Grosmont Dolomite Cores from Various Formation Depths after

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Grosmont Dolomite Cores from Various Formation Depths after Bitumen Extraction: Dissolution in Brine and Interfacial Properties as a Function of Common Lattice Ion and pH Chandra W. Angle* and Tadeusz Dabros CanmetENERGY, Natural Resources Canada, No. 1 Oil Patch Drive, Devon, Alberta, Canada ABSTRACT: Dissolution and ζ potential of dolomitic particles obtained from six cores at various depths in the Grosmont carbonate formation were studied after the bitumen was extracted. Changes in specific conductivity and pH during mixing and after equilibration for 12 h were indicators of dissolution. Zeta potentials were measured as functions of pH and concentrations of common lattice cations Ca2+ and Mg2+ and the anion CO3 2− concentrations in an indifferent electrolyte NaCl background. The negative ζ potential decreased with increases in the added Ca2+ and Mg2+ concentrations and became more negative with incrementally added CO3 2− anion. The ζ potentials for all core solids became less negative when pH was reduced. The data were compared to responses of a model dolomite which displayed an isoelectric point around pH 5.8. None of the core solids showed an isoelectric point. They were not positively charged like the model dolomite when cation concentrations were increased in the suspending medium. The organic film left on the core solids surfaces after bitumen extraction was responsible for the nonideal electrokinetic behavior. There were minor differences in properties of the solid/liquid interfaces from core to core. There was no specific correlation with the depth of origination for the cores. The dissolution and electrical charge responses of the cores solids would suggest possible impact on emulsions that may form during bitumen extraction if water is used for extraction as in steam assisted gravity drainage processes. The properties of the residual core left after bitumen is removed would determine how the aquifer environments are affected.

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INTRODUCTION Most of the presently exploitable Alberta bitumen is hosted by unconsolidated Lower Cretaceous sands in the areas of Athabasca, Cold Lake, and Peace River.1 As these resources are consumed, oil companies are turning their attention to bitumen-bearing carbonate deposits, which lie beneath the more traditional oil sands. The Grosmont and Nisku carbonate formations combined contain more than 25% of Alberta’s crude bitumen on the basis of initial in-place volumes, making carbonates a significant potential source of bitumen in Alberta.2 About 318 billion barrels of bitumen is estimated to be located in the Upper Grosmont units.2,3 Commercial bitumen extraction from these carbonate formations has progressed slowly because of a lack of proven economic extraction methods. High bitumen viscosity, heterogeneity in karsted and fractured reservoirs resulting from erosion, high porosities and permeabilities,4,5 and the presence of gas all combine to make extraction technically challenging.6,7 The use of steam assisted gravity drainage (SAGD) can be environmentally disadvantageous. Effective and efficient extraction of bitumen or heavy oil from these carbonate deposits requires, among other things, an understanding of the interactions and association of the bitumen with the rock matrix, rock/water dissolution, and interfacial effects. The geological stratigraphy, diagenesis, and reservoir properties of the Grosmont have been the subject of numerous studies, especially since the 1980s when there was active interest in exploitation of the formation’s hydrocarbons. An overview of the geology of the Grosmont bitumen deposit by Buschkuehle et al. summarizes previous studies as well as recent work by the Alberta Geological Survey.8 A concise history of Published XXXX by the American Chemical Society

geological research on the Grosmont can be found in that paper. Alberta’s Energy Resources Conservation Board updated its 1990 assessment of the Grosmont in 2010 based on analyses from over 1330 wells. This reassessment2 resulted in a 27.8% increase in the estimated in-place bitumen resources to 64.5 × 109 m3. Dolomite types within the Grosmont Formation were characterized by H. Huebscher in 1996 by macroscopic analysis, thin section petrography, stable isotope composition analysis, X-ray diffraction (XRD) of CaCO3 content, and electron microprobe analysis of elemental composition.9 Huebscher found that there were two types of replacement dolomite within the Grosmont Formation. Type 1, which is most common, especially in units C and D, was probably formed syndepositionally by seawater. It displays loosely interlocking euhedral to subhedral crystals ranging in size from 20 to 100 μm. CaCO3 content varies from 49.2 to 56.9 mol %. Some samples contained trace amounts of Sr, Na, Fe, and Mn.9 The most comprehensive work to date on Grosmont bitumen was completed by Zhao in 2009.10 He performed viscosity measurements, gas chromatography−mass spectrometry (GC−MS) analysis, elemental analysis, and thermogravimetric analysis on both fresh and legacy bitumen samples. Little work has been done with respect to the physicochemical properties of the formation solids. In 1983, Skauge et al. measured the specific heat capacity of Grosmont mineral solids.11 They found that the specific heat capacities were Received: February 19, 2013 Revised: April 30, 2013

A

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Figure 1. Dolomite lattice diagram. Part A is an ideal structure and part B nonideal.14: (1) water; (2) magnesium ion hydrate; (3,4) carbonate.

