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Saskatchewan Geological Survey 1 Summary of Investigations 2009, Volume 1

Studying the Effects of CO2 Exposure on Carbonates from the Weyburn Field Using Microtomographic Techniques: An Update

Ted Mahoney 1, Tom Kotzer 1, 2

Mahoney, T., Kotzer, T., and Hawkes, C. (2009): Studying the effects of CO2 exposure on carbonates from the Weyburn Field using microtomographic techniques: an update; in Summary of Investigations 2009, Volume 1, Saskatchewan Geological Survey, Sask. Ministry of Energy and Resources, Misc. Rep. 2009-4.1, Paper A-8, 13p.

, and Christopher Hawkes 1

Abstract At the Weyburn Oilfield in southeastern Saskatchewan, carbon dioxide (CO2) is being used to enhance oil recovery from Mississippian-aged carbonates of the Charles Formation. In parallel with this enhanced recovery, government, industry, and researchers from around the world are working together to investigate the performance of the Weyburn Field as a site for CO2 geological storage, under the auspices of the IEA GHG Weyburn-Midale CO2 Monitoring and Storage Project. Some of the key questions pertaining to CO2 movement and storage in carbonate rocks are related to the nature, rate, and consequences of CO2-fluid-rock interaction. Synchrotron radiation provides a method to examine the mineralogy and fabric of rocks at a high resolution and in a three-dimensional framework. Using computed microtomography (CMT) in association with conventional micro-beam techniques and standard petrophysical tests, characteristics such as the geometry of pore networks, the distribution of matrix mineralization, and the textural fabric of microstructures can be studied in hydrocarbon and potential CO2 storage reservoirs. By studying CO2-exposed and non–CO2-exposed cores, these analyses can reveal the effects that CO2 alteration has on the nature and continuity of pore geometry and controls on fluid migration at the micro-scale. To date, both CO2-exposed and non–CO2-exposed cores from the Weyburn Field have been analyzed using CMT, with preliminary results showing what appears to be preferential dissolution as a result of CO2 injection. Petrophysical results from a small percentage of the samples collected are also presented.

Keywords: Weyburn Field, Mississippian, Charles Formation, CO2 sequestration, CO2-flood, geological storage, computed microtomography, petrophysics, mineral dissolution, mineral precipitation, CO2-fluid-rock interaction.

1. Introduction

a) General Background At the Weyburn Oilfield in southeastern Saskatchewan, carbon dioxide (CO2) is being injected into the reservoir as a means to improve oil recovery. With this enhanced oil recovery (EOR) technique, CO2 is injected into the reservoir as a dense supercritical fluid (i.e., at pressures and temperatures that exceed the critical values of 7.4 MPa and 31°C, respectively) which interacts with oil to: 1) reduce the capillary forces that trap it within the smallest pore spaces of the reservoir, and 2) reduce its viscosity. Production in the Weyburn Field occurs from fractured Mississippian-aged carbonates, informally subdivided into a lower limestone unit, and referred to as the “Vuggy” and an upper dolostone unit referred to as the “Marly” (Figure 1). The CO2 flood is targeting the high-porosity (16 to 38%), low-permeability [1 to >50 millidarcy (mD)] Marly unit, as production prior to the CO2 flood was preferentially from the higher permeability (10 to >300 mD) although lower porosity (8% to 20%) fractured Vuggy unit (White et al., 2004; Whittaker et al., 2004). Although CO2 is injected to improve oil recovery in the short term, ultimately the CO2 will remain trapped underground over the short term as an ionic fluid and within mineral phases over geological time. Geological storage projects like Weyburn, which has the potential to isolate over 20 million tonnes of CO2 from the atmosphere, are becoming increasingly important in the effort to stabilize CO2 concentrations and hence mitigate climate change effects.

b) Objectives Porosity and permeability are the main controls on subsurface flow so quantifying these parameters is important in any geological storage site. Of equal importance is understanding the potential evolution of these parameters during injection and subsequent storage of CO2. As part of the Final Phase of the IEA GHG Weyburn-Midale CO2 Monitoring and Storage Project, this study aims to understand the mineralogical and petrophysical effects that

1 Department of Civil and Geological Engineering, University of Saskatchewan, 57 Campus Drive, Saskatoon, SK S7N 5A9. 2 Cameco Corporation, 2121 - 11th Street West, Saskatoon, SK S7M 1J3.


