Browsing by Author "Mayer, B."

Now showing 1 - 4 of 4
  • Thumbnail Image
    ItemOpen Access
    An 8-year record of gas geochemistry and isotopic composition of methane during baseline sampling at a groundwater observation well in Alberta (Canada)
    (Springer, 2016-02-01) Humez, P.; Mayer, B.; Nightingale, M.; Ing, J.; Becker, V.; Jones, D.; Lam, V.
    Variability in baseline groundwater methane concentrations and isotopic compositions was assessed while comparing free and dissolved gas sampling approaches for a groundwater monitoring well in Alberta (Canada) over an 8-year period. Methane concentrations in dissolved gas samples (n = 12) were on average 4,380 ± 2,452 μg/L, yielding a coefficient of variation (CV) >50 %. Methane concentrations in free gas samples (n = 12) were on average 228,756 ± 62,498 ppm by volume, yielding a CV of 27 %. Quantification of combined sampling, sample handling and analytical uncertainties was assessed via triplicate sampling (CV of 19 % and 12 % for free gas and dissolved gas methane concentrations, respectively). Free and dissolved gas samples yielded comparable methane concentration patterns and there was evidence that sampling operations and pumping rates had a marked influence on the obtained methane concentrations in free gas. δ13CCH4 and δ2HCH4 values of methane were essentially constant (−78.6 ± 1.3 and −300 ± 3 ‰, respectively) throughout the observation period, suggesting that methane was derived from the same biogenic source irrespective of methane concentration variations. The isotopic composition of methane constitutes a robust and highly valuable baseline parameter and increasing δ13CCH4 and δ2HCH4 values during repeat sampling may indicate influx of thermogenic methane. Careful sampling and analytical procedures with identical and repeatable approaches are required in baseline-monitoring programs to generate methane concentration and isotope data for groundwater that can be reliably compared to repeat measurements once potential impact from oil and gas development, for example, may occur.
  • Thumbnail Image
    ItemOpen Access
    Estimation of fracture height growth in layered tight/shale gas reservoirs using flowback gas rates and compositions–Part II: Field application in a liquid-rich tight reservoir
    (Elsevier, 2016-01) Clarkson, C.R.; Ghaderi, S.M.; Kanfar, M.S.; Iwuoha, C.S.; Pedersen, P.K.; Nightingale, M.; Shevalier, M.; Mayer, B.
    While hydraulic fracturing is the key to unlocking the potential of unconventional low-permeability hydrocarbon resources, challenges remain in the monitoring of subsurface propagation of fractures and the determination of which geologic intervals have been contacted. This is particularly challenging for wells that are completed in multiple hydraulic fracture stages (multi-fractured horizontal wells or MFHWs) where fracture spacing may be very close and fracture geometry complex. Understanding the fracture extent is important not only for assisting with hydraulic fracture design, but also for mitigating unwanted fracture growth into non-target geologic intervals that do not contain hydrocarbons (e.g. zones with high water saturation). Popular current technologies used for hydraulic fracture surveillance include microseismic (surface and subsurface monitoring) and tiltmeter surveys. While these methods have proven useful for characterizing the extent of created hydraulic fractures, they do not necessarily lead to an understanding of what portions of the geologic section (bounding and target intervals for MFHWs, for example) are in direct hydraulic communication with the well. A solution for establishing the extent of hydraulic fracture growth from target to bounding zones is to first obtain a fluid composition fingerprint of those intervals while drilling through them, and then compare these data with fluid compositions obtained from flowback after hydraulic fracturing. In the current work, a MFHW completed in a liquid-rich tight reservoir is used to test this novel concept. Gas samples extracted from the headspace of isojars® containing cuttings samples, obtained during drilling of the MFHW well, were used to geochemically fingerprint geologic intervals through which the well was drilled. The cuttings samples were collected at high frequency in the vertical, bend and lateral sections of the well over a measured depth range of 4725 ft (1440 m). A compositional marker was identified in the bend of the horizontal well above which the average methane to ethane (C1/C2) ratio was 15.7, versus 2.6 below it. The flowback gas compositions were observed to be intermediate (average C1/C2 = 7.4) between the reservoir above and below the marker, suggesting fracture height grew above the compositional marker. In order to estimate fracture height growth from the geologic interval and flowback compositions, a compositional numerical simulation study was performed. An innovative approach was used to estimate recombined in-situ fluid compositions, on a layer-by-layer basis, by combining the cuttings gas compositional data with separator oil compositions. The resulting numerical simulation model, initialized through use of the layered fluid model and a detailed geological model developed for the subject well and offset drilling locations, was used to history match flowback rates, pressures and gas compositions. The gas compositions of the fingerprinted geologic intervals were therefore employed as a constraint on fracture height growth, estimated in the model to be 175 ft (53 m, propped height). However, because of the uncertainty in model input parameters, a stochastic approach was required to derive a range in hydraulic fracture properties. The current study demonstrates for the first time that it is possible to constrain fracture height growth estimates from flowback data, combined with gas compositional data obtained from cuttings data, provided that the geochemical fingerprints are distinct.