tend to be incorporated in the magnesium layer creating calcian-dolomite. He explained that the carbonate ions are unhydrated but must have sufficient energy to displace water next to the layer of cations.14 Carbonates can make excellent reservoirs because their solubility facilitates the generation of the porosity and permeability that allows hydrocarbon emplacement and storage. Carbonate rocks contain approximately 50% of the world’s hydrocarbon reserves, and about 50% of the world’s carbonate reservoirs are dolomites.15,16 About 80% of the carbonate oil and gas reservoirs in North America are hosted in dolomitic rock.16 Although dolomite can be precipitated directly from aqueous solution to form sedimentary deposits, it is more common for dolomite to precipitate as a cement in pore spaces or to replace previously sedimented calcite in a process known as dolomitization.17 Dolomitization requires large volumes of water. In almost all cases, advection transports magnesium ions into undolomitized rock and transports calcium ions out. Dolomitization usually begins as a selective replacement of the rock matrix and then progresses to larger grains and contained biochems and allochems.17 Dolomitization can be responsible for both increases and decreases in rock porosity. Porosity can increase because dolomite has a smaller molar volume than calcite, so pore space is created during replacement. Porosity can also increase when dolomitic solution dissolves unreplaced calcite such as calcified biochems and leaves empty molds or vugs. When there is an excess of dolomitizing solution, dolomite can precipitate into created pore spaces and reduce porosity.17 Dolomitization can preserve original limestone porosity because dolomite is less susceptible than calcite to compaction.9 Besides dolomitization, another diagenetic process affecting carbonates is karstification, which is the dissolution of carbonate rocks by water during periods of subaerial exposure. Karsting processes enlarge previously existing voids and fractures and can create caves and network channels.18 Karstification can affect porosity, permeability, and reservoir seal effectiveness.6 Our earlier study presented a background review of the geology for the Grosmont cores as well as photographic and the microscopic characterization data for the cores under study. The data on mineral composition, oil−water−solids contents,

within 1% of previously published specific heat capacities for dolomite. In anticipation of future exploitation of bitumens in Alberta’s carbonate triangle,12 and cognizant of minimizing the environmental impacts for the bitumen extraction from carbonate cores, CanmetENERGY supported the following fundamental study. Knowledge of the interfacial properties is necessary for future extraction processes when cores contact water and brines and for reducing environmental problems such as those of tailings ponds. The interfacial properties of these core minerals after exposure to saline environments and varied pH have not been reported to our knowledge. The mineral dissolution effects on water quality and the corresponding charge developed on the particle surfaces would indicate how extraction using steam or a solvent would affect the produced water if used for bitumen recovery. Our earlier studies showed that the amount of water in the cores was negligible, and solvent extraction provided ease of bitumen removal but left a thin coating of organic materials on the particles surfaces.13 The results reported in this study would also tell whether this organic coating left on the mineral affects the interfacial properties. Background Mineralogy. Bitumen in the Grosmont Formation is hosted in heterogeneous carbonate rock. Carbonate rocks are sedimentary rocks formed in marine environments. Those dominated by the mineral calcite (CaCO3) are known as limestones, while those dominated by the mineral dolomite (CaMg(CO3)2) are called dolomites (also referred to as dolostones in North America).14 The minerals calcite and dolomite have similar crystal structures, with hexagonal lattice structures described by the trigonal crystal system. Calcite is composed of alternating planes of calcium and carbonate (CO32−) ions perpendicular to the c-axis. In the case of dolomite, every other plane of calcium ions is replaced with a plane of magnesium ions (see Figure 1). Warren in his work represents dolomite lattice as an ideal structure of stoichiometric layers of carbonate separating alternating layers of calcium and magnesium ions as in Figure 1A.14 In Figure 1B he describes a nonideal structure that shows how the water molecules are preferentially bonded to cations on the surface of the dolomite crystallites. He explains that because Ca ions are not strongly hydrated as the Mg ions they B

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and Fourier transform infrared (FTIR) spectra of extracted and unextracted cores were reported.13 This study presents fundamental data on the core solids/liquid interface. Size distributions of ground core particles needed for studies of dissolution and equilibrium development are given. The dissolution kinetics and equilibrium development were measured using pH and specific conductivity. The ζ potential was measured as a function of various electrolyte concentrations and pH for each core solids sample. The aim of the present study is to gain an understanding of the interfacial behavior for core solids−water interactions. The properties and behaviors of the core solids are compared with those of model dolomite, which were reported in an earlier study on the effects of bitumen association.13,19,20

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Figure 2. Photomicrographs of core 25 dolograinstone particles (a) before bitumen extraction, (b) after bitumen extraction, (c) confocal micrograph of extracted particles under fluorescence light shows the residual organic coating on particles. The shiny spots indicate that the concentration of organic coating on particle surfaces were higher and not uniform. (d) FE-SEM image of the deposited solids from a suspension in deionized water. The elemental data for the marked particles in (d) and (e) are presented in Tables 3 and 4. (e) shows the finest particles analyzed.

EXPERIMENTAL SECTION

Materials and Methods. Seven segments of 4-in. diameter oilbearing dolomitic cores and their geological origins were obtained from Laricina Energy Limited21 in October of 2008. The core from which the segments were taken was drilled in the northern Alberta Carbonate Triangle. The segments were received in sealed plastic bags. In order to preserve sample integrity and prevent deterioration of the samples, they were removed, purged, and sealed in gaseous nitrogen, then stored at room temperature in the dark until analysis. Table 1

in the extraction and characterization appear in a previous paper.13 The neat solids before and after bitumen extraction were analyzed previously by photoacoustic (PA) Fourier transform infrared (FTIR) spectroscopy in order to confirm the differences in the samples. The details of these analyses are reported in another study.13 However, the typical FTIR spectra before and after bitumen extraction for core 25 are shown in Figure 3. The field emission

Table 1. Summary of Core As Received13 core identifier number

formation depth (m)