Figure 1 - Schematic geological cross section of the Weyburn Midale beds [courtesy of EnCana Corporation as presented in Glemser (2007), including reservoir flow units present in the Marly (M0, M1, and M3) and Vuggy (V1 to V4 and V6) units according to EnCana’s 2001 geological model for the Phase 1A CO2-flood area]. Samples collected from this research are from the M3, V1, and V6 flow units.

injection of CO2 may induce in the Midale beds, using conventional and synchrotron micro-beam techniques. In a more general sense, the results of the project will provide data to further expand the knowledge base of long-term CO2 storage not only in the Weyburn reservoir, but also in potential geological CO2 storage sites worldwide.

c) CO2 Rock-Fluid Interaction The predominant minerals of carbonate rocks are calcite (CaCO3) and dolomite [CaMg(CO3)2], which are highly susceptible to dissolution, especially by carbonic acid (H2CO3). Dissolution processes are important to understand, as they may significantly alter the porosity and permeability of the rock. The short-term chemical processes expected with injection of CO2 into the Weyburn reservoir are:

CO2 dissolution into the reservoir brine, given by (1), produces a slightly acidic solution.

CO2 + H2O ↔ HCO3- + H+ (1)

As a result of the decreased pH of the reservoir brine, dissolution of calcite and dolomite along with increased alkalinity [HCO3

-] occur as represented by the equations in (2) and (3) respectively.

H+ + CaCO3 ↔ Ca2+ + HCO3- (2)

2H+ + MgCa(CO3)2 ↔ Mg2+ + Ca2+ + 2HCO3- (3)

At Weyburn, geochemical monitoring and analyses of produced fluids have confirmed the dissolution of CO2 in the brine and subsequent mineral dissolution. Within months (10 months) of the beginning of injection, CO2 dissolution in brine was confirmed isotopically, as the significantly depleted δ13C signature of the injected CO2, was reflected in the δ13C values of produced reservoir fluids. Mineral dissolution was recognized by increases of total dissolved solids, [Ca2+] and alkalinity continuing over 10-, 21-, and 31-month sampling periods after injection started. These results also showed good spatial correlation to the highest CO2 injection volumes (Wilson and Monea, 2004).

Although dissolution can enhance effective porosity and permeability, precipitation of carbonate minerals can significantly reduce those characteristics. Carbon dioxide precipitation in solid form is also important because it represents a relatively secure sequestration mechanism. As such, understanding the conditions in which dissolution

9.1mdolostone limestone

anhydrite cemented

anhydrite


and precipitation will occur is important for predicting the evolution of porosity and permeability (and resultant increases or decreases in injectivity), and long-term storage mechanisms. The key to precipitating CO2 in solid form lies in reactions occurring between basic silicate minerals and CO2 given by the equations in (4), (5), and (6) from Gunter et al. (2000).

Anorthite + 2H2O + CO2 = Calcite + Kaolinite (4)

7Albite + 6H2O + 6CO2 = 6HCO3- + 6Na+ + Na-Smectite + 10Quartz (5)

3Annite + 11CO2 = 9Siderite + 2HCO3- + 2K+ + Muscovite + 6Quartz + H2O (6)

Although the Weyburn reservoir is a carbonate reservoir, reactive silicate minerals are abundant enough to potentially react with CO2-charged fluid, particularly in the upper Marly unit (Raistrick et al., 2009).

