Capillary trapping

On a microscopic scale (the inset represents about 1 - 2mm ), even in parts of the reservoir which have been swept by water, some oil remains as residual oil. The surface tension at the oil-water interface is so high that as the water attempts to displace the oil out of the pore space through the small capillaries, the continuous phase of oil breaks up, leaving small droplets of oil (snapped off, or capillary trapped oil) in the pore space. Typical residual oil saturation (S ) is in the range 10-40 % of the pore space, and is higher in tighter sands, where the capillaries are smaller. 

Chemical processes work either to change the mobility of a displacing fluid like water, or to reduce the capillary trapping of oil in the rock matrix pores. Reducing the mobility of water, for example by adding polymers, helps to prevent fingering, in which the less viscous water bypasses the oil and... 

The purpose of alkali flooding (Jennings et al., 1974) is to introduce alkali into a reservoir where it can react with organic acids in the oil to produce organic salts, which act as surfactants. The surfactants (or petroleum soaps ) generated reduce the surface tension between the oil and water and this in turn reduces the level of capillary trapping of the oil. Thus, more oil is recovered because less of it remains trapped in the formation s pore spaces. 

Breath, bl ood uri ne Breath collected on Tenax, blood and urine subjected to purge-and-trap, concentrated on cryogenic capillary trap, thermally removed to GC. GC/MS No data No data Barkley et al. 1980... 

Ai r Collect on Tenax GC, thermal desorption, cryogenic collection on a capillary trap, thermal transfer to GC GC/MS 0.47 parts per tri 11 i on No data Krost et al. 1982... 

Utilization of a part of capillary column as an extraction element and thermal desorption for the release of analytes (Coated Capillary Microextraction - CCME), (Thick Film Open Tabular Trap - TFOT), (Thick Rim Capillary Trap - TFCT)... 

The mobilization of a trapped phase during displacement has been the subject of extensive research (ll>24-26,28-32). These papers all dealt with the concept of viscous forces overcoming capillary trapping forces. Their results can be applied to understand the behavior seen in displacement experiments performed over a range of Ca by several researchers to study residual oil structure and saturation. As Ca increased, the maximum trapped blob size decreased, until eventually singlets were mobile. 

LSD analogs and metabolites were used as model compoimds by Cai and Henion [73] in the development of on-hne immimoaffinity extraction coupled to capillary-colutrm LC-MS-MS. The system consists of a 2.1-mm-lD protein G iimmmoaffinity colutrm with a noncovalently immobilized antibody, which can be operated at flow-rates of up to 4 ml/min, a short packed-capillary trapping column, and a packedcapillary analytical column, operated at 3.5 pl/mia The method enables low ng/1 determination of LSD and related compoimds in urine, which is twenty-fold better than previous methods based on SPE and LC-MS-MS [73]. 

Figure 11.2 Illustration of capillary trapping in small (a) and large (b) water-wet pores. When at the same height above the free water level, the radii of the oil-water interfaces will be the same. The larger pores contain less water as a percentage of pore volume.

POLYMER-SURFACTANT INTERACTION AND ITS EFFECT ON THE MOBILIZATION OF CAPILLARY-TRAPPED OIL... 

Middle-phase microemulsions, which can exist in equilibrium with almost pure brine and oil, have been shown by many authors (1) to be most suitable for mobilization of capillary-trapped waterdrive residual oil. This is explained on the basis of the extremely low interfacial tension between this type of microemulsion and oil (- 10" mN/m), causing almost complete mobilization of capillary-trapped oil, and on the basis of the equally low interfacial tension between such a microemulsion and brine, which prevents the trapping of a surfactant-rich microemulsion phase. 

Secondary oil recovery by spontaneous imbibition of water into low-permeable fractured chalk is a well accepted method to improve the oil recovery from water- to mixed-wet rock material [2], Normally, this process is driven by capillary forces, and it may seem a little strange to lower the capillary forces by adding surfactants to the injected water. In the same way as the viscous forces will mobilize capillary trapped water-flooded oil, the gravity forces may be active in displacing the oil by spontaneous imbibition at low IFT [3]. A crossover from a capillary forced spontaneous imbibition (counter-current flow) to a gravity forced imbibition (cocurrent flow) is observed by decreasing the IFT. The status and recent advances in the research in this area will be included in this presentation as well. 

Indeed, a modification of the chromatographic system is required if the on-line methods are going to be applied in the conventional off-line method the chromatographic system does not need to be ehanged, but additional instrumentation for sample treatment such as a vacuum manifold was used. The CS-capillary LC-DAD approach requires a capillary trapping column and an additional switching microvalve over an standard capillary LC equipment. The IT-SPME based methods only require the replacement of the iimer loop of the injection valve by a capillary extractive column. Thus, the implementation of the IT-SPME-LC-FLD method only requires a minimum modification of conventional LC equipments. Thererefore, this is the most eost-effective approach. 

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