Capillary and Wettability Impacts on Recovery • Capillary pressure does not have a single, simple relationship to the forces such as pore structure, saturation and wetting that affect it. • Capillary and wettability do have enormous effects on fluid movement through the pores of a rock and on ultimate recovery of reserves as well as load fluids and stimulation fluids. • Changes in wettability have been shown to affect electrical properties, capillary pressure, waterflood behavior (and recovery), relative permeability, tracer dispersion, as well as residual and irreducible fluid saturations. Anderson, SPE 15271 8/25/2015 1 George E. King Engineering GEKEngineering.com
Transcript
Capillary and Wettability
Impacts on Recovery
• Capillary pressure does not have a single, simple relationship to the forces such as pore structure, saturation and wetting that affect it.
• Capillary and wettability do have enormous effects on fluid movement through the pores of a rock and on ultimate recovery of reserves as well as load fluids and stimulation fluids.
• Changes in wettability have been shown to affect electrical properties, capillary pressure, waterflood behavior (and recovery), relative permeability, tracer dispersion, as well as residual and irreducible fluid saturations.
Anderson, SPE 15271
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Simple Definitions
• Imbibition – adsorption of a fluid into the pore of a rock.
• Drainage – release of a fluid from the pore of a rock.
• Capillary pressure –difference in pressure across an interface of two immiscible fluids. DP is proportional to surface tension and inversely proportional to effective pore radius at the fluids interface.
• Continuum pressure – an indication of the force exerted by hydraulic connection of a fluid phase in a pore.
• Wettability – tendency of one fluid to spread on or adhere to a surface in the presence of another immiscible fluid.
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Wettability
• Wettability is a major factor controlling the location, flow, and distribution of fluids in a reservoir. The wettability of a core will affect almost all types of core analyses, including capillary pressure, relative permeability, waterflood behavior, electrical properties, and tertiary recovery.
• Wettability of an originally water-wet rock can be altered by adsorption of polar compounds and/or deposition of organic material originally in the crude oil.
• The degree of alteration is determined by interaction of oil constituents, mineral surface and brine chemistry
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Water-Wet, Oil-Wet and Neutrally-
Wet • Wettability is “the tendency of one fluid to spread on or
adhere to a solid surface in the presence of other immiscible fluids”. In a rock/oil/brine system, it is a measure of preference that rock has for either oil or water.
• When rock is water-wet, water is the continuous phase and occupies the small pores and film contacts the majority of the rock surface.
• When rock is oil-wet, oil is the continuous phase and will occupy the small pores thus contacting the majority of the majority of rock surface.
• However, the term wettability describes the wetting preference of the rock, but does not necessarily refer to the fluid that is in contact with tie rock at any given time.
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Wetting From a Fluid Point of View
• Consider a sandstone core saturated with clean oil. Even though the rock surface is coated with oil, the sandstone core may still preferentially be water-wet.
• Wetting preference is demonstrated by allowing water to imbibe into the core. If water will displace the oil from the rock surface, then the rock surface prefers to be in contact with water rather than oil.
• Similarly a core saturated with water is oil-wet if oil will imbibe into the core and displace water.
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Wetting From a Fluid Point of View
• Depending on specific interactions of rock, oil, and brine, the wettability of a system can range from strongly water-wet to strongly oil-wet.
• When the rock his no strong preference for either oil or water, the system is said to be of neutral or immediate wettability.
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A General Preferential Wetting?
• Historically, all sandstone petroleum reservoirs were believed to be strongly water-wet based on two factors: – First, almost all clean sedentary rocks are strongly water-wet.
– Second, sandstone reservoirs were deposited in aqueous environments into which oil later migrated.
• In the 1930’s it was realized that some producing reservoirs were, in fact, actually strongly oil-wet.
• In one case, a sandstone in had adsorbed heavy hydrocarbons in layers about 0.7pm thick (about 1,000 molecules) so firmly that they could not be removed by solvents.
• Wettability contact measurements in carbonates suggest about 80% are oil-wet.
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Fractional Wettability?
• Fractional wettability may be created by inconsistent or uneven adsorption of the crude oil components created by variations in heterogeneous reservoir rock.
• Generally, the interstitial structure of a porous rock is composed of many minerals with different surface chemistry and adsorption properties, which can lead to variations in wettability.
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Mixed Wettability?
• Mixed wettability is a term for a special type of fractional wettability in which the oil-wet surfaces form continuous paths through the larger pores. Oil-wet deposits would not be formed in the small water-filled. pores, allowing them to remain water-wet and containing no oil.
• The fact that all of the oil in a mixed-wettability core is located in the lager oil-wet pores causes a small but finite oil permeability to exist down to very low oil saturation.
• This permits the drainage of oil during a waterflood to continue until very low oil saturations are reached.
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Changes in the Wettability -
Historical • Factors in wettability changes include: oil composition
(natural surfactants), pressure, temperature, mineral surfaces and brine chemistry (ionic composition and pH).