  • Thumbnail Image
    ItemOpen Access
    Tracing nitrate sources with a combined isotope approach (δ15NNO3, δ18ONO3 and δ11B) in a large mixed-use watershed in southern Alberta, Canada
    (Elsevier, 2020-01) Kruk, M.K.; Mayer, B.; Nightingale, M.; Laceby, J.P.
    Rapid population growth and land-use intensification over the last century have resulted in a substantial increase in nutrient loads degrading marine and freshwater ecosystems worldwide. In mixed-use watersheds, elevated nitrogen loads from wastewater treatment plant (WWTP) effluent or agricultural runoff often drive the eutrophication of waterways. Accordingly, the objective of this research was to identify sources of riverine nitrate (NO3), a deleterious dissolved species of nitrogen, with a combined isotopic tracing technique in the Bow River and the Oldman River in Alberta, Canada. Riverine NO3 and boron (B) concentrations, mean daily flux and δ15NNO3, δ18ONO3, and δ11B values were determined at 17 mainstem sites during high and low discharge periods in 2014 and 2015. The data for mainstem sites were then compared to results for effluent from seven WWTPs, eight synthetic fertilizers, cow manure, and three predominantly agricultural tributary sites to estimate point and non-point NO3 sources. The NO3 flux, δ15NNO3 and δ18ONO3 values indicated the city of Calgary’s Bonnybrook WWTP effluent accounts for the majority of the NO3 flux in the Bow River downstream of Calgary. δ15NNO3 and δ11B values in the Bow River highlighted an increase in agricultural NO3 loading downstream of irrigation return-flows. A three-fold decrease in the NO3:B flux ratio indicated NO3-removal processes are active in the lower reaches of the Bow River. For the Oldman River, δ11B values revealed elevated nutrient loading from the Lethbridge WWTP effluent (10% of downstream B flux). Furthermore, the agricultural tributaries contributed 25% of the local B flux to the Oldman River. Overall, δ11B was proven to be an effective co-tracer for discriminating between urban and agricultural sources of NO3 in these large mixed-use watersheds. This combined isotope tracing approach has significant potential to identify point and non-point NO3 sources driving eutrophication around the world.
  • Thumbnail Image
    ItemOpen Access
    Vadose Zone Gas Migration and Surface Effluxes after a Controlled Natural Gas Release into an Unconfined Shallow Aquifer
    (John Wiley & Sons, Ltd, 2018-12) Forde, O.N.; Mayer, K.U.; Cahill, A.G.; Mayer, B.; Cherry, J.A.; Parker, B.L.
    Core Ideas Subsurface gas migration results in localized surficial CH4 releases. Surficial CH4 emissions show pronounced temporal variations. Methane concentrations in soil gas exceed lower explosive limits at low leakage rates. Increasing CO2 effluxes and stable C isotope signatures indicate vadose zone CH4 oxidation. Instantaneous surficial effluxes do not indicate the magnitude of subsurface gas leakage rates. Shale gas development has led to concerns regarding fugitive CH4 migration in the subsurface and emissions to the atmosphere. However, few studies have characterized CH4 migration mechanisms and fate related to fugitive gas releases from oil or gas wells. This paper presents results from vadose zone gas and surface efflux monitoring during a natural gas release experiment at Canadian Forces Base Borden, Alliston, Ontario, Canada. Over 72 d, 51 m3 of natural gas (>93% CH4) was injected into a shallow, unconfined sand aquifer at depths of 4.5 and 9 m. Methane and CO2 effluxes in combination with soil gas concentrations and stable C isotopic signatures were used to quantify the spatiotemporal migration and fate of injected gas. Preferential gas migration pathways led to vadose zone hot spots, with CH4 concentrations exceeding the lower explosive limit (5% v/v). From these hot spots, episodic surface CH4 effluxes (temporally exceeding 2500 μmol m−2 s−1 [3465 g m−2 d−1]) occurred during active injection. Higher injection rates led to increased average CH4 effluxes and greater lateral migration, as evidenced by a growing emission area approaching 25 m2 for the highest injection rate. Reactive transport modeling showed that high CH4 fluxes resulted in advection-dominated migration and limited CH4 oxidation, whereas lower CH4 effluxes were diffusion dominated with substantial CH4 oxidation. These results and our interpretations allowed us to develop a conceptual model of fugitive CH4 migration from the vadose zone to the ground surface.