Modelling the stability of thin water films using SEM images. SPE EUROPEC/EAGE Annual Conference and Exhibition, Amsterdam, The Netherlands. SPE-121250: 22 pp.
Understanding reservoir wettability – including wettability alteration mechanisms – is important to model and to optimise oil recovery. When oil enters an originally water wet porous rock under primary drainage, the oil will gradually displace the water in the pores and a thin water film will be left between the pore walls and the oil. Under certain conditions, the water film may become unstable and collapse. Depending on the chemistry of the oil, water, and porous rock, this collapse may result in wettability alteration. Stability of the water film will control the wettability alteration of the porous rock. Equilibrium saturation configurations in the oil-water system at primary drainage are simulated in 2D for a realistic pore space geometry obtained directly from high resolution scanning electron microscope images and thus retaining geometric features of the porous rock under consideration. The eventual collapse of the wetting phase film is controlled by the curvature of the surface and the disjoining pressure isotherm. Given the critical disjoining pressure value depending on the temperature and the chemical properties of the system at hand, the model facilitates computation of the "wettability index", i.e., the fraction of the rock surface which will come in direct contact with oil and hence potentially become oil wet. Both synthetic and realistic images are analysed. The generic studies show the influence of the surface roughness of the pore walls on the fraction of the surface which can potentially be converted to oil wet for different drainage pressures.
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Induced geometry in chalk during hydrochloric acid stimulation. In: Burlion, N. & Rougelot, T. (eds.), Thermo-Hydromechanical and Chemical Coupling in Geomaterials and Applications: Proceedings of the 3rd International Symposium GeoProc '2008, Lille, France. Wiley: 187-194.
The oil bearing high porous chalk formations in the North Sea are characterized by high CaCO3 content and hence highly soluble in acids. This combined with low permeability results in a frequent use of acid treatments for stimulation. Previous investigations concerning the created geometry suggest the acid to create wormholes in the chalk. Casing deformations and production-log results combined with the stimulation rates and pressures used, indicate that other geometries may develop in the reservoir. The objective has been to test existing theories by stimulating chalk samples using different core setups in a tri-axial cell. This study confirms the importance of rock properties and flooding regime for the resulting acid etched geometry. SEM-studies are also included. One suggest the acid is most likely forming a cavity around the liner for the typical field stimulation rates and pressures used in North Sea chalks.
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Laboratory observation with implications for depletion of chalk reservoirs. In: Ling, H.I., Smyth, A. & Betti, R. (eds.), Poromechanics IV: Proceedings of the 4th Biot Conference on Poromechanics. DEStech Publication, Inc.: 953-958.
Failure mechanics and fluid flow are of great importance when considering weak oil bearing formations. During primary production, and displacement of in-situ hydrocarbons by waterflooding, the reservoir rock is going through different loading and deformation phases. Major reservoirs in the North Sea consist of weak high porosity chalk, which is very compressible and has low matrix permeability (1-10 mD). High porosity chalk is also behaving inelastic before yield, and tend to yield in pore collapse during depletion. The porosity, which largely governs the yield strength of chalk, varies significantly in the North Sea chalk reservoirs. As the pore pressure is depleted beyond the quasi-elastic phase during production, a dramatic reduction in porosity occurs due to pore collapse. As the reservoir flow is restricted due to the low permeability and the limited number of wells, partially drained conditions will then occur that may result in pore pressure increments at various locations within the reservoir. This increase in pore pressure will provide more drive energy than traditionally predicted by standard reservoir engineering methods. When loading rock specimens from in-situ conditions by depletion in the laboratory, the pore pressure is significantly impacted when the specimen yields and undergoes larger deformations. This study provides evidence of pore pressure increase when the material has collapsed mainly in the plastic phase, but also in the quasi-elastic phase. The results are analysed in terms of depletion method and rate, as well as pore fluid and rock properties. These findings are of significant importance for the interpretation of laboratory depletion tests and further application to the field.
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Chemical water weakening of various outcrop chalks at elevated temperature. In: Ling, H.I., Smyth, A. & Betti, R. (eds.), Poromechanics IV: Proceedings of the 4th Biot Conference on Poromechanics. DEStech Publication, Inc.: 543548.
During seawater injection into North Sea chalk reservoirs, a weakening effect leading to increased compaction followed by serious seabed subsidence was observed. To further understand this water weakening effect, mechanical testing with continuous flooding with various aqueous fluids at elevated temperatures was conducted. The tests reveal that for both high and lower porosity chalks, the impact of varying the flooding fluid caused a weakening in the following order: distilled water < synthetic seawater without sulfate < synthetic seawater < synthetic seawater without magnesium. Flooding with synthetic seawater without magnesium led to increased weakening, suggesting a chemical weakening process involving mineral precipitation and enhanced dissolution of calcite.
Results from chalk containing slightly higher amounts of silica indicate that non-carbonate minerals in chalk seem to have a pronounced effect on the water weakening of chalk; however, the process seems to be governed rather by dissolution at silicate-calcite contacts than precipitation-dissolution processes.
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Mechanical properties of high and lower porosity outcrop chalk at various wetting states. The 43rd U.S. Rock Mechanics Symposium and 4th U.S.-Canada Rock Mechanics Symposium, Asheville, USA. ARMA 09-139: 1-7.
In order to study the influence of different wetting states on the mechanical properties of chalk, tests on both high porosity (45-49 %) and lower porosity (37-40 %) outcrop chalks had been conducted. The initial water saturation for the chalk cores was low. The cores were flooded with oil with different acid numbers and aged at reservoir temperature following a procedure designed to achieve a pre-determined wetting condition. The prepared cores were mounted in triaxial loading cells at reservoir temperatures in order to investigate mechanical parameters. Two types of fluids such as formation brine and synthetic seawater were flooded through the cores during the experiments. The results revealed that intermediate-wet cores were stronger than preferential and completely water-wet cores. The mechanical strength of chalk at various wetting conditions might be attributed to: (1) acid number of oils and the effect of oil films close to the intergranular contacts and/or (2) chalk dissolution and precipitation processes.
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