• The importance of the mineral surface is shown by the contact-angle measurements
• Multivalent cations can enhance the adsorption of surfactants on the mineral surfaces. Brine pH is also important in determination of the wettability and other interfacial properties since the pH can affect the way that surfactants behave.
• Alkaline flooding, for example, uses surfactants that alter the wettability of the rock and can sharply increase hydrocarbon recovery.
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Changes in the Wettability –
Short Term • Although surface active agents or surfactants are
found in a variety of crude fractions, they are more prevalent in heavier fractions of crude, such as resins and asphaltenes.
• Natural surfactants in a crude are often sufficiently soluble in water to pass through the water and adsorb onto the rock surface.
• Surface active agents are concentrated at the oil/water interface. Many of these materials can adsorb on the rock and will change the wettability, usually in the short term. Anderson, SPE 13932
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Changes in the Wettability –
Short Term • Surface active agents in brines can be natural or
added to kill or stimulation brines.
• Soaps and cleaners can water-wet a oil-wet surface, but the natural surfactants in produced oil and water, and the rock surface will usually revert the wetting to the preferred state.
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Imbibition
• In addition to wettability, imbibition rates also
depend on relative permeability, viscosity,
interfacial tension, pore structure and the
initial saturation of the core.
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Recovery of oil in naturally
fractured reservoirs • Production from naturally fractured reservoirs
controlled by co-imbibition and counter-current imbibition of water.
• Imbibition is dependent on the wettability that is allowed by capillary forces that control fluid movement in a pore.
– Waterflooding naturally fractured reservoirs is successful in water-wet reservoirs (incremental recovery addition of 30%+ in West Texas).
– Waterflooding naturally fractured oil-wet reservoirs is often unsuccessful (incremental recovery of 2% in Ghaba North in Oman)
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Effect of Natural Fractures • Fractures represent only a very small fraction of the
total porosity – usually about 1 to 3%, but:
– Open natural fractures dominate permeability
– The fracture network increases surface area for imbibition.
– In water-wet reservoirs, oil is produced from the matrix to the fracture system by imbibing water and expelling oil.
• In general, higher permeability of the fractures increases oil recovery.
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Effect of Natural Fractures • The size of the contact area (specifically the fracture
extent and the number and size of the pores that intersect the fracture) controls the transmissibility of oil and therefore the ability to transport fluids across the fracture.
• The fluid flowing in the film of oil at the boundaries controlled the rate of drainage across matrix blocks adjacent to each other and separated by a natural fracture.
O’Meara SPE Res. Eng, Feb 92
Firoozabadi SPE Res. Eng, Aug 94
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Movement of Fluids Across
and in a Fracture in a Matrix
System • Fluids move through capillaries – usually much faster in
the large pores than the smaller pores.
• Capillary continuity from matrix block to matrix block in a fractured reservoir is usually favorable in gravity drainage.
– Wetting-phase controls the bridges that form to link capillaries across natural fractures.
– In oil-wet reservoirs, this bridging may increase ultimate recovery, particularly in gas-assisted gravity drainage.
– In water-wet reservoirs, bridging may reduce the amount of capillary trapped oil.
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Movement of Fluids Through
the Matrix in a Fractured
System • Fluids move through capillaries – usually much faster in
the large pores than the smaller pores.
• Capillary continuity from matrix block to matrix block in a fractured reservoir is usually favorable in gravity drainage.
– Wetting-phase controls the bridges that form to link capillaries across natural fractures.
– In oil-wet reservoirs, this bridging may increase ultimate recovery, particularly in gas-assisted gravity drainage.
– In water-wet reservoirs, bridging may reduce the amount of capillary trapped oil.
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The Number of Bridges • The number of bridges depends on the frac width, the
matrix wettability and the rate of flow of the bridging fluid.
• The process of liquid bridging is strongly dependent on rock wettability.
SPE 108699
Sw = water saturation (Sw=1.0 for fully water saturated)
Iw = water wet condition (Iw = 1.0 for strongly water wet)
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Fluid Movement • Matrix-to-fracture-to-matrix communication changes
with in-situ saturation.
• During waterfloods of oil-wet stacked matrix blocks, water moves through with a sharp saturation front. Water fills the fracture from the inlet block before it flows to the outlet block.
• When flowing oil, the oil invaded the outlet block by forming liquid bridges before the fracture was filled.
• Capillary continuity by liquid bridges improved oil recovery.
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Capillary Bridges Across
Fractures • Formation of bridges is strongly related to wettability.
• Wettability is the dominating property in creating droplets.
• Wettability controls bridge radius.
• If applied pressure exceeded the pressure capacity of a bridge, it would coalesce with an adjacent bridge, until the bridge was large enough for the fluid flux.
• The fracture width must not be so wide that a stable bridge cannot be maintained.
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What flows in the bridge? • Some oil bridges formed in a water wet core during
waterflooding, but although there were oil droplets in the core, no oil droplets passed across – no oil phase continuum pressure, so not enough pressure to make the oil droplets large; thus water slipped around the oil.
