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Dissolution of minerals and precipitation of an aluminosilicate phase during experimentally-simulated hydraulic fracturing of a mudstone and a tight sandstone in the Powder River Basin, WY Ryan Herz-Thyhsen, John P. Kaszuba, and Janet Coker Dewey Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b04443 • Publication Date (Web): 22 Apr 2019 Downloaded from http://pubs.acs.org on April 24, 2019
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Dissolution of minerals and precipitation of an aluminosilicate phase during experimentallysimulated hydraulic fracturing of a mudstone and a tight sandstone in the Powder River Basin, WY Ryan J. Herz-Thyhsen*,a, John P. Kaszubaa,b, Janet C. Dewey a
a
Department of Geology and Geophysics 1000 E. University Avenue, University of Wyoming, Laramie, Wyoming 82071, USA bSchool
of Energy Resources, 1000 E. University Avenue, University of Wyoming, Laramie, Wyoming 82071, USA
Key Words: hydraulic fracturing, geochemical water-rock interactions, unconventional reservoirs, aluminum geochemistry, formation damage, hydrocarbon production, Powder River Basin, dissolution, precipitation
ABSTRACT
Hydrothermal experiments were conducted to evaluate mineral dissolution and precipitation that may occur during hydraulic fracturing of unconventional reservoirs. The B Bench of the Niobrara
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Formation and the Wall Creek Member of the Frontier Formation (Powder River Basin, WY) were reacted with a hydraulic fracturing fluid (pH = 2.5, ionic strength = 0.05) at 115°C and 35 MPa for one month. Data collected from both experiments indicate that calcite began to dissolve in less than 50 hours, Al-bearing solids began to dissolve immediately upon contact with hydraulic fracturing fluid, and an aluminosilicate precipitate began to form in less than 27 hours. Data from the Wall Creek experiment suggest potential dissolution of chert. We estimate that calcite dissolution increased porosity of the B Bench from 1.7 % to 2.8 % and of the Wall Creek from 3.5 % to 4.8 %. Dissolution of calcite increases porosity and storage space for injected fluids. However, removal of calcite may weaken the mechanical integrity of the rock, leading to proppant embedment and fracture closure. Precipitation of an Al-bearing phase may decrease rock and fracture permeability. Aqueous Al in our experiments behaves similarly to aqueous Al collected during flowback from hydraulically-fractured wells, suggesting that Al-bearing phases precipitate during timescales of hydraulic fracturing.
INTRODUCTION Hydraulic fracturing combined with horizontal drilling dramatically improves hydrocarbon extraction from previously untapped unconventional reservoirs.1 These combined technologies have driven recent growth in crude oil and natural gas production in the United States.2 However, hydrocarbon production experiences a sharp decline shortly after initial hydrocarbon recovery from hydraulically-fractured wells.3, 4 To resolve this problem, operators commonly increase well length and number of stages per well to maximize reservoir surface area.5 By maximizing reservoir surface area, injected hydraulic fracturing fluid (HFF) contacts a large area of mineral surfaces surrounding fractures. Thus, efforts to improve production increase the importance of interactions that may occur between minerals and injected HFF.
