Low-permeability (unconventional) hydrocarbon reservoirs exhibit a complex nanopore structure and micro (µm) -scale variability in composition which control fluid distribution, displacement and transport processes. Conventional methods for characterizing fluid-rock interaction are however typically performed at a macro (mm) -scale on rock sample surfaces. In this work, innovative methods for the quantification of micro-scale variations in wettability and fluid distribution in a low-permeability oil reservoir was enabled by using an environmental scanning electron microscope.
Hanford J. Deglint, Christopher R. Clarkson, Chris DeBuhr & Amin Ghanizadeh
Department of Geoscience, University of Calgary, 2500 University Drive NW, Calgary, Alberta, T2N 1N4, Canada
Received: 08 February 2017, Accepted: 11 May 2017, Published online: 28 June 2017
© The Author(s) 2017
Live imaging of controlled water condensation/evaporation experiments allowed micro-droplet contact angles to be evaluated, while imaging combined with x-ray mapping of cryogenically frozen samples facilitated the evaluation of oil and water micro-droplet contact angles after successive fluid injection. For the first time, live imaging of fluids injected through a micro-injection system has enabled quantification of sessile and dynamic micro-droplet contact angles. Application of these combined methods has revealed dramatic spatial changes in fluid contact angles at the micro-scale, calling into question the applicability of macro-scale observations of fluid-rock interaction.
Exploitation of unconventional hydrocarbon resources (UHRs), such as shale gas/oil, has been made possible through the application of technologies including horizontal wells completed in multiple hydraulic fracturing stages1. However, efficient recovery of UHRs has in part been hampered by a lack of understanding of fluid storage, distribution, displacement and transport processes occurring within the matrix nanopore structure where the majority of the hydrocarbon resource resides1, 2. With interest growing in the application of improved oil recovery (IOR) techniques, achieved for example through water and gas injection, and the possibility of permanently storing greenhouse gases (GHGs) in UHRs as an important environmental byproduct of IOR processes3, a premium has been placed on understanding fundamental pore-scale physical processes which control injected and produced fluid distribution, displacement and transport mechanisms.
Recent advances in imaging technologies such as focused-ion-beam scanning electron microscopy (FIB-SEM)4 have revealed important details regarding the nanopore structure in shales5, an important UHR. This pore structure information has been used in the population of pore-scale models to predict important shale reservoir properties affecting fluid storage and flow properties such as porosity and permeability6. It is possible that these pore-scale models may even be able to predict important reservoir properties affecting multi-phase flow of gas, water and oil in UHRs, such as relative permeability and capillary pressure. However, while imaging of rock nanopore structure is now routine, imaging of fluid-rock interaction at this scale, necessary for quantifying multi-phase fluid distribution and flow, is not.
Indeed, characterization of fluid-rock interaction for UHRs is currently limited to the macro (mm) – scale7. Routine methods for assessing macro-wettability of fluids on rock surfaces (as determined from contact angle measurements) include the sessile drop and captive bubble techniques8, 9, which are commonly used by research/commercial laboratories mainly due to their simplicity. However, these two techniques can provide quite different contact angles depending on the degree of heterogeneity/roughness of the solid surface, the drop (bubble) size, and environmental vibrations10. Further, contact angle measurements performed at the macro-scale can only be used to characterize the average wettability of a rock-fluid system.
Figure 1(a) Thin-section (2.8 cm × 1.6 cm) of a low-permeability Cardium sample for which back-scattered electron (BSE) images were acquired at high resolution (<250 nm) using an FEI Quanta FEG 250 environmental field emission scanning electron microscope (E-FESEM). The Cardium sample was first impregnated with epoxy, then finely polished and mounted on a slide. A zoom-in of the area outlined with the orange box illustrates the high spatial resolution. Fine-scale (micro- or nano-meter) pore structure information, such as that illustrated in the zoomed image, is often used to populate pore-scale models for permeability and porosity estimation. However, wettability measurements are still routinely performed at the macro-scale (see hypothetical macro-droplet superimposed on the bottom half of the thin section), which cannot be used to model fluid flow at the pore scale. (b) Image (collected using E-FESEM) of micro-droplet formation on a Cardium sample during a condensation/evaportation experiment illustrating water contact angle variability at the grain- (micron) scale.
