First EAGE Workshop on Evaluation and Drilling of Carbonate Reservoirs

In this work, we investigate the effect of alteration of fracture total permeability, relative permeability and capillary pressure with the change of normal net stress (due to fluid injection or depletion), on estimated oil recovery factor from fractured carbonate reservoir. The numerical experiments are performed in two steps: first, we predict two-phase fluid-flow properties of fracture for different levels of normal effective stress using our in-house geomechanical model; and second, we study the effect of normal net stress in the reservoir on oil recovery factor, using conceptual simulation models and a commercial simulator.

Our main conclusions of this study:

• Oil recovery and water production in fractured carbonate reservoirs is controlled by, not only the fracture permeability (and other matrix related parameters), but also fracture rock curves.
• Complex behavior of fluid flow in fractures cannot be captured only by varying fracture permeability. Shape of relative permeability and its dependence on the effective stress have a significant impact on flow of water vs. oil.

The carbonate package of Upper Permian (Zechstein) age in Mid-Norway is underexplored. Sedimentological analysis of the four wells that penetrated this interval confirm the model of platformal carbonates located on depositional highs and more restricted carbonates deposited within the basins. Only one intriguing well situated on the Nordland ridge exhibits +250 m of heavily dolomitised carbonates carrying no clues about depositional settings. The original depositional character of these unusual foliated dolomites is a mystery and their coarse nature along with other evidence is indicative of fault-related hydrothermal activity. In order to gain insight into the internal heterogeneity and architecture of the Upper Permian carbonates, advanced sedimentological and geophysical technologies deemed necessary. The geological information extracted from the outcome of these technologies in particular analysis on IsoMetrix high resolution 3D seismic is far more diagnostic than conventional reflection seismic data interpretation. An Upper Permian carbonate play may be matured further only by utilizing these advanced technologies.

This research was conducted in Wonosari Formation because those carbonate rocks making up good representatives as analogue because it has been suffered diagenesis intensively. The research methods by stratigraphic measurement in the field. Rock samples were prepared for thin section analysis and pororsity measurement. The main object which is investigated are lithofacies, diagenetic products (such as cemen type, mineralogy and other products that may be encountered such as paleosoil, karst and caliche), pore type and porosity. Diagenetic history of carbonate rocks in Wonosari Formation will be understood by the result identification of diagenetic product and the interpretation of diagenetic processes.

Early Hypothesis, carbonate rocks of Wonosari Formation experienced some diagenetic process in marine, burial and meteoric conditions in early stages, then continued to burial diagenesis until exposed to the surface and has suffered meteoric diagenesis till now. Based on diagenesis history that has been explained, carbonate rocks of Wonosari Formation would have increasing or decreasing of porosity quality. The significant porosity reduction occurred by cementation process, whereas the significant porosity enhancement occurred by the dissolution process. This dissolution process as a diagenesis product with huge potential as an oil and gas reservoir.

Brazilian deep water pre-salt reservoirs are located over 200 km from the coastline and 5000 m below sea level, with 2000 m water depth. To reach these oil rich limestone reservoirs, operators drill more than 1000 m of siliciclastic and limestone rocks before encountering the 2000 m thick salt layers. Before entering the reservoir, there is an intermediary borehole section, usually of 17 ½” size, drilled through the salt. Casing has to be set inside competent formation (anhydrite or limestone) with minimal reservoir entrance to guarantee wellbore stability.

Combining local geoscience expertise and high technology drilling services, the drilling operation is optimized to identify changes in drilling dynamics and to perform proper formation characterizations, where resistivity at the bit technology enhances the geological interpretation for an effective and accurate decision while drilling. Such technologies have been successfully applied, minimizing operational risks and reservoir entrance.

Analysis of a regional dataset of (old and new) wells and seismic data in the Dutch subsurface, in comparison with nearby outcrops and with analogue carbonate systems elsewhere have yielded new, more positive, insights into the original reservoir development and the possible present-day range in reservoir quality of the underexplored Devonian Kyle Limestone and the Dinantian Carbonates. Large variations are present, both between the two systems and within the Dinantian Carbonates. These insights support predictions of reservoir quality in these strata. At the same time, the results of modelling of variations in seismic character of these carbonates, as a function of composition, porosity, pore fill and overburden properties, can be used in the reprocessing and interpretation of existing seismic and in the acquisition of new seismic data. This will be part of a nation wide study into the potential of Ultra Deep Geothermal projects in carbonate reservoirs.

The Upper Jurassic Malmian in the Southern Germany Molasse Basin developed in the last 15 years to an increasingly intensive used carbonate reservoir for producing geothermal energy. The over 20 geothermal wells (reaching depths of up to 5.000 m TVD) which were investigated by ERDWERK, provide a significant amount of information to optimize the future exploration strategies. (3D-) seismic data, drilling parameters, cuttings, geophysical logs as well as outcrop analogues studies allow for conclusions to be drawn concerning the lithofacies, diagenesis, carstification, geodynamic anomalies and strucural framework of the carbonate reservoir, which are the drivers for the inflow zones and so the overall productivity of the geothermal wells. The study presents results from the structural and facies analysis of the Malmian carbonate reservoir and exhibits the requirements for a decision-making workflow for an optimal exploration.

A large proportion of carbonate reservoirs contain natural open fractures, which have a significant impact reservoir fluid flow and ultimate recovery. In this study, high resolution X-ray computed tomography (µCT) has been used for imaging, interpretation and analysis of open fracture in carbonate in true 3D. The samples are form the Shetland Group in the Gullfaks Field, which has produced oil since late 2012. The µCT data shows that the tensile fractures occur as calcite veins or are partly filled with calcite cement. Fractures tend to be most frequent in low porosity parts of the formation, and bioturbations also have a significant effect on the shape, orientation and occurrence of fractures. The µCT gives the detailed geometry and aperture distribution of the fractures in 3D.