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Large volumes of HFF are injected into the subsurface during hydraulic fracturing operations. However, only 20-30 % of the HFF flows back to the surface.6 Capillary tension draws HFF remaining in the subsurface into pores of the rock7-9, where it contacts mineral surfaces. Recent investigations show that HFF chemically reacts with calcite, Fe-bearing minerals (e.g., pyrite, Feoxides), and feldspars.10-15 Dissolution and precipitation of these minerals may alter rock properties controlling fluid storage and transport (e.g., porosity and permeability). Therefore, evaluating mineral dissolution and precipitation reactions during interaction between minerals and HFF may provide insight into fluid storage and transport during and after stimulation by hydraulic fracturing. Dissolution of feldspar has been inferred from trends of aqueous Al and SiO2 during interaction between rocks and HFF despite no visual evidence of dissolution.10, 11 However, aqueous Al may derive from clay minerals, and SiO2 (aq) may derive from clay minerals or silica polymorphs (e.g., quartz, chert). Furthermore, previous experiments did not include proppant, although bauxite – an Al-bearing proppant – reacts with water at elevated temperatures and pressures.16-18 Therefore, the reactivity of Al- and Si-bearing minerals during interaction between HFF and rocks remains relatively unconstrained. Understanding the reactivity of Al- and Si-bearing minerals is important because the size and connectivity of pores associated with clay minerals control the permeability of water-saturated, clay-rich rocks (e.g., mudstones, shales).19-22 The goal of this study is to evaluate the geochemical behavior – including the behavior of Aland Si-bearing minerals – of two different rocks during simulated hydraulic fracturing. We conducted two hydrothermal experiments that react HFF with the B Bench of the Niobrara Formation (a calcareous mudstone) and the Wall Creek Member of the Frontier Formation (a tight sandstone) from the Powder River Basin of Wyoming (Figure 1). The two formations will be
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referred to as the B Bench and the Wall Creek for the remainder of this document. These formations represent compositionally-different, basin-centered unconventional reservoirs that are currently being developed in Wyoming. Reacting these rocks with selected fracturing chemicals showed that calcite, Al-bearing, and Si-bearing minerals react within the first 27 hours of initiating the experiments and that secondary aluminosilicate precipitation occurs. Porosity change
Figure 1. The map depicts basins that are currently experiencing active exploration and production of hydrocarbons in part of the Rocky Mountain province of the United States. Within these basins, the B Bench of the Niobrara Formation (a calcareous mudstone) and the Wall Creek Member of the Frontier Formation (a tight sandstone) harbor unconventional reservoirs of hydrocarbons. Core samples from each formation were collected from a well (indicated by a black circle) in the Powder River Basin, WY. WY = Wyoming. CO = Colorado. associated with calcite dissolution is comparable to observations made during other experimental studies.10, 12 Porosity change and precipitation of secondary minerals may have implications for reservoir permeability if secondary precipitates block flow pathways in hydraulically-fractured unconventional reservoirs. MATERIALS AND METHODS
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Experimental apparatus and procedure. We conducted the experiments in rocking furnacepressure vessel assemblies – also known as rocker bombs – using methods established for hydrothermal experiments.23 A rocker bomb houses a flexible gold reaction cell ported with a capillary tube and metered sampling valve. This configuration allows for periodic withdrawal of fluid samples from the reaction cell with minimal perturbation of in situ conditions of the ongoing experiment. The apparatus controls experimental temperature and pressure externally to maintain reservoir conditions of the experiment. The experimental apparatus and procedure are described in detail in the Supporting Information. Each experiment reacted HFF developed in our laboratory with representative samples of the B Bench and the Wall Creek (HFF and rock samples are described in the Materials section, below). HFF and rocks were combined in the reaction cell, which was subsequently sealed, pressurized, and heated. The HFF was not flushed with inert gas to remove dissolved O2 because HFF injected in the field contains dissolved O2 in equilibrium with the atmosphere. Both experiments were conducted at a reservoir temperature of 115°C (as determined by bottom-hole temperature logs) and pressure of 35 MPa (as determined by shut-in pressure of the Hornbuckle 1-11H well, Figure 1). Experiments were conducted for one month, which was the typical duration between the onset of hydraulic fracturing and the time wells were opened for fluid production. Experimental parameters are provided in Table 1. Aqueous samples collected from the experiments were analyzed (analyses are described in the Analytical Methods section, below) to quantify the evolution of HFF chemistry through time (Table 2). To assess potential reactions induced by combining HFF and rock in the reaction cell at ambient temperature and pressure, we collected an aqueous sample (sample 0.5 in Table 2) from the Wall Creek experiment after the reaction cell was sealed into the pressure vessel and before
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the apparatus reached experimental temperature and pressure. After reaching in situ conditions, aqueous samples were collected after 1, 2, 5, 14, and 28 days during both experiments. A sixth fluid sample was collected after each experiment was terminated to assess retrograde reactions between HFF and minerals that may have occurred during cooling and depressurizing. Additional sampling details are in the Supporting Information. Materials. The experiments used core samples of the B Bench and the Wall Creek collected from the Hornbuckle 1-11H well (API# 490092820) in the Powder River Basin of Wyoming (Figure 1). Seven sub-samples of the B Bench core (depth range of 12075 – 12165 ft) and six subsamples of the Wall Creek core (depth range of ~12450 – 12560 ft) were ground to ~40 µm in diameter using a SPEX tungsten-carbide shatterbox, and two rock chips measuring ~3 mm3 were collected from each sub-sample. Powders from each formation were equally homogenized to create the rock used in the experiments. Samples were homogenized because HFF is expected to interact with different facies of both formations in vertically-extending, stimulated fractures. Rocks used in the experiments consisted of 75 % powders and 25 % chips. We acknowledge that using rock powders alters the kinetics and reactivity of rocks. However, our results provide value by constraining the range of mineral dissolution and precipitation reactions that can occur during hydraulic fracturing. Using rock chips and powders is an established approach for evaluating mineral dissolution and precipitation during experiments (e.g., Marcon et al.11; Harrison et al.12; Jew et al.13). Bauxite powders and chips were incorporated into experiments because bauxite is frequently used as a proppant during hydraulic fracturing operations. Approximately 9 g of rock and bauxite, and 200 g of synthetic HFF were used in each experiment (Table 1). Rock used in both experiments consisted of ~98 % formation samples and ~2 % bauxite.