d) Computed Microtomography Previous studies have observed physical changes on core samples exposed to CO2 (e.g., Mathis and Sears, 1984; Svec and Grigg, 2001; Olsen and Stentoft, 2004). These studies were done in an attempt to better understand the relevant reactions between injected CO2, the pore fluids present, and the host rock matrix. Short-term experiments involving laboratory-controlled exposure of carbonate rocks to CO2 demonstrated small changes in rock permeability and observed mineralogy and morphology (Olsen and Stentoft, 2004). In the natural subsurface environment, however, these changes are likely to be far more subtle and at even smaller scale. To address this, detailed, high resolution three-dimensional imaging of the internal pore network and mineralogy of CO2-exposed rocks is being done to allow for a more complete investigation of CO2-induced reactions, including carbonate dissolution and precipitation effects. This is accomplished by using a non-destructive imaging technique known as X-ray computed microtomography (CMT). CMT has enabled researchers to peer into the internal structure of rock with submicron resolution and in a three-dimensional framework (e.g., Bonse and Busch, 1996; Coles et al., 1998; Knackstedt et al., 2004, 2007; Youssef et al., 2007). In particular, Glemser (2007) utilized CMT to provide detailed information regarding mineralogy and pore networks in several Vuggy and Marly carbonate samples from the Weyburn Field at the micron and sub-micron scale.

In comparison with bench-top tomography, synchrotron radiation provides an even more powerful means to examine internal structures of carbonate rocks on the submicron level. Conventional X-ray sources predominantly produce polychromatic electromagnetic radiation which can cause image artefacts as a result of preferential adsorption of low-energy photons. The major advantage of synchrotron radiation is that its imaging beam is monochromatic, highly collimated and very brilliant (~106 times greater; Wildenschild et al., 2002), thereby greatly increasing the imaging resolution and avoiding image artefacts commonly seen in previous bench-top studies (Bonse and Busch, 1996). Glemser (2007) used synchrotron-based CMT to image Weyburn Field carbonates at resolutions on the micron to sub-micron scale. Within this study, the CMT technique enabled quantitative and qualitative information on pore-space geometry and description of porosity-scale characteristics including pore and pore-throat diameters and tortuosity. These data were integrated and compared with conventional petrophysical laboratory tests as a means to evaluate the effectiveness of CMT as a tool for reservoir evaluation. Overall, the study revealed that CMT analyses coupled with laboratory measurements are a valuable tool for the petrophysical characterization of carbonate rocks.

e) Summary As demonstrated in experimental studies and field observations, injection of CO2 has varied effects on carbonate minerals and their associated pore systems, which in turn can affect injectivity and storage capacity. Dissolution of carbonate minerals as a result of CO2 injection can increase both the injectivity, by virtue of increasing permeability, and the storage capacity by enhancing porosity. Conversely, precipitation of carbonate minerals has been observed to profoundly block pore systems and decrease permeability; this is detrimental to injectivity, and may also affect storage capacity (Svec and Grigg, 2001). Therefore, pore-scale information is necessary to fully understand the petrophysical mechanisms of fluid flow within potential CO2 storage sites. CMT provides a non-destructive technology to qualitatively and quantitatively characterize the pore-space network and distribution of reactive and non-reactive mineral phases of both pre- and post-flood core samples in three dimensions at the micron scale. The objective of this study is to use the high-resolution capabilities of this technique to observe and evaluate geochemical reactions between injected CO2 and reservoir minerals in their very early stages. This enables a significant improvement on previous investigations of geochemical reactions, which have either required prolonged exposure times or artificial means of accelerating reaction rates.


2. Preliminary Results

a) Sample Selection Sample selection is a critical aspect of this study. Within this project, two types of core samples were targeted: 1) cores that have been exposed to injected CO2 in the Weyburn Field; and 2) the closest non–CO2-exposed core samples that are available, in order to minimize sample differences that are unrelated to CO2 exposure. Based on published results (Olsen and Stentoft, 2004; Izgec and Demiral, 2006), dissolution effects are expected to be more pronounced near CO2 injection points. For this reason, within the suite of samples that have been exposed to CO2, samples proximal to injection locations have been collected from both the Marly and Vuggy units (Wilson and Monea, 2004). Data collected by Glemser (2007) on Weyburn Field samples are also being integrated within the sample suite.

