Wettability-Alteration油藏润湿性的讨论.docx

WETTABILITY ALTERATION
Kai Lang Huang, China University of Petroleum, Beijing Campus (CUP)
Abstract
Wettability of rocks is a combined character reflecting the interaction between the rock mineral and the reservoir fluids, as important as other parameters such as porosity, the permeability, the saturation, and the pore pressure. Wettability alteration, refers to the reversal of wettability of rock from oil-wet to water-wet or from water-wet to oil-wet similarly, making a great contribution to enhancing oil recovery, caused by the injected soluble surfactants mainly researched in this paper. When the formation is water-wet, a large amount of oil can be drive out. This paper mainly research the mechanisms for wettability alteration, and its ‘application in EOR and what type and kind of surfactants can be used to improve oil recovery.
Introduction
Wettability is evaluated by measuring water-oil contact angles, when measured at different stages, different angle can obtain thus indicating the rock is oil-wet or water-wet. The term “wettability alteration”, also called “wettability reversal” refers to reversal of the wettability of a solid surface where surface-active substances are adsorbed. For example sand, are water-wet originally, but when the rock core was saturated with surface-active substance, due to adsorption, sand grain can become oil-wet. Similarly, for some rock, oil-wetting originally can also undergo a wettability alteration. Wettability alteration can take place during active production stage and also depending on the variations of surface properties of grains and fluid compositions changes across the reservoir.
It has been recognized that half of the world’s known oil reservoir are in carbonate reservoirs. Many of these carbonate reservoir are naturally fractured, however most carbonate reservoirs(about most 80%) are oil-wet or mix-wet , thus if the formation is water-wet , the positive capitally forces helps in imbibing water into the matrix and expelling the oil out, enhancing the oil recovery by the wettability alteration during surfactant flooding or Surfactant-polymer flooding in carbonate reservoirs. Hence fractured reservoir with high low matrix permeability need EOR technique to achieve maximum production, dilute surfactants with appropriated amounts are developed to dramatically improve the oil recovery from the fractured carbonates.
The mechanism of the wettability alteration
Wettability is a major factor in controlling the location, flow, and distribution of fluids in a reservoir. Polar crude oil components can be adsorb on mineral surfaces and alter their wetting properties. It’s essential to recognize that the polar components of the crude oils can be adsorb by several distinct mechanisms, depending on factors that include brine composition and the solvent quality of the oil for its asphaltenes. It’s recognized that there exit four types of mechanical interaction. ( I ) polar interaction that predominate in the absence of a water film between oil and solid, ( ii ) surface precipitation .dependent mainly on crude oil solvent properties with respect to the asphaltenes, ( iii ) acid/base interaction that control surface charge at oil/water and solid/water interfaces, ( iv ) ion binding or specific interactions between charged sites and higher high valence ions. Fig a clearly shows the mechanism of the function of surfactant, it indicates that molecules of the anionic surfactant (red circles show) can be adsorb by several distinct mechanisms into the interface, while the polar components of the crude oil are also be adsorb into the interface, during the spontaneous imbibition of anionic surfactants into the oil-wet carbonate, active surfactants reduce the contact angle thus alert the wettability of the rock, the capillary pressure as driving force, increase the oil production.

The Effect of Wettability alteration
As saturation is one of the key parameters in the calculations of the volumetric estimates of hydrocarbon reserves.it appears that all the physical properties that are related to the saturation are affected by the rock’s wettability. For instance, capillary pressure curves behave differently depending on the degree of wettability as depicted in fig.2 for the both water-and oil-wet rocks.

In the formation the wettability affect many parameter, when the wettability reverse it will definitely cause great effect on the reservoir. Here list the relationship between the rock physical parameters (porous plate capillary pressure. Nuclear Magnetic Resonance electrical resistivity, acoustic velocity etc.)And saturation measurements under laboratory conditions. When the wettability reverse, it will cause great change the parameters calculation for the formation.

Wettability depends on the brine, oil and mineral compositions as well as temperature and other factors, if the wettability is altered to water-wet, large amount of oil can be recovered through spontaneous imbibition.
Impact of wettability alteration on recovery factor
Half the world’s known oil reservoir are in carbonate reservoirs. Many of these carbonate reservoir are naturally fractured. About half of the world’s Wettability alteration during surfactant flooding of carbonate reservoirs. According to pervious researches, recovery from fractured, oil-wet/mixed-wet, carbonate reservoirs by water-flooding is poor. And the reason can be easily classified as in fractured reservoir, the production mainly relies on the imbibition of the injection fluid into matrix blocks which expel oil into the fracture network which, in turn, transport it to the production wells. The oil recovery from conventional water flooding of fractured carbonate reservoirs is low due to poor imbibition of water into oil-wet matrix. On average, primary and water flooding methods leave about 80% of the original oil in the fractured reservoirs.
Hence fractured reservoir with high low matrix permeability need EOR technique to achieve maximum production, chemical enhanced oil recovery in carbonate reservoirs has been studied for many years and has gained more interest in recent year due to the high oil prices (although the oil prices are in volatility in short period). Wettability alteration has received more attention because carbonate reservoir are much more likely to be fractured and oil-wet, surfactant solutions are injected with the aim of changing the wettability of the matrix to a more water state, and hence enchanting the spontaneous imbibition process, leading to a higher oil recovery. There are a wide range of choices in the selection of surfactants: nonionic surfactant, cationic surfactants and anionic surfactants.
Here an experiment show the process of the wettability reversal by nonionic surfactant (A Nonionic Surfactant Ethomeen T/25 was from Akzo Nobel.), Eight anionic surfactants (A1 to A8: alkyl ether sulphates and internal olefin sulphonates) and three nonionic surfactants (N1, N2 and N3) were tested in this study. ethoxy groups (most in N3, least in N1) The picture of oil droplets on cristobalite plates submerged in surfactant/brine solution in an optical cell, after a certain period time ,we can see the contact angle change, thus the reversion of the wettability taking place.

Here fig3 is another experiment called the imbibition experiments performed with the surfactants which altered the wettability of Cristobalite plates.
The picture vividly shows an oil-wet reservoir core. A water drop placed on top of the aged core does not imbibe, con firming the oil-wetness of the core.
Fig 3
Fig 4, shows one of the cores placed in a surfactant solution. Oil droplets come out of the core on all sides as brine imbibes. This suggests that the dominant imbibition mechanism is the counter current imbibition due to the capillary pressure gradient caused by the wettability alteration. High values of inverse bond number imply high capillary forces compared to buoyancy forces. If the wettability is altered, these capillary forces aid in the imbibition of brine through the periphery of the core (Gupta et al. 2008).
Fig 4
A wide range of choices in the selection of surfactants: nonionic surfactant, cationic surfactants and anionic surfactants.
And Fig 5 indicate the relationship between the extent of wettability alteration with oil recovery.
Figure 5: Effect of extent of wettability alteration on oil recovery.

Conclusion
Recovery factor is one of the key elements in determining commercial viability of hydrocarbon filed, as mentioned before, recovery apart other parameter, depends on wettability of the reservoir rocks .When the matrix is oil-wet or mix-wet, little oil can be recovered by imbibition.at the same time, wettability also affect calculation of other parameters used in the volumetric equation. So in order to enhance oil recovery, the originally oil-wet solid surface is reversed to water-wet by injection of a certain deal of active water into the formation, which makes it easier for the oil to be driven away and ensure a higher recovery.
References
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