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B Bench samples used in the experiments consist primarily of illite with intergranular quartz and calcite; accessory minerals include chlorite, pyrite, albite, and kaolinite.24 Wall Creek samples consist primarily of quartz, chert, calcite, and illite; accessory minerals include chlorite, pyrite, albite, and kaolinite.24 These mineral assemblages are consistent with previous work25-31 (using optical microscopy, scanning electron microscopy, electron microprobe, and X-ray diffraction) that assessed both the Niobrara and Frontier Formations. While amorphous silica or aluminum hydroxide may cement soils and some sedimentary rocks32, we found no evidence that either of these phases exists in rocks used in our experiments. HFF used in both experiments (Table 2) was developed using 1) a chemical analysis of the Cheyenne River (Table 2, “mix water”) tabulated in the United States Geological Survey (USGS) water database33 and 2) a list of chemical additives as compiled in completion reports of the Hornbuckle 1-11H well.34 Chemistry of the Cheyenne River was determined by the USGS for a sample collected in April 2011 (1 month before the well was hydraulically fractured). We used the water chemistry of the Cheyenne River to make our “mixing water” because Cheyenne River water was used at the well site for this purpose. The mixing water was synthesized in our laboratory using reagent-grade salts, Si standard solution, Fe standard solution, and de-ionized water. To synthesize the HFF, we added chemicals into the mixing water that met two or more of the following criteria: 1) the chemical is intended to impact mineral dissolution and precipitation (e.g., scaling, precipitation of a Fe-bearing phase); 2) the chemical comprised greater than 0.1 wt. % of the fluid; and 3) the chemical was the most abundant of multiple chemicals that served a similar purpose. Chemical additives used during hydraulic fracturing of the Hornbuckle 1-11H well – and the reasons why they are used in the field – are listed in Table S1. Methanol, sodium erythorbate, hydrochloric acid, petroleum distillates, and tetramethylammonium chloride met our selection
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criteria; exceptions to this approach were made for Na-erythorbate and biocides. Na-erythorbate was used because it was the only additive that prevented the precipitation of metals. Biocides were excluded for laboratory safety. Analytical Methods. Aqueous samples were analyzed for major/minor cations and anions, pH, and total dissolved inorganic carbon. Cations were analyzed using inductively coupled plasma optical emission spectrometry, and anions were analyzed using ion chromatography. Dissolved inorganic carbon was immediately measured using coulometric titration and ex-situ pH was measured using an Orion pH meter. Detailed descriptions of analytical methods and sampling protocol are in the Supporting Information. We examined unreacted and reacted rock samples to identify minerals, determine rock chemistry, and evaluate petrographic textures indicative of mineral dissolution and precipitation. All samples were analyzed using X-ray diffraction (XRD), scanning-electron microscopy (SEM), and an energy dispersive analysis system (EDS). In addition, all core sub-samples were analyzed by XRD and X-ray fluorescence (XRF); those data are available in Figure S1 and Table S2, respectively. Geochemical Calculations. Geochemical calculations were performed to interpret experimental results by determining in situ pH, aqueous species concentrations/activities, and saturation states for unreacted HFF as well as individual aqueous samples collected from each experiment. All calculations were performed using Geochemist’s Workbench 10.0 using the b-dot ion association model and the thermo.tdat database.35 Thermo.tdat was chosen as the thermodynamic database because it is internally consistent and is more adept at handling aluminum speciation than other available databases.36 Detailed descriptions of the geochemical calculations are presented in the Supporting Information.