9

371.1−371.2

12

377.6−377.8

NA - dolomite with details not available laminite

15

383.6−383.8

laminite

20

391.3−391.6

breccia

25

402.4−402.6

dolomite grainstone

32

415.3−415.5

dololaminite

36

423.0−423.2

vuggy dolomite

lithology

rock unit Upper Ireton Upper Grosmont Middle Grosmont Lower Grosmont Lower Grosmont Upper Grosmont Upper Grosmont

D D D D C C

gives the details of the samples. A model dolomite powder (minus 400-mesh) obtained from IMASCO Minerals, Surrey, BC, was used for comparing some of the experimental results.19 All chemicals used were ACS-grade (NaCl, CaCl2, MgCl2, NaHCO3, Na2CO3, and buffers 4, 7, 10) obtained from Fisher Scientific. Deionized water of 18 MΩ cm resistivity was obtained from a Millipore reverse osmosis and MilliQ polishing process (Millipore Corp. Bedford, MA). Spectrogradequality toluene from Sigma-Aldrich was used in extractions of the bitumen. Table 2 gives the mineral composition of samples as reported earlier.13 Figure 2a−c shows micrographs of typical core particles under study. Detailed experimental process and techniques employed

Figure 3. Photoacoustic FTIR (PAS-FTIR) spectra of core #25 (C) before and (D) after bitumen extraction13.

Table 2. X-ray Diffraction Analysis (XRD) for Core Particles after Bitumen was Extracted13 rock unit and lithology (core number)/wt % Upper Ireton (9) Upper Grosmont D laminate (12) Middle Grosmont D laminate (15) Lower Grosmont D breccia (20) Lower Grosmont D dolograinstone (25) Upper Grosmont C laminate (32) Upper Grosmont C vuggy dolomite (36)

dolomite

calcite

± ± ± ± ± ± ±

0.9 ± 0.2

87.2 97.1 98.9 96.4 93.3 99.0 99.5

1.5 0.8 0.3 0.8 1.9 0.2 0.4

5.6 ± 1.8

C

quartz

kaolinate

muscovite

± ± ± ± ± ± ±

0.4 ± 0.4

7.8 ± 1.4

3.7 2.9 1.1 3.6 1.2 1.0 0.5

0.4 0.8 0.3 0.8 0.7 0.2 0.4

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scanning electron microscopy (FE-SEM) data and elemental analysis for a population of typical core 25 particles also appear in Tables 3 and

Table 3. Elemental Analysis of Particles in Figure 2d spectrum

C

O

Mg

Si

Cl

Ca

total

1 2 3 4 5 6 7 8 9 10 11 12 mean std deviation max min

40.5 22.0 24.7 18.8 18.1 19.7 18.3 21.9 17.9 18.9 18.7 13.8 21.1 6.7 40.5 13.8

45.7 46.2 50.1 54.1 54.1 58.8 56.1 60.0 57.8 56.7 37.0 47.0 52.0 6.9 60.0 37.0

7.7 12.1 10.2 11.4 11.6 10.3 11.4 8.8 11.0 10.0 11.2 13.7 10.8 1.6 13.7 7.7

0.0 0.0 0.2 0.0 0.2 0.0 0.0 0.0 0.0 0.0 0.0 0.4 0.1 0.1 0.4 0.0

0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.1 0.0 0.0 0.0 0.0 0.0 0.0 0.1 0.0

6.1 19.7 14.7 15.7 16.0 11.2 14.2 9.1 13.3 14.4 33.1 25.2 16.1 7.2 33.1 6.1

100 100 100 100 100 100 100 100 100 100 100 100 100

Figure 4. Size distribution of ground core particles after bitumen extraction.

°C and a reference temperature of 25 °C. Orion and Reagecon conductivity standards were used for calibrations prior to sample measurements. The ζ potentials of the fine-sized ground samples were measured using the BIC Zeta PALS ζ potential analyzer and an “SR-230” electrode (Brookhaven Instruments) and where necessary confirmed using the Malvern Zeta-Nano (Malvern Instruments, U.K.). Accuracy of measurement was confirmed by frequent checks using a reference calibration standard “BI-ZR3” for the ζ potential recommended by Brookhaven Instruments Corporation. Samples were prepared at a concentration of 0.1 g/L in a 0.001 M NaCl electrolyte background22,23 at a suitable opacity for a representative population of particles. All particle sizes were controlled by screening through a minus 400-mesh sieve, prior to measurements, because previous trials had larger error bars when particle sizes were not controlled. The pH of each sample was measured before ζ potential measurement. Common lattice ion (potential-determining ions) effects were studied by varying the concentrations of MgCl2, CaCl2, and Na2CO3 electrolytes from 10−5 M to 10−2 M. The ζ potential was calculated from the Smoluchowski model.22,23 In order to verify the effects of the organic film on the isoelectric pH for each set of core particles, three core samples, 9,12, and 25 (UIR, UGD laminite, and LGD dolograinstone), respectively, were heated at 400 °C for 2 h in air using thermogravimetry, in order to remove organic coatings. The heat-treated solids were prepared as the original solids which were suspended in 0.01 M Na2CO3 and 0.001 M NaCl (buffer) at a concentration of 0.1 g/L and left overnight for equilibration before the ζ potential measurements as a function of pH were conducted. The starting pH of the original buffer and the suspension was about 10. The pH range reported was from 3 to 10. The pH of the suspensions was adjusted using 1.0 M HCl to the desired value. Measurements were conducted immediately after pH adjustments. Zeta potential and conductivity of particles were determined using the Zetasizer Nano ZEN 3600 (Malven Instruments Ltd., Malvern,Worcestershire, U.K.). All measurements were carried out at 25 °C.

