Chirurgia vascolare

Accurate modeling of foam rheology on the field scale requires detailed understanding of the correlation between the fundamental properties of foam and the scalable parameters of the porous medium. It has been experimentally observed that foam experiences an abrupt coalescence when the capillary pressure in the porous medium approaches a certain value referred to as the “limiting capillary pressure”, <i>P</i>_c^<i>*</i>. Current foam models that treat foam texture implicitly mimic this fundamental behavior with a so-called dry-out function, which contains adjustable parameters like fmdry and epdry (in the STARS foam simulator). Parameter fmdry (called <i>S</i>_w* in other models) represents the water saturation corresponding to the limiting capillary pressure, <i>P</i>_c^<i>*</i>, and epdry determines the abruptness of foam coalescence as a function of water saturation. In this paper, using experimental data, we examine the permeability dependence of these parameters. We find that the value of fmdry decreases with increasing permeability. We also find that, for the data examined in this paper, the transition from the high-quality regime to low-quality regime is more abrupt in lower-permeability rocks. This implies that in high-permeability rocks foam might not collapse abruptly at a single water saturation; instead, there is a range of water saturation over which foam weakens. In addition, we address the question of whether <i>P</i>_c^<i>*</i> is dependent on formation permeability. We estimate <i>P</i>_c^<i>*</i> from data for foam mobility versus foam quality and find, as did Khatib et al. (<i>SPE Reservoir Eng.</i>, <b>1988</b>, <i>3</i> (3), 919–926), who introduced the limiting capillary pressure concept, that <i>P</i>_c^<i>*</i> can vary with permeability. It increases as permeability decreases, but not enough to reverse the trend of increasing foam apparent viscosity as permeability increases

Wettability Evaluation of a CO₂/Water/Bentheimer Sandstone System: Contact Angle, Dissolution, and Bubble Size

The success of CO₂ storage in deep saline aquifers and depleted oil and gas reservoirs is largely controlled by interfacial phenomena among fluid phases and rock pore spaces. Particularly, the wettability of the rock matrix has a strong effect on capillary pressure, relative permeability, and the distribution of phases within the pore space and thus on the entire displacement mechanism and storage capacity. Precise understanding of wettability behavior is therefore fundamental when injecting CO₂ into geological formations to sequestrate CO₂ and/or to enhance gas/oil production. In this study, the contact angles of Bentheimer sandstone/water/CO₂ or flue gas have been evaluated experimentally using the captive-bubble technique in the pressure range from 0.2 to 15 MPa. The experiments were conducted using different compositions of aqueous phase with respect to CO₂, i.e., unsaturated and fully saturated. It has been shown that a reliable contact-angle determination needs to be conducted using a pre-equilibrated aqueous phase to eliminate dissolution effects. In the fully saturated aqueous phase, the Bentheimer sandstone/water system is (and remains) water-wet even at high pressures against CO₂ and/or flue gas. In these systems, the data of the stable contact angle demonstrate a strong dependence on the bubble size, which can be mainly explained by the gravity (buoyancy) effect on bubble shape. However, the surface nonideality and roughness have significant influence on the reliability of the contact-angle determination. The results of this study prove that in order to avoid the dependency of the contact angle on the bubble size in these systems, the effect of gravity (buoyancy) on bubble shape has to be considered by calculation of the Bond number; for systems characterized by Bond numbers less than 0.9, the influence of the bubble radius on the contact angle becomes insignificant. The experimental results show that, in contrast to quartz, the phase transition of CO₂ from subcritical to supercritical has no effect on the wettability of the Bentheimer sandstone/water system, which originates from differences in the surface charges of quartz and Bentheimer sandstone. In an unsaturated system, two dissolution regimes are observed, which may be explained by density-driven natural convection and molecular diffusion