Although half the world’s hydrocarbons output comes from carbonate formations, the exploration and especially the production of reserves contained in such reservoirs remains complex, due to their significant and rather unpredictable heterogeneity. An optimized and pertinent characterization coupled with a specific model are crucial for reaching a better understanding of the structures, their dynamic behavior and their future production profiles.
By pooling our geological and engineering expertise, we have developed a disruptive approach based on the use of geologic outcrops as reservoir analogues, which we call dynamic analogues. This is the innovative approach being implemented on the Albion R&D project in France, which will enable us to observe in situ fluid flows in a geological environment similar to that of certain carbonate reservoirs in the Middle East.
Improved prediction accuracy
A great majority of the hydrocarbon reserves in the Middle East – 90% of the gas and 80% of the oil –is found in carbonate reservoirs, i.e., in calcareous or dolomitic rocks. The structural organization of carbonate formations is difficult to appraise due to the significant heterogeneity typical of these rocks, which results from several factors that include the biological component of limestone formation, the chemical instability of the calcium carbonate that makes up the rock, and the brittle deformation phenomena that affect carbonates more than other types of rock. For these reasons, grasping the geology of carbonate reservoirs is always a challenge for the oil and gas industry.
Models are therefore an essential tool for predicting the dynamic behavior of the reservoirs, optimizing field developments and predicting the future production profiles with the related uncertainties. The problem is that modeling has proven more difficult for carbonates than for other reservoirs, precisely because of their heterogeneous petrophysical properties, a legacy of their complex geological history. Extrapolations from characteristics measured at the well are often imprecise, and this will undermine accuracy when predicting dynamic behavior and production levels.
Maximizing the value of resources found in carbonate reservoirs thus demands a clearer understanding of the spatial distribution of storage and flow properties within the rocks in relation to their stratigraphic, sedimentary, diagenetic and structural history. Correlating this geologic history with the fluid dynamics of the fields during production is one key to improving the fidelity of our models. Accordingly, we have spent the past several years searching for dynamic analogues, i.e., reservoirs presenting partially similar properties, or field analogues featuring characteristics comparable to those of the carbonate reservoirs.
Discover the Albion project in the heart of the Vaucluse region
A dynamic field analogue is, above all, a site that will help us answer a number of specific questions by facilitating direct access to the reservoir as well as to the fluids it contains. Naturally, we are not looking here for hydrocarbon flows, but rather for water circulating in aquifers. In other words, being able to observe the reservoir from the surface and reach the aquifers at very shallow depths are the two vital prerequisites when selecting a dynamic analogue. One example is the Albion plateau, in the hills of the Vaucluse département of Southern France, where most of the outcrops consist in Urgonian limestone from the Early Cretaceous (some 125 million years ago). This formation has long been viewed as a very good analogue of some of the carbonate reservoirs of the Middle East – comparable in age, sedimentary facies, paleo-environment and reservoir properties. Moreover, it features two unique objects that make it an exceptional scientific laboratory:
- The Fontaine de Vaucluse. This spring, Europe’s largest, is the only exit point of a 1,100 km² underground basin lying beneath a limestone cliff 230 meters high. Featuring a very hierarchical arrangement of flow paths from the fracture matrix to the karst conduits, the Fontaine de Vaucluse is the catchment for most of the Urgonian reservoir. Decades’ worth of data are available documenting the various parameters that characterize the spring.
- The Laboratoire Souterrain à Bas Bruit (LSBB), or Low Background Inter-Disciplinary Underground Science & Technology Laboratory. For many years, the Albion plateau served as the base for France’s nuclear dissuasion force. Since that time, some of the military facilities have been repurposed as an inter-disciplinary research center. Offering nearly 4 kilometers of sub-horizontal tunnels bored through the limestone rock, along with five vertical boreholes and three horizonal cores, the LSBB allows not only unlimited access to the reservoir rock, but also in situ observation of the internal hydrodynamics inside the tunnels.
A continuum of dynamic analogues
The Albion project was launched in 2014 in partnership with the French Center for Scientific Research (CNRS) and a number of French universities (Avignon, Grenoble, Marseille Montpellier, Nice-Sophia-Antipolis and Pau). As partners in this prestigious scientific undertaking, researchers from Total and academia are contributing to the creation of new multiphysics and multi-scale knowledge:
- Multiphysics: the reservoir and the fluid flows are studied through a characterization that draws on numerous fields of expertise (e.g., sedimentary and structural geology, inorganic geochemistry, petrophysics, fluid mechanics, geomechanics, geophysics, hydrogeochemistry),
- Multi-scale: in addition to the LSBB tunnels and the area surrounding the Fontaine de Vaucluse, other zones of the Albion plateau have been delineated. Investigations have been performed to acquire data from boreholes, outcrops, or using geophysical methods. In other words, Albion is not a single dynamic analogue, but rather a continuum of dynamic analogues that we use to study the reservoir at different scales.
Following the completion of the first phase of the project, the Albion 2.0 project is now under way for the period 2019-2021. Although a few more acquisitions are scheduled during this second project phase, the aim now is to collate and synthesize all data acquired to develop modeling principles appropriate for this challenging context. At that point, we will be ready to apply the new concepts and tools to our reservoirs in the Middle East.
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