Wetting heterogeneity can affect oil recovery, for example, when a reservoir is in contact with adjacent strata and the water-wet layers and oil-wet layers have similar permeabilities. A waterflood that has vertical capillary equilibrium should produce a higher water saturation in the water-wet layers than in the oil-wet layers because the invading water will preferentially displace the wetting-phase water. Depending on the interwell spacing and the layer thickness, a waterflood may leave substantial oil in the oil-wet layers and yield high water-cut in the production stream because the injected water cannot move oil from oil-wet surfaces as easily it can move the free water in the pores.
Wettability also affects the amount of oil produced at the end of oil displacement and recovery by waterflooding as measured by the residual oil saturation, Sor. At the pore level in a water-wet formation, water flows into all accessible pores, and the remaining oil remains isolated in large pores, disconnected from an oil-connected pathway. In a strongly oil-wet formation, oil adheres to surfaces, increasing the probability of a continuous oil-connected path to a producing well and resulting in a lower Sor than that in a water-wet formation. However, this connected oil may have very low mobility because of its confinement to small pores.
The wetting states of layers within formations can also differ because of lithology variations. A tight, low-permeability zone may remain water-wet if little or no oil migrates into it while surrounding formations can become more oil-wet. Wetting can also be affected by temperature variations and the connate water pH.
Because its impact extends from pore scale to reservoir scale, wettability can affect project economics. Reservoir engineers use the parameters Swr and Sor to predict the amount of oil that can be recovered. These parameters are directly affected by wettability. In addition, the relative permeabilities of oil and water vary with formation wettability; relative permeability is the relative ease that one fluid has to flow through the pore space in the presence of other fluids. In projects that have large upfront capital expenditures for facilities, such as those in deepwater areas, failure to understand wettability and its ramifications through both residual saturations and relative permeability can be costly.
Reservoir engineers design enhanced oil recovery processes to alter wettability to water-wet to remove oil contacting the solid surface. Alternatively, rendering the formation to be oil-wet may improve ultimate oil recovery.
Wettability may also influence gasflood performance. The gasflood front or oil bank can move water, if it is mobile, generating flow variation based on oil/water wetting preferences. In addition, if asphaltenes are present in the crude oil, contact by injected hydrocarbon gas alters the equilibrium condition, possibly causing precipitation of asphaltenes, which can plug pores.
Even in a gas reservoir, wettability or its alteration can affect recovery. Blockage by condensate formed near a wellbore—condensate banking—during depletion of a gas reservoir decreases gas productivity. Some recovery methods use chemical means to alter the wettability around the wellbore to produce the fluids and clear the blockage.
Wetting conditions affect some logging measurements. Most resistivity measurements rely on a continuous, conductive electrical path through the rocks, which the water phase provides. In an oil-wet formation, the water may not be continuous, especially if the well is drilled with oil-base mud. In these conditions, a resistivity-based saturation assessment such as that with Archie's equation will likely be incorrect if used without considering phase connectivity.
Nuclear magnetic resonance (NMR) responses also depend on the position of the fluids with respect to the pore surfaces. The nonwetting fluid exhibits NMR relaxation rates similar to those of bulk fluid if the fluid does not contact the pore surface. The wetting fluid has shortened relaxation times because of its interactions with the pore surface.
Wettability is important to drilling-fluid formulation, particularly in oil- base muds. Surfactants keep solids in suspension. If an oil-external mud filtrate containing oil-wetting surfactants invades the near-well formation, there is a potential to alter the wettability of the pores. This process can change the position of the fluids in the pore spaces and may affect some logging responses. Because the alteration may not be permanent, measure-ments may differ in subsequent logging runs.
Fields around the world are maturing, but the industry still needs to extract as much of the hydrocarbon resource as is economically possible before these fields are abandoned. To optimize recovery, operators must understa.nd the effects of wettability on well production and field performance.
Oilfield Review 2016
Copyright © 2016Schlumberger.