• In strongly water-wet conditions, oil fills the fracture – from the drop, displacing water. No bridges form.
• In moderately oil-wet conditions, oil bridges contributed to capillary continuity across the fracture.
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Fluid movement – going to a large scale system
• In waterflooding a water-wet rock, water moves through the porous medium in a fairly uniform front.
• The injected water will displace oil in larger pores and will imbibe into smaller and medium sized pores and displace oil to the larger pores.
• In oil-wet rock the water tends to finger through the oil and displacement is poor.
• The non-wetting phase permeability is often higher since it flows through the middle of the pore.
• There is also a pore flowing size factor since the 8/25/2015 23
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Waterflood Response
• In a water-wet rock, more oil is displaced. The water advances along the walls, displacing most oil. At some point, the connection between the oil outside the pore and inside the pore will break, stranding a drop of oil inside the pore with not enough continuum pressure to move it.
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Some Rules of Thumb (F.F. Craig)
• For Determining Wettability
Craig, The Res. Eng. Aspects of Waterflooding, SPE
Water-Wet Oil-Wet
Interstitial Water Saturation Usually greater
than 20 to 25% PV
Generally less than
15% PV
Saturation at which oil and
water relative permeability are
equal
Greater than 50%
water saturation
Less than 50%
Water Saturation
Relative perm to water at the
max water saturation (i.e.,
floodout) based on effective
oil perm at reservoir
interstitial water sat.
Generally less than
30%
Greater than 50%
and approaching
100%
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Oil/water Relative Permeability Curves
Craig, The Res. Eng. Aspects of Waterflooding, SPE
water
water
oil oil
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Capillary and Wettability
Impacts on Recovery
Anderson, SPE 15271
Water wet interface – a meniscus.
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Cleaned vs. Native State Cores
• The effect of wettability on Archie saturation exponent and formation
factor determined experimentally in cores is strongly influenced by the
condition of the core – cleaned or native state (as in the reservoir)
• The condition of core influences determination of hydrocarbon
saturation of a formation and accuracy of resistivity data obtained from
well logging.
• Archie saturation exponent, “n” may have a value of about 2 in water-
wet formations and cleaned cores. In native-state, non-water-wet
formation it is generally larger than 2. In uniformly oil-wet cores with low
brine saturations, “n” can reach values of 10 or more.
• “n” exponent is higher in oil-wet cores at low saturations because a
portion of the brine is trapped or isolated in dendritic fingers where it is
unable to contribute to electrical conductivity. If a cleaned, water-wet
core is used to measure “n” and the reservoir is actually oil-wet, the
interstitial water will be underestimated during logging.
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Resistivity – some other effects • Resistivity is further increased by any hydrocarbon saturation in the core. The
increase will depend on the wettability, saturation, saturation history, and the factors that control the location and distribution of fluids in the rock.
• In water-wet rock, brine occupies the small pores and forms a continuous film on rock surfaces.
• In an oil-wet rock, the brine is located in the centers of the larger pores.
• Difference in brine distribution caused by wettability becomes very important as brine saturation is lowered. Almost all brine in water-wet rock remains continuous, so resistivity increases due to any decrease in the cross-sectional area (of the brine) that can conduct flow. In an oil-wet rock, a portion of the brine will lose electrical continuity as the saturation is lowered, so electrical resistivity will increase faster.
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Relative Permeability
• Relative Permeabilities are a function of wettability, pore geometry, fluid distribution, saturation, and saturation history. Wettability affects relative permeability by controlling the flow and spatial distribution of fluids in a porous media.
• In a uniformly wetted core, the effective oil permeability at a given water saturation decreases as the wettability is changed from water-wet to oil-wet.
• Water relative permeability increases and oil 8/25/2015 30
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Effects on a Waterflood - 1
• Oil recovery during a waterflood is a function of wettability pore geometry, fluid distribution, saturation, saturation history, and water/oil viscosity ratio.
• As wettability of a system ranges from water-wet to oil-wet, the time to breakthrough decreases and economical residual oil saturations (ROS) level increases, usually leading to less oil recovery.
• Small amounts of oil are produced for a long time after flood front breakthrough; therefore, Anderson, SPE 16471
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Effects on a Waterflood - 2
• During waterflooding of a strongly water-wet system at a moderate oil/water ratio, a large fraction of the oil in place (OIP), is recovered before breakthrough. Little additional oil is recovered after breakthrough.
• During waterflooding in a strongly oil-wet system at moderate oil/water ratio, water breakthrough occurs early and most of the oil is recovered after breakthrough.
• Waterfloods are less efficient in oil-wet systems because more water has to be injected to Anderson, SPE 16471
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Effects on a Waterflood - 3
• Front breakthrough and economic recovery are dependent on both wettability and the oil/water viscosity ratio (the mobility).
• The decrease in oil mobility at high oil/water viscosity ratios causes a decrease in oil recovery at breakthrough and an increase in the time needed for waterflooding at any wettability.