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RESULTS AND DISCUSSION Observations described in the following sections suggest that experimentally-induced mineral dissolution and precipitation reactions are similar for both formations. Due to these similarities, the following sections are organized as mineral dissolution (subsections labeled pH and dissolution of calcite, and Dissolution of Al- and Si-bearing minerals), mineral precipitation (subsection labeled Precipitation of an Al-bearing phase), the evolution of mineral stability (subsection labeled The Evolution of Al- and Si-Geochemistry), and porosity alteration (subsection labeled Calculating Porosity Change using Mass Balance). pH and dissolution of calcite. In both experiments, the pH increased from an initial value of ~2.5 (the pH of the HFF) to near neutral in less than 27 hours where it remained for the duration of the experiments (Figure 2). Increasing pH correlated with increasing concentration of aqueous Ca, although aqueous Ca continued to increase after the experiments reached near-neutral conditions. The HFF was undersaturated with respect to calcite, but all samples collected from the experiments were saturated or supersaturated with respect to calcite. These aqueous data suggest that the majority of Ca released to the fluid derives from dissolution of calcite – the only major Ca-bearing mineral in the rock. Continued increase of aqueous Ca concentration suggests continued dissolution of calcite. The interaction between HFF and rocks in other experimental studies12 has produced acid-generating reactions, such as pyrite dissolution, that dissolve calcite. However, our experiments provided no aqueous or mineral data documenting dissolution of pyrite. As expected, we observed dissolution pits on calcite of reacted rocks (Figure 3A). The same amount of Ca was removed from the rock in both experiments, suggesting the same amount of calcite dissolved in both experiments. Dissolution of calcite is consistent with experimental studies of the interaction between HFF and other carbonate-bearing rocks.10-12, 37 Dissolution of calcite is
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not a surprising result due to the incorporation of HCl into the HFF. However, HCl is a common additive in HFF, and our aim was to simulate the chemistry of HFF. Calcite dissolution is significant because removal of calcite cement may expose other mineral surfaces to HFF.
Figure 2. Plots show the development of aqueous chemistry from experiments over time. Time = 0 represents the chemistry of the hydraulic fracturing fluid before contact with the rock. Five in situ samples were collected from hydrothermal experiments over the course of one month. An additional sample (sample 0.5 in Table 2) was collected early during the Wall Creek experiment; this sample is illustrated by a star symbol (excluding pH). Error bars are smaller than the size of the symbols; uncertainty associated with measurements is recorded in Table 2. Dissolution of Al- and Si-bearing Minerals. Aqueous SiO2 concentrations increased by approximately 10X within the first 27 hours of the B Bench experiment and within the first 50 hours of the Wall Creek experiment (Figure 2). In the B Bench experiment, aqueous Al
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concentration increased from 4 µmol/kg in the HFF to 9 µmol/kg in less than 27 hours (Figure 2, Table 2). Aqueous Al concentration of the Wall Creek experiment behaved similarly, but sample 0.5 (collected at ~6 hours) contains 81 µmol/kg of aqueous Al – over 35X greater than the aqueous Al concentration of the original HFF. We believe that aqueous Al concentrations in the B Bench experiment experienced a similar increase, but our sampling protocol did not capture this behavior. Increasing concentrations of aqueous Al and SiO2 correlate with dissolution of bauxite and albite, although dissolution of other Al- and SiO2-bearing minerals may have occurred during the
Figure 3. SEM images of reacted rock chips from the Wall Creek and B Bench experiments show A) dissolution pits on a calcite grain, B) dissolution textures on the face and side edge of an albite grain, and C) aluminosilicate mineral precipitates on a calcite substrate. Labeled white circles correspond with EDS spectra 1, 2, and 3. All three EDS spectra display Ca and Mg peaks that may derive from the calcite substrate. EDS spectra collected from mineral precipitates (spectrum 1 and 2) display Al and Si peaks. The EDS spectrum collected from the calcite substrate (spectrum 3) displays Ca and Mg peaks, a small Si peak, and no Al peak.