Based on input received from EnCana geological personnel, four cored wells from the Weyburn Field were sampled. The locations of these wells are shown in Figure 2. The core was accessed at the Subsurface Geological Laboratory in Regina. Ten one-inch core plugs were taken from these four wells at various depths and orientations. The locations, depths, orientations, and flow unit for each sample are given in Table 1. Depth picks for the flow units in the sampled wells were provided by EnCana.

Cores from two wells located at 06-08-006-13W2 were selected in order to compare and observe potential mineral dissolution as a result of CO2 injection. An observation well at 31/06-08-006-13W2 was drilled and cored in November 2005. Carbon dioxide injection in this part of the field is conducted through an injection well at 21/06-08-006-13W2. This injection well was drilled and cored in November 2003, with CO2 injection commencing in the fall of 2004. As such, the 31/06-08 observation well provides core that was exposed to CO2 for approximately one year, and the 21/06-08 well provides relatively proximal core that was not exposed to CO2. Because the 31/06-08 core is only about 120 m east of the 21/06-08 injector well, its potential for showing mineral dissolution is postulated to be high as injected fluids would be undersaturated with respect to the ions Ca2+, Mg2+, and HCO3

-. According to EnCana, the reservoir layers in this area should have been preferentially contacted by injected CO2, the lower Vuggy probably more so than the upper Vuggy or Marly. To evaluate the effects of anisotropy in the rock fabrics within the core, two one-inch core plugs were drilled from the V6 flow unit in the 31/06-08 observation well at a depth of 1480.30 m; one drilled perpendicular to core axis (i.e., sub-horizontally), and the other drilled parallel to core axis (i.e., sub-vertically). Correspondingly, as a baseline sample to permit comparison, two one-inch core plugs were drilled from the V6 flow unit of the injector well at 1462.65 m, parallel and perpendicular to the core axis. A one-inch core plug was also drilled from the Marly M3B flow unit at 1465.15 m in the observation well, allowing for a comparison to pre-injection Marly core from the injector well available from Glemser’s (2007) research project.

Figure 2 - Locations of the sampled wells for this project within the Weyburn Field, with the detailed map outlined in red. The detailed map shows the locations within the boundaries of the EOR area (shown in blue).

Detail Map Area

0 6

Miles0 4R12

T5

R13R14

T6

T7

kilometreskilometres


Table 1 - Sample number, location, exposure history, and plug orientation of the Weyburn Field carbonates studied in this project. The Marly dolostone sample is of a different vintage from samples 1A to 1E and was collected as part of Glemser’s (2007) research project.

Cores from other wells offer another opportunity to compare and observe CO2 alteration effects. Rocks in the horizontal well 92/07-05-006-13W2, drilled and cored in January 2005, through the Vuggy V1 reservoir unit, had been exposed to CO2 for approximately 18 months, given that injection in the area had commenced in mid-2003. A one-inch core plug was drilled from the core of the horizontal well at a measured depth of 1557.60 m (1453 m vertical depth). For comparison purposes four one-inch core plugs, both parallel and perpendicular to the core axis, were drilled from the core of 21/14-05-006-13W2 through two porous zones of the V1 reservoir unit at vertical depths of 1446 m and 1450 m.

b) CMT Analyses To date, five of the ten samples collected have been imaged using CMT. Samples 1A, 1B, 1C, 1D, and 1E were imaged at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France, using the ID19 High-resolution Diffraction Topography Beamline with an image resolution of 7.5 µm. At the ESRF, 15 mm diameter by 7 mm length plugs were drilled out of the one-inch plugs provided.

