Letter pubs.acs.org/journal/estlcu
Sampling Dissolved Gases in Groundwater at in Situ Pressure: A Simple Method for Reducing Uncertainty in Hydrogeological Studies of Coal Seam Gas Exploration Eddie W. Banks,*,†,‡ Stanley D. Smith,§ Michael Hatch,∥ Lawrence Burk,†,‡ and Axel Suckow§ †
National Centre for Groundwater Research and Training, GPO Box 2100, Adelaide 5001, South Australia, Australia College of Science and Engineering, Flinders University, GPO Box 2100, Adelaide 5001, South Australia, Australia § CSIRO Land and Water Waite Campus, Waite Road, Urrbrae 5064, South Australia, Australia ∥ School of Physical Sciences, University of Adelaide, North Terrace Campus, Adelaide 5005, South Australia, Australia ‡
S Supporting Information *
ABSTRACT: The ability to accurately and precisely measure dissolved gas concentrations in groundwater is crucial for environmental monitoring purposes. Unfortunately, the collection of dissolved gas samples in groundwater is challenging because of the loss of dissolved gas in the groundwater as a result of inadequate sampling methodologies and the need to avoid exposure of the samples to the atmosphere. In environments where water can contain large amounts of methane and carbon dioxide (e.g., coal basins), there is a high probability of separation between the water and gas phases during sample collection, making it impossible to accurately measure the gas concentrations in a groundwater sample. In this paper, we describe the design and development of a novel downhole sampler that is used to collect dissolved gas samples in groundwater at in situ pressure. The sampler was used to collect samples from 26 bores in a proposed coal seam gas field in regional New South Wales, Australia. Comparison of CH4 concentration data showed that the gas concentrations were higher using our methodology than when using alternate methodologies, especially when CH4 concentrations were >10000 μg L−1. Not surprisingly, results from the new methodology suggest that isotopic fractionation occurs when samples are collected using alternate methodologies.
■
INTRODUCTION Dissolved gas concentrations in groundwater are useful environmental tracers that can be used to determine groundwater residence times, understand geochemical processes, monitor contamination plumes, and identify mineral, oil, and gas reserves.1−6 Collecting dissolved gas samples in groundwater is often challenging because of, among other factors, well construction limitations, low-yield aquifer formations (i.e., low permeability), high concentrations of dissolved gases, and the need to avoid exposure of samples to the atmosphere. To obtain a representative sample from the aquifer, an investigator relies on knowing the well construction details, physical characteristics of the aquifer, and geology and then, on the basis of this information, choosing and using the most appropriate sampling techniques and devices. In many aquifers, the total dissolved gas pressure (TDGP) is near 1 atm and the dissolved gases remain dissolved when a sample is pumped to the surface. If the sample is exposed to the atmosphere, the dissolved gas composition will begin to equilibrate with the atmosphere and dissolved gas concentrations will eventually reflect the solubility and atmospheric concentration of each gas species (Figure 1a). Atmospheric © 2017 American Chemical Society
exchange is avoided by using standard sampling techniques (e.g., collecting samples in sealed copper tubes connected to the pump outlet).7,8 However, in certain settings, such as basins containing coal seams, the dissolved gas pressure may be significantly greater than 1 atm because of the elevated concentrations of CH4 and CO2. The hydrostatic pressure of the overlying water column keeps the dissolved gas in solution. We call the concentration at which gas would start to effervesce at a certain depth the “critical concentration”, and this critical concentration is reached when the sum of gas partial pressures corresponds to the sum of atmospheric and hydrostatic pressure. Below the critical concentration, gases remain dissolved, which is the case at depth even if the partial pressure is >1 atm, but only until there is a reduction in hydrostatic pressure, causing the gas to effervesce. When groundwater is sampled, a decrease in hydrostatic pressure could be caused by, for example, lowering the water level during well purging (degassing occurs within the well bore) or when water is Received: Revised: Accepted: Published: 535
October 11, 2017 November 7, 2017 November 7, 2017 November 13, 2017 DOI: 10.1021/acs.estlett.7b00457 Environ. Sci. Technol. Lett. 2017, 4, 535−539
Letter
Environmental Science & Technology Letters
Figure 1. (a) Relationship between total dissolved gas pressure and depth below the water table. (b) Solubility of methane as a function of temperature and salinity (after Walker and Mallants12).