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experiments. The HFF is undersaturated with respect to Al-bearing and aluminosilicate minerals, but all fluid samples collected from the experiments are supersaturated with respect to gibbsite, albite, chlorite, illite, kaolinite, and Mg-bearing smectite (Table S3). Gibbsite (Al(OH)3) is a common component of bauxite proppant. In XRD diffractograms, the intensity of the (010)gibbsite peak is subdued in both reacted rocks relative to the unreacted rocks (Figure 4), suggesting that bauxite dissolved during the experiments. However, bauxite dissolution cannot account for increasing SiO2 (aq) concentration in both experiments, suggesting that dissolution of SiO2bearing minerals (e.g., aluminosilicates, quartz polymorphs) may also have occurred. No amorphous silica cement was observed in unreacted rocks, thus indicating that increasing concentration of SiO2 (aq) cannot be attributed to dissolution of amorphous silica. Indeed, we observed dissolution of albite in the B Bench experiment (Figure 3B). Dissolution of bauxite and albite is attributed to acidity of the HFF because aqueous Al is more soluble at acidic conditions than near-neutral conditions.38 After an initial increase, SiO2 (aq) concentrations continued to rise during the Wall Creek experiment. In contrast, SiO2 (aq) concentration of the B Bench experiment remained constant. Mineralogy of both rocks is similar with the exception that the Wall Creek contains chert while the B Bench does not, suggesting that increasing SiO2 (aq) during the Wall Creek experiment may result from chert dissolution. Despite these aqueous trends, we did not observe alteration of chert in reacted Wall Creek samples. Precipitation of an Al-bearing Phase. In both experiments, the concentration of Al decreased after its initial increase (Figure 2, Table 2). Removal of aqueous Al from the reacting fluid suggests precipitation of an Al-bearing phase. Fluid samples collected from both experiments are
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supersaturated with respect to albite, chlorite, illite, and Mg-bearing smectite (Table S3), indicating favorable fluid chemistry for precipitation of an aluminosilicate phase.
Figure 4. Selected portions of whole rock diffractograms of unreacted and reacted rock powders from both experiments. The four diffractograms are illustrated in the same order as listed in the key. Relative intensities between each diffractogram have been adjusted so that the diffractograms do not overlap. The (010) gibbsite peak at ~18.5 °2θ has a greater intensity in both unreacted powders than in both reacted powders. Gibbsite is a mineral component of bauxite (proppant), thus indicating dissolution of bauxite due to reaction with HFF. We observed aluminosilicate precipitates on a calcite surface in the Wall Creek experiment (Figure 3C). The spherical precipitates range from 10’s – 100’s of nanometers in diameter. EDS spectra of the precipitates display Ca-, Mg-, Si-, and Al-peaks. However, EDS spectra of the calcite substrate near the precipitates display Ca and Mg peaks, no Al peak, and a small Si peak, suggesting the underlying calcite is the source of an indeterminate amount of Ca and Mg in spectra of the precipitates (Figure 3C). These spectra collectively suggest that the precipitate is an aluminosilicate phase, but do not provide sufficient information to identify the phase. Precipitates
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were not pervasive across all surfaces of reacted rocks of the Wall Creek and were not observed on reacted rocks of the B Bench. Smectite has been hypothesized to precipitate during interaction between HFF and the Marcellus Shale.11 In addition, precipitates – suggested to be Fe-oxides or clay minerals – were observed on samples of the Marcellus Shale and Eagle Ford Shale that reacted with HFF and brine, respectively.10 Those precipitates adhered to mineral surfaces and also appeared to flocculate and form micrometer-sized clusters, similar to mineral precipitates observed in our Wall Creek experiment. The Evolution of Al- and Si-Geochemistry. Here, we evaluate the behavior of Al- and Sibearing minerals by comparing aqueous chemistry of fluid samples collected from the experiments with theoretical mineral stabilities. To do so, we constructed an activity diagram of the MgOAl2O3-SiO2-H2O system (Figure 5). The diagram shows mineral stability relationships at in situ conditions as measured within fluid samples collected from the B Bench experiment (Table 2); aqueous Al concentration is in equilibrium with chlorite. Log 𝑎𝑀𝑔2 + /𝑎(𝐻 + )2 and log 𝑎𝑆𝑖𝑂2(𝑎𝑞) of aqueous samples are plotted on the diagram. The HFFs used in the experiments plot in the gibbsite stability field (Figure 5) and are undersaturated with all Al-bearing minerals of the rock (Table S3). The concentration of aqueous Al increased in