The basic process of CMT involves recording radiographic projections of attenuation of linear X-rays passing through a sample. X-rays emitted from the synchrotron source pass through a sample and are attenuated by scattering or absorption, the extent to which is strongly a function of density and chemical composition of the sample. This is commonly referred to as the Linear Attenuation Co-efficient (LAC). The variably attenuated X-rays subsequently strike a phosphor screen which emits visible light dependent upon attenuated X-ray intensity, which is then recorded by a detector as the sample is scanned stepwise at discrete angles through 180°. The visible light recorded by a single pixel within the detector directly corresponds to the intensity of the attenuated X-rays and is recorded and converted by a computer to a digital signal. This digital signal is coded on eight bits (256 grey levels) with each pixel representing a particular grey level from black (0) to white (255), ultimately depending on the density and chemical composition of the phase being imaged. As seen in Figure 3, the radiographic projections from each angle create two-dimensional slices of the X-ray attenuation map or internal structure of the image scanned. These two-dimensional slices of X-ray attenuation maps are then digitally stacked to create voxel-based, three-dimensional images of LAC, facilitating digital projections of density-dependant mineralogy and pore networks within the samples.

Within this study, CMT data collected on samples are recorded as 950 two-dimensional radiograph slices of pixels along the length of the core, arranged on a 2048 × 2048 grid. As previously indicated, LAC is directly related to electron and phase density, hence in this context can be taken as an indication of the spatial distribution of chemical elements within arranged lattices, or minerals within phase space. A previous study on the Vuggy and Marly carbonates from the Weyburn Field used mineralogical and chemical data from transmitted light microscopy (TLM) and electron microprobe (EMP) to establish a relationship between LAC as observed grey levels within CMT radiographs and mineralogy (Table 2; Glemser, 2007). For the analyses here, this relationship was used primarily to delineate porosity and distribution of carbonate minerals within the overall matrix. As the relationship between LAC

Sample Number

Well Location

Flow Unit

Measured Depth (m)

CO2 Exposure History

Plug Orientation

1A 31/06-08-006-13W2 V6 1480.30 Exposed, over one year Orthogonal to core axis

1B 21/14-05-006-13W2 V1 1446.00 Unexposed baseline Orthogonal to core axis

1C 92/07-05-006-13W2 V1 1557.60 Exposed, 18 months Orthogonal to core axis

1D 31/06-08-006-13W2 M3 1465.15 Exposed, over one year Orthogonal to core axis

1E 21/06-08-006-13W2 V6 1462.65 Unexposed baseline Orthogonal to core axis

2A 21/14-05-006-13W2 V1 1446.00 Unexposed baseline Parallel to core axis

2C 21/06-08-006-13W2 V6 1462.65 Unexposed baseline Parallel to core axis

2D 31/06-08-006-13W2 V6 1480.30 Exposed, over one year Parallel to core axis

2E 21/14-05-006-13W2 V1 1450.00 Unexposed baseline Parallel to core axis

2F 21/14-05-006-13W2 V1 1450.00 Unexposed baseline Orthogonal to core axis

Marly dolostone * 21/06-08-006-13W2 M3 1446.00 Unexposed baseline Unknown

* Digital CMT data only, no physical specimen is available.


Figure 3 - Two-dimensional CMT radiograph slices of Weyburn Field samples showing areal distribution of LAC-related phases, or minerals and pore space. Black areas indicate porosity whereas grey to white areas indicate higher density phases, with brightest being highest density. A) sample 1C; B) sample 1A; C) sample 1D; D) sample 1E; E) sample 1B; and F) Marly dolostone. A through C are CO2-exposed samples, D through F are baseline non–CO2-exposed samples. Radiographic slices shown are 15 mm in diameter.

Table 2 - Variation in LAC values for mineral phases and porosity within Weyburn Field samples as established by Glemser (2007). LAC is a dimensionless parameter, ranging from 0 (lowest density, shown as black) to 255 (highest density, shown as white).

and mineralogy can be affected by isostructural solid solution within the minerals, similar mineral-LAC relationships are being established for the present study. As the samples from the Glemser study are closely related to samples from this study, this first approach is reasonable.