4. Variations in elemental composition from particle to particle were observed, but overall average composition shows that the magnesium to calcium ratios was not entirely stoichiometric as in ideal dolomites depicted in Figure 1. The finest particles appear to be more stoichiometric. The particle size distributions for ground solids of each core were measured using a Mastersizer 2000 and a Hydro-2000MU sampler, with a 1 wt% sodium hexametaphosphate carrier fluid. The bitumensaturated samples were first prewetted with alcohol, predispersed in carrier fluid, and sonicated for 60 s before the size measurements. These size data are reported earlier.20 The particle size distributions of bitumen-free ground cores are shown in Figure 4. However particles for which ζ potential measurements were conducted passed through a −400 mesh Tyler screen to ensure common size values among samples. In the present study we focus on dissolution and the ζ potential of core solids in electrolytes, after extraction of the bitumen. The bitumen-free (extracted) solids were stored under nitrogen before ζ potential measurements and a dissolution study. The dissolution kinetics and solubility of the solids were monitored using specific conductivity (SC) and changes in pH as a function of time. About 1 wt % solids were suspended in either deionized water or 0.001 M NaCl electrolytes placed in a rectangular glass cuvette with a square base of internal width 4.5 cm. The liquid height was also 4.5 cm. The sample was mixed at 300 rpm using a 3.4-cm diameter marine-type impeller at an off-bottom clearance of 1.5 cm. The pH was monitored using pH/ATC electrode no. 300792.1 (Denver Instruments, Bohemia, NY) attached to a Fisher Accumet Research AR50 dual-channel pH meter. The specific conductivity (SC) was measured with an Orion 5 Star portable conductivity meter and an Orion 013010MD conductivity cell with a cell constant 0.475. Linear temperature compensation was utilized using a coefficient of 2.1% per

Table 4. Elemental Analysis (in wt %) of Circled Particles in Figure 2e spectrum

C

O

Na

Mg

Al

Si

S

K

Ca

total

1 2 3 mean std deviation max min

51.4 16.6 14.9 27.6 20.6 51.4 14.9

39.1 53.7 43.4 45.4 7.5 53.7 39.1

2.6 0.0 1.4 1.3 1.3 2.6 0.0

0.7 11.7 13.3 8.5 6.8 13.3 0.7

0.0 0.1 0.0 0.1 0.1 0.1 0.0

0.4 0.0 0.3 0.2 0.2 0.4 0.0

2.3 0.0 0.4 0.9 1.2 2.3 0.0

0.2 0.0 0.0 0.1 0.1 0.2 0.0

3.3 17.9 26.4 15.9 11.7 26.4 3.3

100 100 100 100

D

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Table 5. Size Distribution Data for Core Particles Used in Dissolution Experiments particle size

core 9 UIR

core 12 UGD laminite

core 15 MGD laminite

core 20 LGD breccia

core 25 LGD dolograinstone

core 32 UG2C laminite

core 36 UGC vuggy dolomite

d32 μm d10 μm d50 μm d 90 μm

3.80 1.56 8.02 24.78

4.74 1.45 34.94 134.64

4.64 1.67 15.50 60.27

6.59 2.78 23.05 164.91

7.43 6.04 15.80 35.21

9.63 4.76 43.12 118.53

9.76 6.29 28.18 75.58

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RESULTS AND DISCUSSION Size Distribution of Ground Bitumen Free Core Particles. Figure 4 shows the size distribution analyses for preground bitumen-extracted core particles. Table 5 shows a summary of the size data. The size distributions of these ground core particles vary widely. Since a given mass of smaller particles will have a greater collective surface area than the same mass of larger particles, variations in particles size would be expected to influence results of experiments in which surface area plays a role, i.e., dissolution and ζ potential tests. As those particles from which bitumen was not extracted clumped together, the size data were not useful. The size distributions were broad for most extracted core solids. However the finer Upper Ireton particles would suggest a softer more friable core, but there was no correlation between size distribution and core friability. Lower Grosmont D dolograinstone was more uniformly distributed and sizes shifted to the larger particles as the Grosmont C dololaminite particles were measured. These results could indicate that finer grinding is required for analysis of ζ potential and dissolution kinetics. However, our results indicated that these size differences had minimal effects on dissolution rates. Dissolution Kinetics of Solids. Dolomite is a sparingly soluble salt-type mineral and, consequently, its lattice ions begin to react with water through dissociation and hydration upon contact.24 Since the surface charge of the mineral is affected by these reactions, the ζ potential of carbonates immersed in aqueous solutions changes rapidly with time until equilibrium is reached.25 To obtain meaningful ζ potential data, it was important to determine the time required for equilibration of the core solids with water. The dissolution rate of carbonate minerals can be expressed by the evolution of solution pH as a function of stirring time because the pH of the solution changes as lattice ions dissolve. In an open system, atmospheric CO2 will also affect the pH of a carbonate mineral solution.24 SC is a measure of a liquid’s ability to conduct electricity. Since conductivity increases with increasingly dissolved ion content, SC is a reflection of the amount of total dissolved ions in solution and, when measured against time, can provide information on dissolution rate. The mineral dolomite dissolves according to the following equation:25 CaMg(CO3)2 = Ca 2 + + Mg 2 + + 2(CO32 −)

Figure 5. pH vs mixing time of 1 wt % core particles and model dolomite in 0.001 M NaCl.

dissolution kinetics for samples ranging from the shallow Upper Ireton to the middle Upper Grosmont D (UGD) and Upper Grosmont C (UGC) laminites in comparison to UGC vuggy dolomite drilled from the deepest region, as followed by pH changes with time. Figure 6 shows the dissolution kinetics for

(1)

Figure 6. Dissolution kinetics of 1 wt % core particles and model dolomite in 0.001 M NaCl measured by specific conductivity.

