Mechanistic Model Describes Wettability Change in Sandstones and Carbonates
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Previously proposed models of wettability change have not been tied to the chemistry that controls wettability but instead were driven by simplistic criteria such as salinity level or concentration of an adsorbed species. In this paper, after testing proposed models in the literature on sandstones and carbonates, the authors propose a mechanistic surface-complexation-based model that describes observations quantitatively for ionically treated waterfloods. To the best of the authors’ knowledge, this is the first surface-complexation-based model that describes fully ionic compositional dependence observed in ionically treated waterfloods in both sandstones and carbonates.
While some debate remains about the underlying mechanisms of ionically tuned waterflooding, the geochemical reactions that control the wetting of crude oil on the rock surface are likely to be central to a detailed description of the process. Models of wettability change often have been simplistic and not tied to the chemistry that controls wettability, instead driven by a simplistic criterion such as salinity level or concentration of an adsorbed species. Such models are inadequate for modeling the effect of compositional changes in brine, which is key to optimizing ionic design. One problem has been the lack of reservoir models that included geochemistry. The oilfield reactive-transport simulators are simplistic; in these, either aqueous reactions are assumed to be ideal (i.e., with aqueous activity coefficients of unity) or they lack important geochemical features such as kinetics or surface-complexation reactions.
In a previous work, the authors assumed that decrease in the total ionic strength is the driving mechanism for wettability change during low-salinity waterflooding. The reason was that the total ionic strength is the controlling parameter for the double-layer expansion. Double-layer thickness increases as the brine total ionic strength decreases. The thickness of the double layer is an inverse function of the square root of the total ionic strength. However, the proposed model was too simple and was unable to predict many low-salinity or modified-salinity waterflooding observations, such as the detrimental effect of divalents on oil recovery in sandstones, the positive effect of sulfate in chalks, the positive effect of high pH in sandstones, and the detrimental effect of sodium chloride in chalks.
In this study, the authors identified and tested an alternative approach based on a stability analysis of thin water film, which yields a dimensionless group, a ratio of electrostatic-to-Van der Waals (VdW) forces called the stability number. The stability number can be used as a measure of wettability. The authors use simulation software to model surface reactions that can estimate surface charge and potential from surface-complexation models, demonstrating that this process can be used in a model to match a wide variety of data in sandstones and carbonates found in models in the literature. The complete paper provides details of these comparisons.
Ionically Tuned Waterflooding in Sandstones
In this section of the complete paper, the authors discuss mechanistic modeling of wettability change using surface complexation of ionically tuned waterfloods in sandstones. Two surface-complexation-based approaches proposed in the literature are tested, and the mechanistic modeling approach used by the authors is described. The proposed model is validated qualitatively against experimental observations.
Modeling Low Salinity Through Bond Product Sum (BPS). The BPS is a surface-complexation-based approach for mechanistic modeling of modified-salinity waterfloods. Broadly speaking, a surface complexation is essentially an ion-exchange reaction in which ionic adsorption to surface depends on potential at the surface, surface potential depends on surface charge, and surface charge depends on adsorbed species. BPS is hypothesized to indicate high interaction between oil and rock species—hence, an oil-wet state—and low values of the BPS show a water-wet state. The authors reproduce the research behind the BPS model using their surface reactions.
Estimating Contact Angle From Disjoining Pressure. Another mechanistic model proposed for ionically tuned waterfloods is estimation of contact angle from disjoining pressure. Disjoining pressure is a function of oil/brine and rock/brine potentials. Two charged layers at distance h exert three forces on one another: VdW, structural, and electrostatic. The overall force per unit area of the oil/brine and rock/brine interfaces is called disjoining pressure. If the disjoining pressure is positive, the thin water film between two layers stays stable or metastable (with a contact angle); otherwise, the film ruptures and the surface is wet by oil. VdW and structural forces are nearly independent of brine salinity, whereas surface charges at the oil/brine and brine/rock interfaces are salinity-dependent (i.e., electrostatic force changes as the ionic content of the brine changes because changes in the surface charge are strongly dependent on brine composition). In the literature, calculations show that estimating contact angle from disjoining pressure gives the right trend for selected experiments. However, those authors miscalculated reduced potentials in electrostatic-force formulation.
Mechanistic Modeling of Wettability Alteration Using Stability Number. Here, the authors describe their new approach in which wettability alteration is modeled from the oil/brine and rock/brine zeta potentials using the stability number.
The oil/brine/rock system can be visualized as shown in Fig. 1. If the thin water film between the oil and rock interfaces becomes unstable, it ruptures and oil adsorbs to the rock surface, which makes the rock surface oil-wet. However, if the thin water film is stable, water is in contact with the rock surface and the wettability of the rock is water-wet. Oil/brine and rock/brine interfaces are both charged interfaces. Depending on the aqueous composition between the two interfaces, mineralogy of the rock, and the amount of acidic and basic components in the oil phase, the oil/brine and rock/brine interfaces can be positively or negatively charged.