Figure 2. (a) Sample collection in the field using the downhole sampler. (b) Puncturing the butyl-rubber septa sample port of the downhole sampler with the double-ended syringe. (c) Filling the evacuated sample bottle.
being sampled, well construction, sample depth, aquifer recovery to pumping, and other factors.13 These range from simple techniques such as the “inverted bottle” technique in which the water/gas sample is pumped from the well and collected at the surface from the pump outlet for analysis14,15 to more sophisticated methods that are deployed and collect samples at the well screen [e.g., passive diffusion samplers,16 snap samplers (ProHydro, Inc.), and positive displacement samplers (Leutert Positive Displacement Sampler-PDS Sampler or One Phase Sampler)] to more complex methods that involve measurements made in situ with gas detection sensors within the well screen (e.g., ProOceanus Mini-Pro CH4). The inverted bottle, passive sampling, and snap sampling techniques do not preserve gas in the sample fluid when the TDGP is >1 atm. The
pumped to the surface (degassing occurs within the pump head or the sampling hose as the sample approaches the surface). The resulting groundwater sample may be partially stripped of its original dissolved gas content, or the sample may contain both a liquid phase and a gas phase, which may not be representative of the original composition. This leads to the collection of a nonrepresentative sample and inaccurate reporting of both gas concentration and isotopic ratio data in the groundwater sample (if fractionation also occurs). This may mean that critical information is lost in areas that are being developed for shale and other unconventional gas resources.9−12 A number of different sampling techniques are available for dissolved gas collection in groundwater, depending on what is 536
DOI: 10.1021/acs.estlett.7b00457 Environ. Sci. Technol. Lett. 2017, 4, 535−539
Letter
Environmental Science & Technology Letters in situ samplers are designed to be used in large bore wells typically seen only in the oil industry and are often much longer (3.5−4.5 m) and require a significantly greater financial investment. They are therefore not practical when samples are taken from smaller bore wells, typical of many hydrogeological and baseline environmental monitoring studies. The growth in exploration for unconventional gas resources such as coal seam gas (CSG) and shale gas has led to an increased level of baseline environmental monitoring of groundwater and other water resources in the proximity of these developments.6 One of the key variables that is used to assess the potential impacts of these developments on nearby water resources is the measurement of both methane concentration and the stable isotope ratios of carbon (δ13C) and hydrogen (δ2H) in methane both before and during prospect development.15 Baseline data of methane in these environments can be under-reported if the sampling technique chosen to collect water samples allows dissolved gas to escape before the sample is collected.17 The objective of this study was to develop a sampling instrument and a methodology for collecting dissolved gases from a representative groundwater sample, preventing sample gas loss by maintaining sample pressurization. The design of the sample device required that it could be deployed easily in the field and that the collected sample could be transferred to a suitable sample container, appropriate for analysis by a portable gas analyzer (in this study a Picarro 2201-i Cavity Ring-Down Spectrometer) to determine the dissolved methane concentration and isotopic ratios in water.
Figure 3. Example from monitoring well S4MB02 showing the purging and recovery of the bore followed by the deployment, flushing, and recovery of the downhole sampler: (a) depth of the sampler and water levels (WL) in the bore and inlet and outlet tubes and (b) pneumatic, hydrostatic, and total dissolved gas pressures. The sampler pressure is modeled on the basis of the approximate water level within the bore, recorded pneumatic pressures at the wellhead, and sampler depth; the TDG pressure was estimated from the combined CH4 and CO2 concentrations.
■
water level dropped to 46.27 m. At that point, the hydrostatic pressure at screen depth was still >50 m. Had there been a TDGP in the groundwater much higher than 5 atm, it would have partially equilibrated, because effervescence to reach equilibrium needs several minutes to an hour, depending on the gas. The measured concentration of the dominant gas constituent, methane, corresponds to a partial pressure of 0.5 atm (Table S1). It is therefore safe to assume a TDGP of 160 m deep well (Figure 4). We cannot exclude effervescence during well purging under all sampling and well conditions, because this would imply an a priori knowledge of the TDGP in groundwater (which may or may not be similar to an in situ TDGP measured at the well screen depth). Figure 3 shows modeled hydrostatic sample pressures based on the system pressures logged at the surface. The true water level and sampler depth pressure may be slightly different from the values that are indicated because the water level in the inlet line does not recover as quickly as indicated when the line pressure is decreased. To limit large pressure drops in the sampler (i.e., to prevent degassing of the sample during purging of the sampler), the inlet line was vented slowly over a period of ∼5 min, allowing the inlet water level to rise as the pressure dropped.
MATERIALS AND METHODS The design of the new gas sampler is shown in Figure S1 followed by a detailed description of the operating principles, preparation of the sample bottles, and the laboratory analysis that was conducted. The sampler in use is shown in Figure 2. Field Testing Site. Samples were collected from 26 groundwater and coal seam gas monitoring wells near Gloucester, New South Wales, Australia. Well depths ranged from 6.5 to 230 m below the ground surface and were typically completed with short screen intervals (typically