The equilibrium constant is then ⎛ [Ca 2 +][Mg 2 +][CO 2 −]2 ⎞ 3 ⎟ Keq = ⎜ [CaMg(CO3)2 ] ⎝ ⎠

the same samples using SC of the water phase as the indicator. The control was a background electrolyte of 0.001 M NaCl in Figure 6. As all samples were only 1 wt % solids, the differences between them would be the result of the sample sizes and solubilities. Figure 6 shows that the model material, clean −400-mesh dolomite solids suspended in 0.001 M NaCl followed the same curves as the UGD laminite. Although initially all samples spiked to pH 10 or so, with time there was a slow decline in pH, leveling out between 8.4 and 8.8 and

(2)

where brackets [ ] denote the activities of indicated compounds. Warren14 reported Keq values in the range 10−15.01 to 10−17, and Sherman reviewed the dissolution rates and concluded in his experiments that first CaCO3 dissolves followed by MgCO3 in a two-step process26 with a Keq of 10−17. Figure 5 shows the E

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Table 6. Soluble Ca2+ and Mg2+ Ions of Supernatants from Samples in Figure 5 ions, ppm sample

mixing solution

mixing time (min)

control control Upper Ireton UGD laminite UGC laminite UGC vuggy dolomite UGC vuggy dolomite

DI water 0.001 M NaCl 0.001 M NaCl 0.001 M NaCl 0.001 M NaCl DI water 0.001 M NaCl

0 0 600 600 600 540 540

Ca

K

Mg

Na

Cl

0 1 22 21 12 15 16

0 0 25 33 28 29 20

0 0 2 11 5 7 7

0 23 28 27 25 0 23

0 36 58 65 59 26 51

(3)

CO2 (g) ⇌ CO2 (aq)

(4)

CO2 (aq) + H 2O ⇌ H 2CO3

(5)

CO32 − + H 2O ⇌ HCO3− + OH−

(6)

HCO3− + H3O+ ⇌ H 2CO3 + H 2O

(7)

H 2CO3 + OH− ⇌ HCO3− + H 2O

(8)

The equilibrium reactions governing the interaction between Ca2+CO32−and water are Ca 2 + + HCO3− ⇌ CaHCO3+

(9)

CaHCO3+ ⇌ H+ + CaCO3

(10)

Ca 2 + + OH− ⇌ CaOH+

(11)

CaOH+ + OH− ⇌ Ca(OH)2 (aq)

(12)

Ca(OH)2 (aq) ⇌ Ca(OH)2 (s)

(13)

0 0 4 4 1 5 5

5 5 69 103 63 72 75

0 0 0 0 0 0 0

0 0 0 0 0 0 0

pH

ion balance

mole ratio Ca/ Mg

6.1 6.8 8.0 8.0 7.8 8.0 8.0

0.23 0.95 1.11 1.09 1.03 1.03 1.02

10.24 1.77 2.15 1.95 2.09

ions lead to preferential dissolution of Ca2+ and Mg2+ cations from the dolomite crystals. The cations then bond with OH− ions, and this lowers the pH of the solution until equilibrium is obtained. The facts that magnesium ions have a stronger hydration-bonding ability than calcium ions and that carbonates must overcome the hydration bonding to form the carbonates indicate that once dissolved, the cations remain hydrated in the pores.25 Positively charged surfaces will be a result of the bicarbonate of MgHCO3+ or CaHCO3+ formed. Specific conductivity increases as ions dissolve into solution. Figure 6 shows solids from different cores having different dissolution rates compared to model dolomite that dissolved the most. Figure 6 also shows that of all the selected cores, the UGD Laminite (core 12) 13 has the highest specific conductivity, thus indicating that it dissolved the most, followed by Upper Ireton (core 9) and UGC vuggy dololaminite (core 36), with the latter being the least soluble. Eventually all samples show a decline in the rate of dissolution as indicated by a plateau in specific conductivity vs time. Thomas et al. 1993 found that carbonate dissolution was inhibited by organic compounds that absorbed strongly (such as fatty acids and carboxylated polymers) because the compounds served to isolate the minerals from the solution.39 A thin coating of organic material on the surfaces was observed by fluorescence microscopy (Figure 2c), but our data shows that it does not significantly affect the dissolution rate. However, our earlier model studies on adsorbed asphaltenes on fine dolomite showed significant differences compared to that with adsorbed and desorbed bitumen.19 Variations in exposed surface area due to size differences may also have affected dissolution rates. In all cases, dissolution proceeded quickly for the first hour and then began to level off. It can be seen that the concentration of ions in solution increased continually throughout the experiment. An analysis of the ion composition of the equilibrated electrolyte is given in Table 6. Although, theoretically, dolomite should have a calcium-to-magnesium mole ratio of 1:1, the data indicate that there are differences in the dissolved ions from sample to sample. The Upper Ireton sample contains the most calcium at a mole ratio of 10.2, and the UGC vuggy dolomite at 2.0 is closer to the ideal dolomite composition. All others were closer to a mole ratio of 2, indicating that Ca2+ ion solubility exceeded that of Mg2+. The values were also confirmed by our earlier XRD data.20 The dissolution of the cores would suggest that the quality of water left after extraction will change if the cores are steam injected to extract the bitumen or are treated with mild brine. The composition of the water-continuous phase would influence the particles net charge and hence ζ potential. In many cases calcium may be preferentially dissolving. This may impact on the porosities or cores over long periods of time.

