The influence of CO₂ saturation time on the coal gas flow: Fractured bituminous coal

Abstract

The permeability reduction due to the coal matrix swelling induced by the adsorption of CO₂ is the major concern during enhanced coalbed methane recovery (ECBM) as well as CO₂ sequestration in coal reservoir. Many problems arise in the context of this process regarding the coal permeability variations, and very few studies have been made with consideration of long-term CO₂ saturations on gas flow behaviour alterations. The main objective of this study is therefore to examine the influence of both subcritical and supercritical CO₂ flooding durations on N₂ and CO₂ permeability variation of naturally fractured low volatile bituminous coal under various tri-axial conditions. Five CO₂ flooding durations, up to 65 h, both subcritical (6 MPa) and supercritical (14, 20 MPa), were introduced to the sample under three confinements, respectively. Results show that for 11 MPa and 17 MPa confining environment, most permeability reduction occurs during the first CO₂ flooding process, and supercritical CO₂ flooding causes significant higher N₂ permeability reduction (from 14.38% to 50.18%) than subcritical CO₂ (from 4.11% to 11.25%). The permeability reduction rate decreases with time but this reduction continues even after a total of 153 h flooding. Longer permeability reduction process is observed upon supercritical CO₂ adsorption due to the much higher viscosity of supercritical CO₂. Under deep underground condition (>1 km), CO₂ exhibits a N₂-like flow behaviour because of the dominant poroelastic effect. The influence of CO₂ flooding on permeability alterations has been significantly compromised with much smaller permeability reduction after 20 MPa CO₂ flooding compared with 14 MPa CO₂ flooding (7.33% and 38.75% for 7 MPa CO₂ injection pressure, respectively), along with no apparent permeability reduction for the further CO₂ flooding scenarios.

Introduction

Coalbed methane (CBM) is considered as a more clean-burning fuel, compared with traditional natural fuel counterparts such as coal and oil [1], [2], [3]. For example, the amount of carbon dioxide produced from a gas power plant is generally less than half (40–50 percent) of the carbon dioxide emitted from a coal power plant when generating the same amount of electricity [4]. Methane is originated during coalification process and can be basically found anywhere there is coal deposit [3]. The majority of CBM are strapped within microstructures through physical adsorption [2], therefore, unlike conventional natural gas reservoir, where the gas can flow towards the production wells to the surface without additional stimulation, coal seam water which is known as formation water needs to be pumped out to reduce the reservoir pressure before methane can be liberated from the coal seam. However, since coal seams are generally less permeable, methane recovered using this primary pressure depletion method is generally less than 50 percent [5]. Therefore, in order to achieve an economical methane harvest, additional stimulating method is required to be employed to increase the methane recovery rate.

To date, hydraulic fracking and enhanced coalbed methane (ECBM) recovery techniques are two options that can be adopted to increase the methane recovery rate, while the former is the most widely used technique to enhance the gas flow rate. However, despite the great effectiveness of increasing the methane recovery, controversies over environmental impacts of hydraulic fracking have been in discussion for years [6], [7], [8]. Enhance coalbed methane recovery technique is performed by injecting CO₂, N₂ or gas mixture into the target coal seam to replace the methane. Of the various ECBM methods, CO₂-ECBM technique has an extra reward of immobilizing significant amount of CO₂ into coal seam with the mechanism of residual trapping [9], which contributes to CO₂ mitigation. During CO₂-ECBM process, the more reactive CO₂ acts as a displacement medium to admit the CH₄ in-place to be desorbed from coal matrix, flow through seam fractures and eventually recovered via the production well with the CO₂ to be sequestrated in the coal seam at the same time. Overall, a successful implementation of CO₂-ECBM would harvest methane in an economical and environmental-friendly manner.

