The rise in core (RIC) method is an alternate reservoir wettability characterization method described by S. Ghedan and C. H. Canbaz in 2014. The method enables estimation of all wetting regions such as strongly water wet, intermediate water, oil wet and strongly oil wet regions in relatively quick and accurate measurements in terms of Contact angle rather than wettability index. During the RIC experiments, core samples saturated with selected reservoir fluid were subjected to imbibition from a second reservoir fluid. RIC wettability measurements are compared with and modified – Amott test and USBM measurements using core plug pairs from different heights of a thick carbonate reservoir. Results show good coherence. The RIC method is an alternate method to Amott and USBM methods and that efficiently characterizes Reservoir Wettability.
Cut-off values vs wettability index One study used the water advancing contact angle to estimate the wettability of fifty-five oil reservoirs. De-oxygenated synthetic formation brine and dead anaerobic crude was tested on quartz and calcite crystals at reservoir temperature. Contact angles from 0 to 75 degrees were deemed water wet, 75 to 105 degrees as intermediate and 105 to 180 degrees as oil wet. Although the range of wettabilities were divided into three regions, these were arbitrary divisions. The wettability of different reservoirs can vary within the broad spectrum from strongly water-wet to strongly oil-wet. Another study described two initial conditions as reference and non-reference for calculating cut-off values by using advancing and receding contact angles and spontaneous imbibition data. Limiting value between water wet and intermediate zones was described as 62-degree. Similarly, cut-off values for advancing contact angle is described as 0 to 62 degrees for water wet region, 62 to 133 degrees for Intermediate-wet zone, and 133 to 180 degrees for Oil wet zone. Chilingar and Yen examined extensive research work on 161 limestone, dolomitic limestone, calcitic dolomite, and dolomite cores. Cut-off values classified as 160 to 180 degrees for strongly oil wet, 100 to 160 degrees for oil wet, 80 to 100 degrees intermediate wet, 80 to 20 degrees water wet and 0 to 20 strongly water wet. Rise in core uses a combination of Chilingar et al. and Morrow wettability cut-off criteria. The contact angle range 80 – 100 degrees indicate neutral-wetness, the range 100 – 133 degrees indicate slight-oil wetness, the range 133 – 160 degrees indicate oil-wetness while the range 160- 180 degrees indicate strongly oil-wetness. The range 62 – 80 degrees indicate slight water wetness, the range 20 – 62 degrees indicate, water-wetness, while the range 0 – 20 degrees indicate strong water-wetness.
Technique RIC wettability characterization technique is based on a modified form of Washburn's equation (1921). The technique enables relatively quick and accurate measurements of wettability in terms of contact angle while requiring no complex equipment. The method is applicable for any set of reservoir fluids, on any type of reservoir rock and at any heterogeneity level. It characterizes wettability across the board from strongly water to strongly oil wet conditions. The step of deriving the modified form of Washburn equation for a rock/liquid/liquid system involves acquiring a Washburn equation for a rock/air/liquid system. The Washburn equation for a rock/air/liquid system is represented by:
t = μ C ρ 2 γ cos θ m 2 {\displaystyle t={\mu \over C\rho ^{2}\gamma \cos \theta }{m^{2}}} (Eq.1). Herein, "t" is the penetration rate of liquid into a porous sample, "μ" is the liquid's viscosity, "ρ" is the liquid's density, "γ" is the liquid's surface tension, "θ" is the liquid's contact angle, "m" is the mass of the liquid that penetrates the porous sample and "C" is the constant of characterization of the porous sample. evaluating a value of "γos" using a young’s equation for a rock surface/water/air system (Figure 2) and a value of "γws" using young’s equation for a liquid/liquid/rock system is represented as:
γ o w cos θ = γ o s γ w s {\displaystyle \gamma _{ow}\cos \theta =\gamma _{os}\gamma _{ws}} (Eq.2). "γow" is the surface tension between the oil and water system, "γos" is the surface tension between oil and solid system and "γws" is the surface tension between water and the solid system. Using Young's equation for a rock surface/ water/air system and substituting in equation (2) to obtain equation 3:
cos θ w o = γ o cos θ o ⋅ γ w cos θ w γ w o {\displaystyle \cos \theta _{wo}={{\gamma _{o}\cos \theta _{o}}\cdot {\gamma _{w}\cos \theta _{w}} \over {\gamma _{wo}}}} (Eq. 3). Rearranging equation (1) to factor out γLV obtains equation (4), wherein γLV a liquid-vapor surface tension is:
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