This latter point is illustrated conceptually in Fig. 1a where back-scattered electron (BSE) images of a low-permeability (Cardium Formation) sample were acquired using an SEM1. Imaging over a thin-section-scale sample (2.8 cm × 1.6 cm) was performed at high imaging resolutions (<250 nm). The inset image illustrates the fine-scale variation in mineralogy (evident from gray-scale variation) and ultra-fine pore structure – pore structure images of this resolution may be used to populate pore-scale models. However, conventional macro-scale measurements of wettability, illustrated with a conceptual macro-drop placed on the bottom half of the thin section image, would provide an average wettability that cannot be used in modeling fluid flow through the fine pore-structure in contact with mineral grains with variable composition. For example, Fig. 1b is an SEM image of a Cardium Formation sample taken during a condensation/evaporation experiment (see “Results” section) illustrating variability in droplet contact angles at the micro-scale for different mineral grains.
In the present work, quantification of micro-scale variations in wettability and fluid distribution in a low-permeability oil reservoir for use in pore-scale modeling is made possible with an environmental field emission scanning electron microscope (E-FESEM). The studied samples were taken from the Bakken reservoir in Viewfield Saskatchewan, Canada, which is an active low-permeability oil development area11. Importantly, in-field piloting of IOR processes is being undertaken in the middle member of the Bakken12 (hereafter referred to as “middle Bakken”) and the present work and other reservoir characterization efforts will allow field-scale models to be constructed to evaluate both hydrocarbon recovery efficiency and CO2 storage potential. Pore-scale models will first be generated to derive important reservoir properties such as relative permeability and capillary pressure curves13, but a key uncertainty to be addressed in the current work is the effect of micro-scale rock compositional heterogeneity on fluid-rock interaction.
A logical order of experimentation with the E-FESEM was used to assess micro-wettability in the mineralogically-heterogeneous middle Bakken tight oil samples. First, live imaging of distilled water condensation/evaporation experiments, due to relative experimental simplicity, was undertaken to provide a base dataset for: 1) testing micro-droplet profile extraction and contact angle-fitting algorithms and 2) assessing the variability in micro-droplet contact angles on sample surfaces. Once the algorithms were vetted, cryogenically frozen samples were then analyzed after sequential imbibition of reservoir oil then synthetic brine to evaluate “native” fluid distribution and contact angles. The combination of SEM imaging and X-ray mapping of the cryogenically frozen samples allowed for both fluid identification and fluid-rock interaction analysis. Finally, live imaging of the injection and subsequent fluid-rock interaction of nanoliters of fluid, using a micro-injection system, was performed. Here live imaging refers to a sequence of time-lapsed images taken at 1 frame per second. These latter experiments, the first of their kind, were used to determine if sessile and dynamic micro-droplet contact angles of arbitrary fluids could be measured.
This work builds on that of previous studies using environmental scanning electron microscopy (ESEM)14,15,16, cryogenic scanning electronic microscopy (cryo-SEM)17,18,19,20,21,22 and other imaging studies23 focused on the wetting behavior of micro- (and nano-) droplets on a variety of substrates. As a result, the current advances put forward, particularly in the area of controlled micro-injection of nanoliters of fluid, have broader applicability in various scientific fields where the study of wetting phenomena at a fine scale is necessary.
Condensation and evaporation
By using a Peltier stage for heating/cooling the samples, condensation and evaporation of distilled water can be carefully controlled and imaged. As such, the (time-lapse) evolution of sessile droplet formation onto samples can be observed, allowing for the appropriate moment to select droplet profiles for contact angle estimation.
Figure 2 Time-lapse images (compiled over 2:27 minutes) of a condensation/evaporation experiment performed on a middle Bakken sample using the E-FESEM. Micro-droplets of distilled water can be seen forming on the large dolomite grain, annotated by the green circle (Frame 1). As the experiment progresses, the droplets coalesce into a larger micro-droplet (Frames 2–4) which can then be used for contact-angle measurement (green box in Frame 4). The micro-droplet in Frame 4 was chosen for this purpose because, later in the condensation process (Frames 5–6), pinning and the merging of the micro-droplet with a micro-droplet in an offset grain affects the contact-angle measurement. Evaporation occurs during Frames 8–9. The sample temperature was held at around 2.0 °C, and chamber pressure around ~700 Pa. The scale bar is 150 µm.