averaging 8.5 after 6 h. The pH values are typical for carbonate/ bicarbonate buffers. Results were also similar to the equilibrium pH of dolomite obtained by Predali and Cases who reported final dolomite equilibrium pH values between 8.18 and 8.36 and that found by Chen and Tao, who reported an equilibrium value of pH 9.2.24,25 The initial rapid increase in pH during the first 15 min of mixing can be attributed to the release of CO32− ions that were protonated to form HCO3− and H2CO3, removing hydronium ions from the solution. As pH increases, atmospheric CO2 gas becomes more soluble. Dissolution of CO2 into solution results in the generation of carbonate ions through reactions that consume OH− ions25 are illustrated in eqs 3−8. 2H 2O ⇌ H3O+ + OH−

SO4 HCO3 CO3 OH

Similarly, the reactions for Mg2+CO32− can be described in eqs 9−13 by replacing Ca2+ with Mg2+. However, the solubility of dolomite is lower than that for calcite and its reactivity with acids is also lower than those for calcite or magnesite ores.25,27−29 Published data on the ζ potential of calcite are highly variable. The sign of the ζ potential has been found to be positive,30−33 negative,34,35 and variable.36,37 Also the potential determining ions for calcite are not agreed upon, being a selection of H+, OH−, Ca2+, CO32−, and HCO3−. Dolomite mineral dissolution rates and ζ potential are also a function of its purity.26,38 Equation 7 indicates a decrease in pH, which is quickly compensated for by interaction of dissolved carbonate with the water and cations, thereby increasing pH. The excess of CO32− F

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ζ Potential. ζ Potential As a Function of PotentialDetermining Lattice Ion Concentration. The ζ potential of the fine core particles following bitumen extraction was measured as a function of pH in a fixed indifferent background electrolyte 0.001 M NaCl and as a function of common lattice ion concentrations (CaCl2, MgCl2, Na2CO3), where Ca2+ and Mg2+ are lattice cations and CO32− is the common lattice anion. The results are compared to that for model dolomite from an earlier study.19 Figures 7, 8, and 9 show graphs of the ζ

Figure 9. Effects of CaCl2 concentrations on ζ potentials of model dolomite and core particles in 0.001 M NaCl adjusted to pH 9.0. Lines indicate the instability region.

compression of the double layer.22,23,25 Since the ζ potential results for the core solids are more negative than those of model dolomite at low electrolyte concentrations, the difference is likely due to residual adsorbed bitumen on the core solids, as opposed to a difference in exposed particle surface area.13,20 The Upper Ireton from the top of the formation and the UGC vuggy dolomite at the deepest point in the formation showed similar but not identical responses as model dolomite. However, there are distinct differences in responses for Upper Ireton samples as compared to LGD dolograinstone and UGC vuggy dolomite. This is consistent with the mineral composition differences illustrated in Table 2. In addition, these cores were associated with different quantities of bitumen as shown from our earlier work.13,20 Figures 8 and 9 show the model dolomite particles begin at a low negative ζ potential of −30 to −25 mV and becomes more positive with Mg 2+ and Ca2+ additions, crossing the zero ζ potential before 0.002 and 0.001 M, respectively. The core particles, similar to the model system, at increased concentration of Mg2+ and Ca2+ ions (Figure 8 for MgCl2 and Figure 9 for CaCl2) caused the ζ potential of core solids to become less negative, contrasting the effects of addition of CO32− ions (in the form of Na2CO3 in Figure 7), that caused the ζ potentials to become more negative. The curves show ζ potentials decrease as concentrations of MgCl2 and CaCl2 increase but did not cross zero charge. The Upper Ireton, LGD dolograinstone and UGC vuggy dolomite came closer to zero ζ potential at 0.01 M Ca2+. All others were around −10 mV. This could be because the adsorbed organic material rendered the particles to be more negatively charged and charge reversal was not possible. The majority of extracted core solids behaved similarly, even though their dolomitic composition differed slightly. In many cases the responses to increased concentrations of Ca2+ and Mg2+ were nearly identical. However at high concentrations, Ca2+ was more effective than Mg2+ for charge neutralization. ζ Potential As a Function of pH. Figure 10 shows the ζ potential of model dolomite as a function of pH with isoelectric points (ieps) between 5 and 7.2 and becoming more positively charged with increasing acid additions as measured by two different instruments. Literature values28 for the iep of dolomite

Figure 7. Effects of Na2CO3 concentrations on ζ potentials of model dolomite and core particles in 0.001 M NaCl adjusted to pH 9.0.

