Samples and Sample Preparation
Samples used for micro-wettability evaluation in this study were sub-sampled from core plugs, which in turn were extracted from the middle member of the Devonian- to Mississippian-aged Bakken Formation of the Williston Basin in the Viewfield area of Saskatchewan, Canada38. The middle member is comprised primarily of dolomite-cemented siltstones that are actively being developed as a low-permeability, light oil resource39. The core plug samples were taken from the middle member of the Bakken primarily for the purpose of characterizing porosity, permeability, and mechanical property trends in support of reservoir simulation studies of enhanced oil recovery in the Viewfield area. The current study was intended to extend the previous middle Bakken characterization work by our research group40, 41 to include wettability/micro-wettability characterization.
For condensation/evaporation experiments, a sample wafer was cut from a core plug, and a small fragment of this wafer was broken off with tile nippers. The sample fragment was then mounted into an aluminum sample cup using a thermally – conductive epoxy. Once the epoxy was cured, excess material from the fragment was broken off to expose a fresh surface for imaging. The sample cup was then fitted to the Peltier heating/cooling stage. For analysis, sample temperatures were held between 1.9 and 2.1 °C, and SEM chamber pressures ranged between 650 and 800 Pa. Condensation/evaporation of water vapor was controlled by varying sample temperature with the Peltier stage.
For cryogenic experiments, a sample fragment was again extracted from the wafer. The sample fragment was submerged in formation oil (obtained at a Viewfield Bakken well location by the operator), and allowed to imbibe oil for 6 weeks (42 days). After oil imbibition, some fragments of the sample were examined using the procedure described below, while other fragments were subjected to 3½ weeks (25 days) of spontaneous imbibition of synthetic brine prepared to match as closely as possible a formation water analysis from a nearby offsetting well.
The following steps were used for analysis of these samples:
- The sample was placed into an aluminum sample holder with “cryo glue”: a viscous fluid at room temperature consisting of Polyvinyl Alcohol, Carbowax and water, that serves as an adhesive to bond the sample to the sample holder at cryogenic temperatures.
- The sample/sample holder was submerged in a bath of nitrogen slush until reaching thermal equilibrium.
- The frozen sample was transferred under vacuum to the Cryo-Preparation chamber.
- In the preparation chamber, the sample was broken to expose a fresh surface using the point of a #10 scalpel blade on a wobble stick manipulated from outside the preparation chamber.
- The sample was then transferred into the E-FESEM chamber for electron imaging and X-ray mapping to reveal mineralogy and fluid type and distribution in the pore system of the sample.
The synthetic brine was generated as follows:
- The masses of various salts required to match the ionic concentrations in 1 L of formation brine were first calculated.
- Bicarbonate and sulphate were introduced as NaHCO3 and CaSO4, respectively, while Na, K, Ca, and Mg were added as chlorides. A trace concentration of iron in the formation brine was ignored.
- Deionized water was then added to approximately the 900 mL level, and the flask agitated until all salts were dissolved. Once dissolution was complete, the flask was topped up to the 1 L level with additional deionized water.
For micro-injection experiments, sample preparation was very similar to the condensation/evaporation experiments described above; however, a specially-made sample holder was used in order to orient the sample vertically so that the sample surface was orthogonal to the silica micro-injection tube, and parallel to the microscope’s axis of observation (Fig. 7). This modification allowed the contact angles to be evaluated more confidently.
Contact Angle Evaluation
Contact angles were obtained by fitting a parameterized version of the Young-Laplace Equation to extracted micro-droplet profiles. The Young-Laplace Equation relates the pressure difference across a curved interface42 as follows:
Where Δp is the pressure difference across the homogeneous fluid interface, γ is the interfacial tension, and R1 and R2 are the principal radii of curvature at point P on the surface of the micro-droplet. Assuming an axisymmetric interface, Equation 1 can be expanded into a system of ordinary differential equations as a function of arc length s26:
where b is the curvature of the interface at the origin, and b and c= (Δρ/γ)*g are fitting parameters.
- The micro-droplet profile is first extracted from the SEM image with user-guided software (red dotted line in Fig. 3, for example).
- Once the profile has been extracted, it is rotated to be horizontal in image coordinates (pixels).
- The profile is converted to physical coordinates (μm).
- A fourth order Runge-Kutta method, with truncation error to control the solution, is used to numerically solve and fit the set of parameterized equations to the micro-droplet profile in physical coordinates. From the solution, the left and right contact angles are determined.
- For comparison with the imaged micro-droplet, the model fitted profile is converted back to image coordinates and rotated back to the original droplet profile orientation (see green dotted line in Fig. 3, for example).
As noted in the “Discussion” section, sub-optimally-oriented samples can cause significant errors in contact angle and interfacial tension determination using this procedure.