: Le Gallo Y. , Bildstein O. , Brosse E. 
Published: December 2007

Within the same geological facies, properties such as permeability can be strongly altered by mechanical compaction and later on chemically modified by water-rock interactions during diagenetic periods.

A new and original reaction - transport and flow code DIAPHORE was developed to address multi-dimensional mass transport and water-rock interactions at the reservoir scale. DIAPHORE was designed to simulate short-term diagenetic episodes, i.e., mineral transformations that occur in less than a few million years. More and more data show that a wide range of diagenetic transformations can be considered completed in this limited period of time. A good example is provided by the illitization process in many deep reservoir sandstones of the Northern North Sea. Accordingly, DIAPHORE is not suited to simulate processes involving sediment compaction: neither porosity-depth evolution due to mechanical rearrangement of grains nor pressure-solution phenomena due to coupling between mechanical stress and dissolution/precipitation.

The DIAPHORE software is a three-dimension two-phase fully-implicit reservoir engineering simulator. In reservoirs, interstitial water and rock minerals are usually not in thermodynamical equilibrium. Thus, the water-rock interaction module couples kinetically controlled slow mineral reactions for dissolution and precipitation (up to 60 different minerals) with fast equilibrium reaction among chemical species in water phase (up to 140 aqueous species). The kinetic rate laws used for the mineral dissolution and precipitation derive from theoretical studies: Transition State Theory and Surface Coordination Chemistry. They also include empirical parameters to better fit experimental observations. The mass transport modules couples the flow of dissolved chemical species in the reservoir (based upon mass balances) with the kinetic geochemical reactions. The model is based upon macroscopic mass balance equations for oil and water phases using the finite volume approximation. The DIAPHORE code also includes the feedback of chemical reaction upon texture (reactive surface areas and porosity) and permeability. Several permeability models implemented are Carman-Kozeny, Brinkman and Fair-Hatch. The DIAPHORE code is applied to model the influence of mineral diagenesis transformations in a Brent Group sandstone reservoir from North Sea. In the Dunbar field, porosity and permeability have been reduced during relatively deep burial by diagenetic illite and quartz. The prediction of this phenomenon at the reservoir scale is a serious challenge when building a reliable flow-unit architecture needed during field development.

The results presented concern the upper part of the Brent Group from Ness B Unit to Tarbert Formation where lies most of the hydrocarbon reserves in the Dunbar field. The Ness B is composed of sandy tide dominated near shore complexes. At Dunbar, Tarbert is composed of an unusual succession of three units. In the reservoir sandstones of the Brent Group at Dunbar, illite is abundant whereas kaolinite and K-feldspar are only residual and very seldom co-exist in the same thin section. By contrast at Alwyn North, illite formation is rare and the sandstones are composed of quartz with minor proportions of K-feldspar (generally less than 15 vol.%) and kaolinite (generally less than 10 vol.%). Kaolinite is a diagenetic mineral which precipitated partly in conditions of shallow burial and partly in conditions of moderate burial as documented in numerous other fields of the Brent Province. Illite, calcite, pyrite are frequently observed but at levels less than 2 to 3 vol.%. From geochemical and petrophysical controls available at Dunbar, it turns out that most illite and secondary quartz formed at ca 100 C, 300 bars and 3.6 km concomitant with dissolution of detrital kaolinite and K-feldspar. The relevant geochemical system for modeling and understanding the illitization process in sub-arkosic sandstones during burial considers at least these four minerals (quartz, K-feldspar, kaolinite and illite). In our simulations, albite and calcite are also considered. The four mineral phases taken into consideration lead to conservation equations for nine elements: Si, Al, Na, Cl, K, Ca, C, O, and H.

From thermodynamics the illitization process is predictable even at relatively low temperature. Stability diagrams show that with such mineral precursors the overall illitization reaction is the transformation of kaolinite and K-feldspar into quartz and illite:

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