Figure 8. Effects of MgCl2 concentrations on ζ potentials of model dolomite and core particles in 0.001 M NaCl adjusted to pH 9.0. Lines indicate the instability region.

potential vs concentrations of the common lattice ions added to the solutions and equilibrated for 24 h. All systems are made up at 0.1 g/L in 0.001 M NaCl, adjusted to a starting pH of pH 9.0. ζ potentials are plotted as functions of the concentrations of the salts for common divalent lattice ions CO32−, Ca2+, and Mg2+, respectively. Figure 7 shows that the carbonate anion caused an increase in negative ζ potential as expected for potential determining anions. The model dolomite exhibited less negative ζ potentials than all core particles after interaction with CO32− ions. A sharp decrease in the ζ potential that occurs at 0.01 M Na2CO3 for all core solids was likely due to G

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were contacted with asphaltenes and subsequently washed with toluene, the particles had more negative ζ potentials than clean model dolomite at a given pH.19 However, after adsorbing followed by desorbing bitumen the system was completely reversible and negligible residual bitumen that was left did not affect the surface charge.19 Predali and Cases observed that the equilibrium pH of dolomite dissolution solutions was close to the iep of dolomite.25 If bitumen adsorption was responsible for lowering the iep of the core solids, it would be reasonable to assume that the equilibrium pH would also be affected. However, the equilibrium dissolution pH was similar to reported values. Our measured iep for our model systems did not exactly coincide with their iep values. Figure 12 shows the ζ potential vs pH for three different core samples, but in this case the particles were suspended in 0.01 M Figure 10. ζ potential vs pH for model dolomite in 0.001 M NaCl by using two instruments.

vary but should fall within the range of pH 6.0−8.8. It is expected that slow dissolution is occurring at the low pH as well, thus changing the particle surfaces. Figure 11 shows the ζ

Figure 12. ζ potential vs pH for Upper Ireton, UGD Laminite, and LGD dolograinstone in buffer for better control of pH.

Na2CO3 plus 0.001 M NaCl to obtain better pH control. The fact that the iep occurs at a lower pH than expected indicates that the specific adsorption of soluble anions40 plays a role. Isoelectric points ranged between pH 3 and 5. This suggested that eqs 9 and 10 may be at play forming CaHCO3+ on the core lattice. Figure 13 shows the ζ vs pH comparative data for isotherms of model dolomite and the model dolomite with asphaltenes

Figure 11. ζ potential vs pH for Upper Ireton and LGD dolograinstone in 0.001 M NaCl.

potential vs pH data for the Upper Ireton and Lower Grosmont D dolomite grainstone measured under similar conditions as model dolomite in a background electrolyte 0.001 M NaCl. The ζ potential values were more negative than those of the model system at all pH values. There appeared to be virtually no differences in behaviors between the two core solids. The isoelectric pH was not reached for these samples. Small significant differences between the two are observed between pH 7.5 and 8. This behavior may be a result of the bitumen coating on the particle surfaces. Even above pH 10, where there is a possibility of precipitation of dissolved cations to form Mg(OH)2 or Ca(OH)2, the particles are still negatively charged. However, no evidence of hydroxide precipitated particles was observed in the controls when monitored by light scattering using the Malvern Zeta Nano instrument. Residual adsorbed bitumen may be responsible for the lack of an isoelectric point. Our earlier studies on asphaltenes adsorption on model dolomite showed that when model dolomite samples

Figure 13. Comparing ζ vs pH isotherms for model dolomite with and without asphaltenes adsorbed for samples suspended in buffer.19. H

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adsorbed when suspended in the buffer. This clearly shows a shift in the isoelectric point from a pH of ∼5 for the model control to none for the latter. The ζ vs pH was much more negative after asphaltenes adsorption. This observation supports the original hypothesis that adsorbed materials would tend to make the dolomite surfaces more negatively charged. We next compare ζ vs pH isotherms for the heat treated core samples relative to the untreated samples suspended in the same buffer. Figure 14 shows the ζ vs pH isotherm curves for

Figure 16. ζ vs pH of LGD (core 25) particles in buffer before and after heat treatment.

mV at pH 5.5. Near pH 3.0 the heated solids in this case had lower −ζ than unheated. Heating caused some removal of functional groups which contributed to the highly negative charges in basic pH. The results were shifts in behavior in acidic pH as well. The isoelectric pH shifted to lower pH or none at all. UIR core samples showed a distinct shift in iep before treatment from pH 5. The presence of muscovite or quartz did not affect this sample’s responses significantly.41−43 This core 9 sample was taken from the shallowest depth. The core sample UGD (no. 12) showed an iep of pH 4.5 untreated, but after heat treatment no visible iep was noted. The isotherm for LGD (no. 25) which was the deepest of the lot and had the most bitumen before extraction, showed a more uniform response with pH decrease as well. Untreated, the sample was more negatively charged and had no measurable iep. After heat treatment the iep trends toward pH 2.5. The behavior was different from the other samples. In Figure 17a if we compare the ζ vs pH isotherms on the same graph for all heat treated cores with that for the model dolomite with adsorbed asphaltenes, we note that there is a significant shift to less negative ζ potential until pH 4.5 for all the core samples as compared to the model dolomite. Figure 17b show the corresponding SC of the suspensions as a function of pH measured and this indicates that the ionic strengths were identical for all the systems. One may surmise that not all of the organic materials were removed and that the surface properties changed drastically with heat. One can rule out oxides,44 as the temperature of 400 °C was insufficient to cause thermal decomposition of dolomite.45 Therefore, the behavior difference from model systems was indicative of materials coating the core particles. Thus the above data show that Mg2+, Ca2+, and CO32− as well as H+, OH−, and species of MHCO3+ are all potentialdetermining for the solid/liquid interfaces of the dolomitic cores, where M is either Ca or Mg. Minor differences in the mineral compositions of cores made only small differences in the interfacial responses. The ζ potential as a function of pH decrease was not measured for all core solids as the trends were similar in most. Also, because solids dissolution occurs as pH is lowered, we performed all measurements immediately after sample preparation. However, even though the calcite content of LGD dolograinstone solids was much greater than that of the Upper Ireton (Table 2), the responses to decreasing pH were nearly identical. Any trace clays present in Upper Ireton core solids had no significant effects on ζ potential as pH decreased.41

Figure 14. ζ vs pH of UIR (core 9) particles in buffer before and after heat treatment.