The LAC values in Table 2 facilitate segmentation and display of the CMT data within the three-dimensional framework as either porosity or mineral phases. The resulting three-dimensional voxel-based images serve to reinforce the highly variable nature of the pore networks within the Weyburn carbonate rocks. Relatively large, vuggy pores are easily distinguished in the Vuggy samples (Figures 4 to 7). Conversely, Marly samples reveal isolated, scattered vuggy pores (see Figures 8 and 9), with Figure 9 displaying a more evenly distributed pore network. Total porosity values, calculated from the statistical distribution of open pore space within the digital CMT data, are highly variable (Table 3).

Porosity within the Marly unit is known to be generally intercrystalline and homogenous (Wegelin, 1987), although this is not observed within the CMT data at the current imaging resolution of 7.5 µm as these small-scale features are below this level of resolution. Previously, on similar Marly samples, Glemser (2007) was able to resolve this homogeneous, intercrystalline porosity typical of the Marly unit by imaging at a higher resolution of 0.7 µm. This will be similarly done here as a follow-up on selected samples.

Despite the variable nature of the pore space network within the samples analyzed, the reconstructed CMT data are being used to evaluate their potential use toward providing insight into the physical nature of possibly subtle, short-term alteration effects of CO2

Sample Constituent

Observed LAC Range

Vuggy Marly

Porosity 0 to 85 0 to 85 (0 to 50)*

Quartz N/A 92 to 112

Dolomite 118 to 130 130 to 170 (53 to 93)*

Calcite 146 to 186 179 to 204 (104 to 120)*

Anhydrite 214 to 233 N/A

Celestine N/A 223 to 255

* Marly dolostone values of a different vintage.


Figure 4 - Sample 1A (CO2-exposed Vuggy V6). Three-dimensional sub-volume of LAC-related phases on left and segmented pore space shown as a gold surface on the right. Note the linear tube-like structures in the porosity image on the right, highlighted within the red circles. Field of view for an individual sub-volume is 10 mm x 10 mm x 7 mm.

Figure 5 - Sample 1E (non–CO2-exposed baseline Vuggy V6). Three-dimensional sub-volume of LAC-related phases on left and segmented pore space shown as a gold surface on the right. Field of view for an individual sub-volume is 10 mm x 10 mm x 7 mm.


Figure 6 - Sample 1C (CO2-exposed Vuggy V1). Three-dimensional sub-volume of LAC-related phases on left and segmented pore space shown as a gold surface on the right. Field of view for an individual sub-volume is 10 mm x 10 mm x 7 mm. Note linear tube-like structures in porosity image on the right, highlighted within the red circle.

Figure 7 - Sample 1B (non–CO2-exposed baseline Vuggy V1). Three-dimensional sub-volume of LAC-related phases on left and segmented pore space shown as a gold surface on the right. Field of view for an individual sub-volume is 10 mm x 10 mm x 7 mm.


Figure 8 - Sample 1D (CO2-exposed Marly M3B). Three-dimensional sub-volume of LAC-related phases on left and segmented pore space shown as a gold surface on the right. Field of view for an individual sub-volume is 10 mm x 10 mm x 7 mm. Note linear tube-like structures in porosity image on the right.

Figure 9 - Marly dolostone (non–CO2-exposed baseline Marly M3B). Three-dimensional sub-volume of LAC-related phases on left and segmented pore space shown as a gold surface on the right. Field of view for an individual sub-volume is 8.7 mm x 8.7 mm x 6.5 mm. Data are of a different vintage from samples 1A to 1E and were collected as part of Glemser’s (2007) research project.