However, significant reduction of CO₂ injectivity has been found both in lab-scale experiments [10], [11], [12], [13], [14], [15], [16], [17] and field scale projects [18], [19], [20], [21]. This permeability reduction is associated with coal matrix swelling during the adsorption of CO₂ into coal matrix. As depicted in Fig. 1(a), coal possesses a unique porosity structure which can be viewed as a “dual porosity” system. This dual porosity system consists of a matrix porosity (micropores) and a cleat porosity (cleats or fractures), the latter accounts for most of the flow characteristics and therefore, controls the flow behaviours of the gases during ECBM process [22]. Coal originally has a strained, glassy, cross-linked macromolecular polymer-like structure which can be altered upon the adsorption of CO₂ who plays as a plasticizer when interacting with the coal mass [23]. The structure of plasticized coal mass would be rearranged and large strain is therefore induced between the adsorbed CO₂ gas layer and the surface of pore wall which is commonly known as coal matrix swelling [12]. The swelled coal structure would significantly narrow down the paths through which gas transportation primary occurs (see Fig. 1 (b)), resulting in a reduced coal seam permeability.

To date, a number of studies have been carried out to provide insights into coal mass hydro-mechanical properties alterations upon CO₂ adsorption subjected to various experimental conditions. Table 1 summarises key findings by past authors on coal mass permeability alterations under different test conditions, those condition variables can be classified as coal seam properties, such as coal rank, coal seam buried depth, seam temperature, moisture content, and the properties of injecting gas, such as injection pressure, temperature and gas phase. Besides, the interaction time between coal mass and injecting gas also significantly affect the swelling process and degree.

Based on the information provided by Table 1, high rank coals tend to have much higher swelling effect upon the adsorption of CO₂ compared with low rank coals [13], [15], [24], probably due to the maturely developed cleats system created by a much longer coalification process, which provides more places for housing CO₂ molecule, resulting in a greater permeability reduction than low rank coals. Supercritical CO₂ causes much higher permeability reduction than gaseous CO₂ because of its more chemical reactive adsorption potential to coal [12], [15]. CO₂ exists in its supercritical phase beyond the critical point (31.8 °C and 7.38 MPa), and in deep underground, where the temperature and pressure are generally higher than this critical point, permeability reduction due to supercritical CO₂ adsorption would pose great challenges for CO₂ injection during ECBM process since the favourable sites for implementation of CO₂-ECBM are generally in deep buried coal seams [2]. Coal mass swelling degree with CO₂ adsorption increases with time, and this process is one of the most vital aspects that keeps CO₂ from successfully entering and sequestrating in coal seams, as reported by many in situ CO₂-ECBM schemes [18], [19], [20], [21], for example, around 43% reduction of CO₂ injectivity (from 4.3 t per day to 1.7 t per day) is observed in the Ishikari coal basin of Japan after 22 days of CO₂ injection. This suggests the importance of the related studies on time-dependent permeability changes subjected to CO₂ flooding. However, studies to date with regards to the effect of interaction time between coal and CO₂ on flow behaviour alterations in coal are insufficient, especially for long term CO₂ flooding. Perera et al. [12] observed a reduction trend of permeability with time using a naturally fractured black coal and this reduction ended within 3 days; similar results were also reported by Siriwardena et al. [29], a negative relation between swelling rate and time was recorded and swelling process was concluded within 3–4 days on an artificially-made-fractured anthracite. However, according to Ranathunga et al. [10], the amount of CO₂ being adsorbed into coal increased over time even after 10 days flooding which indicates that permeability variation caused by CO₂ adsorption did not end. As suggested by the model developed by Pan and Connell [30], though coal swelling ratio could reach a plateau and then decrease at very high gas pressures, coal swelling degree increases with the increasing amount of CO₂ being adsorbed into coal mass. Besides, Structural re-arrangement of coal mass due to physical and chemical reactions between CO₂ and coal mass could take place over time which may in turn cause alterations in the hydro-mechanical property of coal [31], [32].

Therefore, the main purpose of this study is to identify the influence of both subcritical and supercritical CO₂ flooding time on permeability variations of naturally fractured low volatile bituminous coal under tri-axial conditions. Low volatile bituminous coal (high rank) was adopted as the favourable sites for carrying out CO₂-ECBM pilots are in deep buried coal seam where high rank coals generally exist.