Reservoir Engineering [Wettability]

1. In a real field scenario, if the pore walls remain not to be smooth, flat surfaces and if the matrix surrounding the pore-wall remains to be composed of more than one mineral species, then, how do secure the details on the spatially and temporally varying ‘true contact angle’ (that is based on the local orientation of the surface), which remains to be significantly different from the laboratory-based, surface-rugosity independent ‘apparent contact angle’?

Would it remain feasible to delineate the parts of pore spaces that remain to be oil-filled and those that remain to be brine-filled as a function of pore and pore-throat geometry and the surface roughness @ field-scale?

Feasible to capture the way, the surface wetting preferences keep changing as the water film gets destabilized @ pore-scale?

2. As the pore-scale details, associated with a wetting scenario, in a real field scenario remains extremely challenging, and (the next level of simplification) as the concept of (saturation-history & mineralogy based) Mixed-wettability also remains challenging @ laboratory-scale, then, what does the contact angle measurement @ laboratory-scale convey (when different solid-grains have varying wetting preferences; and given the fact that the original, water-wet condition got already altered upon oil migration)?

3. Whether the ‘Equilibrium Contact Angle’ obtained by balancing the surface energies according to Young-Laplace Equation remains valid only on a flat, ideal surface in the absence of an inherent roughness, topographic defects and chemical heterogeneity? Or, do we need to apply (thermodynamically stable) Wenzel model that describes the variation of contact angle with surface roughness (leaving aside the complex meta-stable Cassie-Baxter state; or, its associated Cassie-Baxter-to-Wenzel wetting transition through sagging and depinning)?

4. With the composition of crude-oil, pH & composition of brine influencing the disjoining pressure predominantly, would it remain feasible to ensure, whether, it is going to either a Positive Disjoining Pressure (that holds the interfaces apart) or a Negative Disjoining Pressure (that holds the interfaces to remain to be attractive) – in terms of van der Waals, electrostatic and structural interactions, when, rock-brine and brine-oil interfaces are in proximity?

5. Both at the laboratory-scale and at the field-scale, whether the range of pH (resulting from the presence and nature of ionized sites on the solid-rock surface) leading to ‘destabilization of films’ would remain to be different for Carbonates and Sandstone Reservoirs? Or, Does it require the details on Contact Angle Hysteresis (water-advancing contact angle and water-receding contact angle) in terms of Surface Interaction as well as its ‘Saturation History’ (as the saturation in a reservoir is no more static; and such saturation not only changes over geological time-scales, but also, within drilling and production time-scales)?

6. Wettability [describing the preference of a solid rock surface to be in contact with one of the (two) immiscible fluids rather than with another] essentially projects the balance between surface and interfacial forces. In such a case, if the balance of forces in the oil/water/solid system result in a contact angle between the fluids at the solid surface, then, does it not clearly indicate that the respective reservoir condition neither pertains to strongly water-wetting nor strongly oil-wetting?

If so, then, how could we simply treat wettability as a simple binary switch between oil-wet and water-wet (that completely vanishes the complexity of wetting physics in reservoir rock)?

7. At any point of time, in a real field-scale scenario, can we ever end up with a scenario, where, an oil drop gets completely surrounded by water on a water-wet surface (where, the contact angle remains approximately zero); or, where, the oil drop spreads completely on an oil-wet surface, which results in a contact angle of about 180 degrees?

8. In a real field scenario, if the solid rock surface does not have a marked preference either for water, or, for oil, then, does it not amount to Intermediate-wetting or Neutral-wetting? Feasible to distinguish ‘Intermediate-wetting’ that lacks a strong wetting preference from that of a ‘Mixed-wetting’, which has a variety of wetting preferences @ Field-scale?

9. Can we ever secure the details on the degrees of wetting that could be applied along the continuum @ field-scale?

10. Under what circumstances, the original wettability of a formation and altered wettability during and after hydrocarbon migration, ‘significantly’ influence the profile of initial water saturation, and production characteristics in the formation?

11. Why does a Water-wet reservoir exhibit a long transition zone (through which saturation changes gradually from mostly oil with irreducible water at the top of the transition zone to water at the bottom)?

12. At any given height above Free Water Level (FWL), the portion of the pore-size and pore-throat size distribution that can sustain brine at that height will remain brine-saturated only in Water-wet reservoirs?

13. What is the field-scale physics associated between the regions (a) Free water Level (Pc = 0); and (b) Brine-Oil contact – in Water-Wet (where, FWL is located at the beginning of the curve and that falls well-below OWC; and, where, pressure must be applied to force oil into the largest pores) & Oil-Wet Reservoirs (where, FWL lies well-above OWC; and, where, pressure must be applied to force the brine into the largest pores)?

14. What is the physics behind (some of the) Carbonate Reservoirs in the Middle East remain characterized by ‘Layered Wettability Variation’? Why does such a Heterogeneous Wettability affect the Oil Recovery significantly? In particular, why do we fail to extract oil from Oil-Wet Layers?

15. How exactly Wettability affects the amount of oil that could be produced @ pore-scale, in a Water-Wet Reservoir (where, oil remains in the large pores, and where, the oil could get snapped off, or, the oil could become disconnected from a continuous mass of oil, and eventually the oil could get rapped); and in an Oil-Wet Reservoir (where, oil adheres to rock surfaces, and, where, oil is just tending to enhance the probability of a continuous path to a producing well, and in turn, leading to a reduced Residual Oil Saturation)?

16. How could we go ahead with reservoir projects that require huge upfront capital expenditures for complex facilities, if wettability significantly affects the project economics as its impact extends from pore-scale to reservoir-scale; and, if wettability significantly influences Oil Recovery in terms of Initial Water Saturation & Residual Oil Saturation; along with the variation of Formation Wettability as a function of Relative Permeabilities of Oil & Brine?

17. Feasible to measure the intensity of Imbibition Forces (which display the tendency of a formation to draw in the wetting-phase; and which determines the ease with which water could be injected through the formation, particularly, before the occurrence of water breakthrough) associated with a Water-Wet Reservoir?

18. Whether the contact angle increases significantly near the asphaltene-precipitation point – in both Water-Wet & Oil-Wet Reservoirs?

19. Can wettability significantly influence Oil-based mud’s (oil mud filtrate with oil-wetting surfactants invading in the vicinity of well) drilling fluid formulation? If so, then, is it going to be just temporarily?

20. In a mixed-wet reservoir, when water breaks through to a producing well, oil production would continue for a long time, despite the increase in the water cut. Can we expect the same in a strongly water-wet reservoir?

21. Although strongly water-wet and strongly oil-wet reservoirs display certain characteristic relative-permeability curves, then, why does Intermediate-wetting and Mixed-wetting not remain to be a simple extrapolation between the above wettability extremes?

If so, then, don’t we have a reliable method for measuring wettability that would provide absolutely accurate results?

Suresh Kumar Govindarajan, Professor [HAG]

IIT Madras 19-Dec-2024