The stability number determines the stability of thin water film by including the net effect of attractive and repulsive forces between the oil/brine and rock/brine interfaces. The VdW force is always attractive and is a weak function of the interface charges, whereas the electrostatic force is a strong function of surface charges. Electrostatic force is repulsive for similarly charged interfaces and is attractive for dissimilarly charged interfaces. The structural force is ignored because it is a short-range force. Hence, to have a stable thin water film between the oil/brine and rock/brine interfaces, electrostatic force between the two interfaces should be repulsive and sufficiently high to dominate the VdW force. The authors write that they believe there is a smooth transition in wettability from oil-wet to water-wet conditions.
Measured zeta potentials support the stability criterion approach, in which both oil/brine and rock/brine interfaces become more negative during low-salinity waterflooding or when adding, or removing, certain ions to or from the injected water. Both oil/brine and rock/brine zeta potentials become more negative as water salinity decreases.
Dynamic Modeling of Wettability Alteration Using Stability Number: Sandstones. The authors use commercial simulators for modeling the Endicott field trial, corefloods, and imbibition tests. In this software, the oil/brine and rock/brine zeta potentials for gridblocks can be monitored dynamically and the stability number can be calculated throughout the simulation on the basis of which rock wettability changes. Effective matches to parameters in two widely cited sets of experiments were achieved and are detailed in the complete paper.
Ionically Tuned Waterflooding in Carbonates
The proposed surface-complexation-based model predicts most experimental trends in carbonates as well.
Dynamic Modeling of Wettability Alteration Using Stability Number: Carbonates. Low Salinity in Carbonates. Another set of experiments cited widely only reported rock and brine zeta potentials for different diluted seawater data. To model the oil/brine zeta potential, the authors use the tuned surface reaction.
A composite core is used in this experiment with a total length of approximately 1 ft; the capillary end effect is ignored. A high-salinity relative permeability set and three altered sets are defined in the model.
Another set of experiments examined by the authors is representative of low-salinity waterflood in carbonates. Coreflood configuration is vertical. Deionized water (DI) is injected in tertiary mode after a secondary seawater injection. Two bump rates are applied before the low-salinity water slug, which shows the presence of the capillary end effect. The authors of this paper include the capillary end effect in the model and match effluent ion histories along with oil recovery. High-salinity and low-salinity relative permeabilities are tuned along with only the negative part of the capillary pressure curve, which controls the capillary end effect. The match to the sulfate is poor. The authors write that they believe that this is caused mainly by the experimental data quality. The sulfate effluent breakthrough after the DI injection is approximately 2 pore volumes (PV) delayed compared with the other ions. Different delays for sulfate, calcium, and magnesium ions were expected because of different levels of adsorption on the rock surface; however, 2 PV is too large a measurement to be the result of adsorption. The match to calcium effluent history improves if the effluent ion history of sulfate is excluded from the history-matching work flow.
Effect of Enriching Injected Brine With Sulfate. Enriching seawater with sulfate improves oil recovery in carbonates; however, brine sulfate enrichment makes rock/brine less positive. This is how the electrostatic force increases, which subsequently increases stability number. The authors apply the tuned surface reaction, and the tuned surface reactions from the match, to rock/brine zeta potentials. Relative permeabilities are fixed, and only capillary pressures are adjusted. Capillary pressure and molecular diffusion are key components in matching imbibition tests. The inclusion of the negative part of capillary pressures is important to avoid significant oil recovery caused by gravity.
Dilution in Chalks Using Stability Criterion. A stability criterion fails to predict the right trend for dilution in chalks. Unlike the dilution in carbonates, simple dilution has a detrimental effect on oil recovery in chalks. This is believed to be the case because sulfate concentration is the key component for wettability alteration. Diluting the seawater decreases the concentration of the key component, too, so that injected/imbibition brine would be less-effective in modifying rock wettability unless dilution dissolves some anhydrite in situ, which results in higher in-situ sulfate concentration.
Combining surface complexation with simplified colloidal science provides a promising approach for mechanistic modeling of wettability change. The mechanistic approach includes surface chemistry (effects of ionic strength, pH, and preferential adsorption of ions such as sulfate). The authors tested a number of mechanistic models proposed in the literature. The existing mechanistic models fail to predict most ionically tuned waterflood observations. A surface-complexation-based model is proposed that includes wettability fundamentals (balance of electrostatic and VdW forces). The mechanistic model can describe oil/brine and brine/rock zeta potentials in sandstones and carbonates.
For a limited time, the complete paper SPE 190236 is free to SPE members.
Mechanistic Model Describes Wettability Change in Sandstones and Carbonates
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