Table 3 - Preliminary petrophysical results. Samples 1A, 1B, 1C, 1D, 2A, and 2D are saturated in toluene at the time of preparation of this report. The Marly dolostone sample is a digital dataset only.

injection on the Weyburn carbonate rocks. As previously indicated, linear, tube-like pore structures are readily observable in the CO2-exposed samples (especially in Figures 6 and 8; to a lesser extent in Figure 4) which appear to be noticeably absent in the baseline Vuggy samples (Figures 5 and 7).

In this project, integration of standard techniques, such as transmitted-light microscopy (TLM) with synchrotron CMT is being done to provide insight into the origin of these pore structures within the cores, which are suggested here to reflect the early stages of CO2 interaction with the rock. Because segmentation of the CMT data allows for detailed isolation and visualization of pore space in a

three-dimensional framework, the linear features within the CMT reconstructions are not easily distinguished in photomicrographs. This is evident from a comparison of reconstructed CMT porosity (Figure 10) with a photomicrograph (Figure 11) obtained from a Vuggy V1 sample that was exposed to CO2 prior to coring. The fact that tube-like linear features are abundant in the CO2-exposed samples while a limited number of smaller linear features have been observed in some of the non–CO2-exposed samples suggests that these features are a result of preferential dissolution during exposure; i.e., the CO2-rich fluids are interacting with the carbonate minerals around existing features, thereby producing pore structures with defined geometry. What these features are is presently unclear. Working hypotheses include: (i) cleavage planes and crystal terminations in the carbonate crystals (which may be supported by fact that these features frequently appear to be structured rather than random, as shown in Figure 10a); (ii) micro-fractures through the rock; or (iii) microfossils (which may be supported by the occurrence of a few features with shapes as shown in Figure 10b). As these interfaces are exposed to more CO2, these features would become larger and more distinct, hence facilitating the interpretation of their origins. For example, if the linear pore structures are a result of dissolution along cleavage planes and crystal terminations or micro-fractures, they might be expected, with longer exposure time, to propagate into more planar features.

Figure 10 - Porosity images of Sample 1C (CO2-exposed V1) shown as a gold surface. Encircled pores on the left are possibly the result of preferential dissolution along cleavage planes or micro-fractures. The image on the right shows potential dissolution of a microfossil.

Sample Number Total Porosity Effective Porosity Gas Permeametry

1A 0.62% N/A N/A

1B 2.14% N/A N/A

1C 0.90% N/A N/A

1D 0.30% N/A N/A

1E 1.71% 12.0% < 6 mD

Marly dolostone 3.10% N/A N/A

2A No digital data N/A N/A

2C No digital data N/A < 6 mD

2D No digital data N/A N/A

2E No digital data N/A < 6 mD

2F No digital data N/A < 6 mD

(a) (b)


Figure 11 - Photomicrograph of Sample 1C (CO2-exposed V1), showing cleavage planes in calcite crystals.

c) Preliminary Petrophysical Results A component of this research project involves measuring and comparing standard petrophysical properties of CO2-exposed samples to those of non–CO2-exposed samples. This will be accomplished using standard laboratory techniques as well as techniques utilizing the digital CMT datasets. This component of the project was in its initial stages as this paper was being written. To date, a limited number of permeability and porosity measurements have been made on selected samples. A number of the samples were strongly oil stained; all of these samples are currently in the process of being flushed with toluene to remove the oil. Results obtained to date are summarized in Table 3. The total porosities listed were determined by segmentation of pore space in the CMT datasets. The lone effective porosity measured to date was obtained gravimetrically (by water saturation; this same technique will be used for the other samples after they

have been cleaned). The permeability measurements reported for four samples were performed with a Ruska gas permeameter. All of these samples had permeabilities that fell below the measurement threshold of the permeameter, which is 6 mD. An alternative testing apparatus is currently under development, which will be used to re-test these samples, and the others (once they are cleaned).

One of the limitations of CMT is obvious when comparing the CMT total porosity of 1.71% to the effective porosity of 12.0% measured gravimetrically on sample 1E. The significantly lower porosity determined using the CMT data is a consequence of the fact that a significant proportion of the pores in the Weyburn Field samples are smaller than the 7.5 µm image resolution that was used for this first round of imaging. The effective porosity of 12.0% falls within the range of values reported in literature for the Vuggy unit (8 to 20%).