UIR (core 9, Upper Ireton) solids before and after heat treatment. Figure 15 shows those curves for UGD (core 12,

Figure 15. ζ vs pH of UGD (core 12) particles in buffer before and after heat treatment.

Upper Grosmont D) and Figure 16 for LGD dolograinstone (core 25, Lower Grosmont D). There is some variability in the responses for each system. The heating caused all systems to respond to the pH reduction with smaller error bars in measured ζ potential. Figure 14 shows that heat caused the UIR solids to shift the isoelectric point, from 5.5. Figure 15 shows that the UGD solids followed a similar trend where the iep for untreated was at 5.5 and particles fluctuated to positive charge until pH 3.0. The heat treated samples were more negatively charged at all pH, with no visible iep. The behaviors of the unheated LGD dolograinstone compared to its heat-treated shown in Figure 16 indicate higher negative ζ potential from pH 7 and above. This observation was similar to solids of UIR and UGD as well. However, the untreated LGD solids were negatively charged at most pH, reducing −ζ from 50 mV at pH 10 and pH 7.5 to10 I

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potential (iep) as compared to model dolomite when pH decreased in 0.001 M NaCl, although the ζ potentials decreased with pH. (5) All core solids in electrolytes at fixed pH responded with a decrease in negative ζ potential as the divalent cations Ca2+ and Mg2+ concentrations in solution increased. However charge reversal did not occur readily, in contrast to those for the model dolomite. The organic film on core particle surfaces may have affected the interactions. (6) Potential-determining ions for all dolomitic cores measured were Ca2+, Mg2+, CO32−, H+, and OH− ions. (7) Heat treatment of core particles cause a shift in ζ vs pH to less negative values and a shift in the iep to lower pH or none at all. This confirmed the role of the organic coating in contributing negatively charged functionalities, evidenced by lowest ζ of the model dolomite with adsorbed asphaltenes.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to acknowledge support from the Government of Canada’s interdepartmental Program of Energy Research and Development, PERD POL 1.1.1. We thank Laricina Energy for supplying the cores used for this work, Rachel Ghent, who worked diligently on this project during her 16 month professional engineering student internship, Vicente Muñ o z who kindly acquired the SEM and Confocal Microscopy data for us, Dr. Yujuan Hua for confirming some of the electrokinetic results, Dr. Kirk Michaelian for FTIR-PAS, and Dr. Wally Friesen for the TGA analysis of the core samples.

Figure 17. (a) Comparison of ζ vs pH in buffer for all core samples compared to the model dolomite with adsorbed asphaltenes. (b) Specific conductivity vs pH of samples measured in part a.

Thus from the results and evidence above it is safe to conclude that Mg2+, Ca2+, CO32−, H+, and OH− are all potential-determining ions for these cores. The residual organic content after bitumen extractions caused the particles to be more negatively charged than model dolomite and thus generally they did not exhibit isoelectric points as pH decreased in a 0.001 M NaCl. However the addition of Na2CO3 facilitated an isoelectric range for the core particles. Therefore, from the results of this work one can hypothesize that the presence of near-positively charged surface groups with dissolution of dolomites may affect its interactions with bitumen and emulsions that would be created in a SAGD-type process. However, testing this hypothesis requires an examination of the bitumen properties and the associated emulsions, and this research will be presented in another study to follow.

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CONCLUSIONS (1) The rates of dissolution and solubility for core solids measured by changes in specific conductivity and pH as a function of time followed the order UGD laminate > Upper Ireton > UGC vuggy dolomite = UGC laminate. The rates varied, but they were not correlated to the depth of core origin. Particle sizes had some influence on this data, but results for size distributions were close. (2) During equilibration in brine, all slurries showed a sharp pH increase at the start followed by a steady exponential decline, leveling off around pH 8.5. The order of equilibrium pH values was UGD laminite > model dolomite > UGC laminite > Upper Ireton. (3) The mole ratios for dissolved lattice Ca2+ and Mg2+ ranged from 10.2 for Upper Ireton to 1.77 for UGD laminite. Ratios for all other core solids approximated 2. (4) None of the core solids crossed the zero ζ

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NOMENCLATURE ERCB = Energy Resources Conservation Board FTIR = Fourier transform infrared PAS = photoacoustic spectroscopy XRD = X-ray diffraction analysis SAGD = steam assisted gravity drainage CSS = cyclic steam stimulation UIR = Upper Ireton UGD = Upper Grosmont D MGD = Middle Grosmont D LGD = Lower Grosmont D UGC = Upper Grosmont C σ = SC = specific conductivity, mS cm−1 ζ = zeta potential, mV REFERENCES

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