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In this work, unstable displacements were conducted using special equipment designed to run in-situ CT-scanner experiments. All the displacements were conducted on a homogeneous Bentheimer sandstone plug, of 10 cm in diameter and 40 cm in length. Digitations (or fingering) have been observed under varying conditions of injection flowrate, displaced fluid viscosity, and core wettability. They have been characterized at both the core scale, using the core average oil saturation and the water breakthrough; and at the local scale, using the local saturations and had-hoc image processing analysis. It was found that the effect of the different flowing conditions on the front digitations could not be interpreted independently. The oil recovery at brine breakthrough showed a good correlation with the viscous fingering number for the water-wet case. However, a different scaling was observed for the oil-wet case. The interplay of the different flowing conditions mitigates the possibility of constructing a unique scaling number to account for all experimental condition. The local saturation monitoring has provided a new insight to characterize the finger shapes and analyze the production mechanisms. It allowed to distinguish two independent contributions to early breakthrough: viscous dominated digitations and capillary dominated digitations. A two-phases diagram has been constructed to plot and compare these contributions for all flowing conditions. Their evolutions show the main production mechanisms during the flooding. We observed that the viscous digitations were not causing phase trapping at core scale: the core is completely swept after breakthrough. For the water-wet case, we found that the local oil recovery of swept zone remained constant before and after breakthrough while for the oil-wet case it is improving during all the water flooding process.

We report an experimental study of N2-foam injection in a Bentheimer sandstone coupled with X-ray Computed Tomography (CT-scanner). The measurements of pressure drop and gas saturation at different flow rates and foam qualities allowed us to describe the foam dynamics under transient and steady-state flow conditions. The brine displacement by foam shows a transient piston-like displacement pattern taking place in two successive phases. Saturation profiles reveal permanent entrance effects related to the injection procedure, and transient downstream end effects related to the gradual foam build-up. Entrance effects are attenuated with a high foam quality and at low total flow rate. The rheological behavior of foam was studied in terms of apparent foam viscosity and foamed-gas mobility as a function of foam quality and gas interstitial velocity. In the low-quality regime, foam exhibits a shear-thinning behavior that can be modelled by a power function. Furthermore, for a fixed total velocity, the quasi-invariance of strong foam apparent viscosity values is shown to result from the slight increase of trapped gas saturation, within the commonly-admitted assumption of invariant foam texture in the low-quality regime. An increase in gas mobility was observed above a certain value of the foam quality. That transition between low-quality and high-quality regimes was related to a limiting capillary pressure of foam in the porous medium under consideration.

Summary To predict the effects of stress on rock permeability, the authors propose an integrated approach based on an extended rock characterization, an experimental investigation of pressure dependency of directional rock permeabilities and finally a pore-scale simulation of this dependency using equivalent pore network extracted from microtomography analysis. This study has been conducted on two analog reservoir rock types: the high-permeability Bentheimer Sandstone and a dual-porosity bioclastic carbonate, the Estaillades Limestone, having an intermediate permeability. Compression tests have been conducted using a new triaxial cell specially designed to measure directional permeabilities along and transverse to direction of maximum stress application. We measured the pressure dependency of porosity, directional permeabilities, compressibilities, and elastic moduli of the tested samples. We also performed computed microtomography (CMT) imaging of the rock samples, from which we extracted the poral skeletons and the associated characteristics lengths. Then, we calculated the macroscopic transport properties using Pore Network Modeling (PNM) based on the real pore geometry. We included a model of pressure dependence of pore and throat sizes based on pressurized cavity models derived from elasticity theory to simulate the evolution of porosity and permeability with pressure. First, we show that the experimental determination of the evolution of directional permeabilities under hydrostatic and deviatoric loading is feasible. Finally, we show that the PNM coupled with μ-tomography can be a promising tool to forecast the evolution of transport properties under stresses representative of reservoir conditions, at the condition of integrating more advanced pore-scale compaction models.

High Resolution 3D Reconstructions of Rocks and Composites — Ten µm resolution 3D representations of different media, were obtained with a laboratory computer microtomograph develo - ped from an electron microprobe column. From the original electron microprobe, only minor modifica - tions have been required, indeed several of the utilities of the microprobe have been used to ensure high resolution radiography (2 µm). The impact of the electron beam focused onto a thin film is used to form a "point" X-ray source and the radiographic image of the sample is acquired on a CCD camera. A speci - men rotation mechanism allows multiple radiograph acquisition and reconstruction of the X-ray attenua - tion 3D cartography. Since X-ray attenuation is directly related to density and atomic number, the microscanner provides 3D cartographs of the different phases present in the sample. System perfor - mances have been evaluated on various samples, mainly rocks and composites. Comparison with scan - ning electron micrographs was used when possible to validate the reconstructions. Results are mostly qualitative but already show the potential of the technique in describing 3D connectivity and topology of pore networks or 3D orientation of fibres in composites.

Much has been already done and written on the effect of wettability and spreading in three-phase displacement properties and efficiency. However, a direct visualization of the impact of these parameters in the distribution of the three fluids (water–oil–gas) within the pore space and consequently on the recovery mechanisms has been lacking. The objective of this paper is to obtain at the pore scale the necessary information to interpret the macroscopic behavior and to predict ultimate oil recovery for well controlled wetting and spreading conditions. To this end cryo-scanning electron microscopy (cryo-SEM) observations are performed in water-wet and oil-wet Fontainebleau sandstone plugs for spreading and non-spreading conditions. It is demonstrated that cryo-SEM can be used as a tool to study at the pore scale the fluid distributions and efficiency of oil recovery processes involving three phases. It is also proved that in a real porous medium both wettability and spreading affect gas injection efficiency by controlling three-phase distributions at the pore level and phase connectivity.

Cryo-SEM has become a powerful tool to show the distribution of oil and brine in cores, which may be highly influenced by wettability properties. Previous studies have shown special affinity between kaolinite and oil after aging. The present work clarifies the behaviour of illite- or kaolinite-bearing cores, and compares with the measurements made at the plug scale (`bulk' measurements). After aging, fibrous illite has still no affinity for oil, kaolinite has an affinity for oil, but platy illite behaves more like kaolinite. However, a minimal quantity of kaolinite-like surfaces is needed to change the wettability of the bulk sample towards oil-wetness. The `bulk' wettability depends thus, although not linearly, on the nature, morphology, amount and distribution of clay, but one cannot ignore phenomena occurring at larger scales such as trapping. A qualitative agreement is found between the amount of oil and brine, depending on the state of saturation, and the bulk measurements (wettability index, Swi, Sor).