3. Ongoing and Future Work

a) Mineralogical Investigation In order to positively identify CO2-induced alteration effects in the samples collected to date, determination of the mineralogy, both in thin-section and in the CMT datasets, of CO2-exposed and baseline samples is crucial. As such, the relationships established by Glemser (2007) linking a range of LAC values to a particular mineral phase through electron microprobe analyses (EPMA) and TLM is being confirmed for this particular set of samples. Further investigations will be carried out on relationships between the linear tube-like features observed in the CO2-exposed samples through the structural and mineralogical identification of these linear features in the non–CO2-exposed samples.

b) Petrophysical Investigation Petrophysical work has been completed on a small percentage of samples collected, as described in the Preliminary Petrophysical Results section above. Once the residual oil has been satisfactorily removed from the remaining samples, gas permeametry followed by effective porosity measurements and mercury intrusion tests will be conducted. Permeability and tortuosity will be calculated from the digital CMT datasets. The results from these analyses will be examined in light of their history of CO2 exposure, with CO2-exposed samples potentially showing some early stage dissolution effects manifested by increased effective porosity and permeability. Also, the degree to which tortuosity values differ between non–CO2-exposed and CO2-exposed will be indicative of the impact CO2 has had on the fluid flow paths of the exposed samples.

c) Higher Resolution CMT Analyses Higher resolution CMT analyses will be carried out on several samples from this data set at a resolution of 0.7 µm. Increasing image resolution to 0.7 µm will serve to aid in observation and characterization of pore network interconnectivity between observed features in CO2-exposed core samples which appear isolated at 7.5 µm resolution. Similar to the 7.5 µm dataset, total porosity, permeability and tortuosity values will be calculated and examined in light of their CO2-exposure history. Of particular interest is the development and extent of interconnectivity (permeability) amongst the tube-like linear pore features as well as their connection to apparently isolated pores, here assumed to be the loci for these features. This information will be extended to relationships

Cleavage planes

1.5 mm


observed in TLM to further test the hypotheses that these features are a result of: 1) preferential dissolution of carbonate minerals along cleavage planes and crystal terminations, or 2) dissolution along micro-fracture planes, or 3) dissolution of microfossils.

d) Core Samples Experimentally Exposed to CO2 Natural variation in geological samples makes the comparison of non–CO2-exposed and CO2-exposed rocks in their natural setting difficult, but nonetheless valuable. A direct comparison of the same sample before and after laboratory-controlled CO2 exposure would further expand upon the knowledge being built around early CO2-alteration effects in this project. Preliminary plans have been discussed regarding analyses of core samples exposed to CO2 in a laboratory setting. Ideally, this will be carried out on non–CO2-exposed baseline core samples from this research project which have been previously analyzed using CMT and subsequently subjected to CO2 exposures under pressure and temperature representative of the Weyburn Field. Even though the total amount of supercritical CO2 interacting with the carbonate rock cores within this study will be much greater than for the natural samples, even over extended time periods, the mineralogical and geochemical effects observed will be used as a proxy for model comparisons and evaluating the effects within the present data sets.

4. Acknowledgments The authors of this paper would like to acknowledge the IEA GHG Weyburn-Midale CO2 Monitoring and Storage Project for financial support and Geoff Burrowes (EnCana), James Johnson (Schlumberger Carbon Services), Erik Nickel (Saskatchewan Ministry of Energy and Resources), and Steve Whittaker (Petroleum Technology Research Centre) for their input and help, and the peer reviewers who improved this paper with their suggestions and corrections.

5. References Bonse, U. and Busch, F. (1996): X-ray computed microtomography (µCT) using synchrotron radiation (SR); Prog.

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studying the effects of co2 exposure on carbonates from ... › pubsask...saskatchewan geological...

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