This is illustrated in Fig. 2, which provides a time-lapse sequence (2:27 minutes) of a condensation/evaporation experiment for a middle Bakken sample, captured using back-scatter-electron imaging. Focusing attention on the dolomite grain (circled in Frame 1), it can be observed that micro-droplet growth occurs on the grain until Frame 7, after which an evaporation cycle initiates. Frame 4 is selected for droplet profile extraction (Fig. 3a) because the contact angle in later frames is affected by pinning and the micro-droplet wetting and merging with a micro-droplet growing on an offset grain (Frames 5–8). In this example, pinning refers to change in the three-phase contact line during subsequent addition of fluid24.
Figure 3(a) A zoom-in of the dolomite crystal in Frame 4 of Fig. 2 illustrating 1) the extracted droplet profile (red dotted line) used for contact-angle assessment and 2) the fit of the parameterized Young-Laplace equation (green dotted line) used to estimate left and right contact angles, and droplet dimensions. The procedure for micro-droplet contact angle assessment is provided in the “Methods” section. The measured drop height and width is 17.1 μm and 48.4 μm, respectively. The right and left contact angles are 69° and 69°, respectively. (b) Elemental map (created from X-ray detection and analysis), superimposed on the secondary electron image of Fig. 2. The following minerals are interpreted from the elemental mapping: dolomite (determined from Mg, light blue), quartz (Si, yellow), potassium feldspar (K, red-orange), calcite (Ca, pink) and salt (Na, dark blue).
Once the appropriate frame for analysis has been selected, the droplet profile is extracted, and a parameterized version of the Young-Laplace Equation25, 26 is fit to the droplet profile (see “Methods” section for procedure) to obtain the contact angles on each side of the droplet (Fig. 3a). The resulting extracted micro-droplet dimensions are 17.1 μm (height) × 48.4 μm (width), with an average (of left and right) contact angle of 69°.
Using this procedure, additional micro-droplet profiles were extracted from the imaged region of the sample shown in Fig. 2. Micro-scale wettability of this sample is variable. An X-ray elemental map (Fig. 3b), overlaid on the secondary-electron image, reveals compositional variability of the sample, which in turn can be used to assess the compositional controls on wettability.
Figure 4 The first 3 frames are time-lapse images (compiled over 21 seconds) of a condensation/evaporation experiment performed on a second middle Bakken sample using the E-FESEM. Water condensation is seen to occur on a rutile grain (tear-drop shaped grain appearing in the lower half of the green box in Frame 2 and on a potassium feldspar grain whose edge can be seen in the upper half of the green box). The sample temperature was held at around 2.1 °C, and chamber pressure around ~700 Pa. The scale bar is 100 µm. (a) Mineral identification through the use of elemental maps. Color coding for elements/minerals is the same as for Fig. 3b with the addition of titanium (light green) corresponding to rutile. (b) Zoom in of the green box in (a) showing an extracted droplet profile (red dotted line) for a micro-droplet condensed onto the surface of the potassium feldspar grain. The measured micro-droplet height and width is 6.8 µm and 12.2 µm, respectively. The calculated right and left contact angles are 94° and 97°, respectively.
An additional time-lapse example (21 seconds) is provided in Fig. 4. This sample is compositionally distinct from that shown in Fig. 3b, due in part to the presence of rutile (light green on elemental map in Fig. 4a) and potassium feldspar (orange on elemental map), providing an opportunity to further evaluate the relationship between composition and wettability. Micro-droplets such as those condensing on the potassium feldspar grain adjacent to the rutile grain (highlighted in green box, Frame 2 of Fig. 4) can be analyzed using profile extraction and fitting of the Young-Laplace Equation (Fig. 4b). The resulting extracted micro-droplet dimensions are 6.8 μm (height) × 12.2 μm (width), and an average contact angle of 95.5°.
Imaging and X-ray mapping of samples after cryogenically freezing them allows the distribution of native fluids, and native fluid contact angles, to be determined, if preserved samples are available. This same technology can be used to evaluate the distribution of fluids and their contact angles after they are imbibed or injected into the sample. An example of the latter is studied herein.
A middle Bakken sample was immersed in formation oil (obtained from the wellsite) for six weeks, allowing oil to be imbibed into the sample, and was subsequently cryogenically frozen, broken to expose a fresh surface, and imaged (Fig. 5).