In-situ disposal of CO2: Liquid and supercritical CO2 permeability in coal at multiple down-hole stress conditions

Journal of CO2 Utilization 17 (2017) 235–242

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In-situ disposal of CO2: Liquid and supercritical CO2 permeability in coal at multiple down-hole stress conditions Vikram Vishal Computational and Experimental Geomechanics Laboratory, Department of Earth Sciences, Indian Institute of Technology Bombay, Mumbai, 400076, India

A R T I C L E I N F O

Article history: Received 8 August 2016 Received in revised form 12 December 2016 Accepted 22 December 2016 Available online xxx Keywords: Permeability Effective stress Supercritical CO2 Liquid CO2 Indian coal

A B S T R A C T

Geological CO2 sequestration is one of the most effective methods to counter global climate change. Coal matrix shrinkage, swelling, gas diffusion and permeability are the key phenomena associated with geological CO2 disposal in coal. In this study, permeability experiments were approached using the supercritical and liquid phases of CO2 (less understood; most likely insitu phases) for naturally fractured bituminous coal. Experiments were performed under triaxial conditions using four sets of various confinement conditions corresponding to variable depths. Injection pressure was varied gradually and the permeability changes were calculated using the Darcy’s equation for subcritical and supercritical CO2 exclusively. Changes in CO2 phases were obtained by changing the system temperature from 26
C for liquid CO2 to 34
C for supercritical CO2. N2 was alternatively injected at the start and between the injections of CO2 to analyze the changes in the permeability from a relatively very less sorptive medium’s perspective. It was observed that the supercritical CO2 flow reduced the permeability significantly and this behavior was greatly attributed to the highly viscous nature of the supercritical phase and the high volumetric deformation or the swelling of coal under supercritical CO2 as compared to liquid CO2. The injection pressures were observed to reduce the effective stress behavior, which in turn pushed the permeability evolution at each confinement to a positive trend. However, the permeability of CO2 reduced exponentially with increasing effective stresses. Two different empirical equations were proposed for permeability of both the phases of CO2 with effective stresses. © 2016 Elsevier Ltd. All rights reserved.

1. Introduction Anthropogenic contribution resulting in the rise of CO2 has exponentially risen, especially in the past three decades, with atmospheric CO2 levels going above 400 ppm in April 2014 at all World Meteorological Organization (WMO) stations in the northern hemisphere [1]. Geological CO2 sequestration is one of the most effective and readily available methods to keep the rise in CO2 levels in control and achieve climatic targets. In India, which is the world’s third largest emitter of CO2 after China and U.S.A respectively, the unmineable deep coal seams are among the most promising candidates to physically sequester CO2, with or without coalbed methane recovery [2,3]. According to a survey in 2009, coal’s contribution via power and steel plants to the Indian carbon emissions was 72% [4]. Thus, in context of operational proximity and availability of storage reservoirs, coalbed sequestration in India is a more favorable option when compared to mineral

E-mail addresses: [email protected], http://www.geos.iitb.ac.in/index.php/en/vik (V. Vishal). http://dx.doi.org/10.1016/j.jcou.2016.12.011 2212-9820/© 2016 Elsevier Ltd. All rights reserved.

trapping in basalt formations, offshore sequestration in sedimentary basins, and depleted oil and gas reservoirs. In coal, CO2 may be sequestered in three states: free-space (gas, liquid or supercritical fluid), in solution in mine water, and adsorbed to the remaining coal in the mine [5,6]. Geological media suitable for CO2 storage must have the capacity to store the intended volume of CO2. This enables them to accept the injectant at the intended flow rate and confinement potential for safe storage in geological time scales [7]. The structure of coal makes it one of the prime options for CO2 storage in the above context. Also, CO2 has greater affinity to adsorb onto the coal's microstructure among CO2, CH4 and N2 [8] making it a feasible injectant for enhanced coalbed methane (ECBM). Potential for CO2 storage in coal reservoirs with/without CBM recovery has been justified in a diverse range of reservoirs throughout the world [9–19]. Previous permeability studies on Indian coal have been carried out with the sub-critical phase of CO2 at varying confinements for both intact and naturally fractured coal specimens [2,20]. However, in case of deep seam sequestration, the pressure temperature (P-T) conditions could lead to either liquid or supercritical stage of CO2. CO2 exists in supercritical state above

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the pressure of 7.38 MPa and temperature of 31.1
C, which is likely to be encountered at depths more than 800 m below the surface. The critical point of CO2 is the transition zone for CO2 in the P-T regime, where significant changes in the hydro-dynamic properties of CO2 occur for relatively small fluctuations in the P-T conditions near the critical point. In the Indian context, typical depths for underground coal mining range between 300 m and 1200 m below ground level. Confining seam conditions in this range can induce phase transitions in CO2, which further affects the coal-fluid interactions. According to Krooss et al. [21], CO2 has higher adsorption affinity into the coal matrix under super-critical conditions as compared to sub-critical conditions whereas White et al. [10] mentioned that the supercritical CO2 has a higher potential to displace the existing gases from coal. Thus, it is vital to investigate the influence of liquid and supercritical CO2 flow at varying confinements, corresponding to different depths of coal seam. The aspect of permeability evolution in coal upon CO2 injection in subcritical phases have been studied in the past [22–30] and there have been a few studies on the behavior of supercritical CO2 flow upon injection into fractured and intact coals. Toribio et al. [31] and Perera et al. [32] observed that there was an increase in sorption when the CO2 phase condition changed from subcritical to supercritical state. This is due to the fact that supercritical CO2 has greater affinity for coal sorption when compared to subcritical CO2. Also, Perera et al. [33] described that the high improvement in viscosity of the fluid at high downstream pressure could be the reason for the sudden decline in the permeability apart from the usually induced swelling of coal. While many studies have gone into injection of liquid/supercritical CO2 for CBM extraction [34,10,35], very limited studies have been carried out in context of observational permeability evolution with respect to varied

experimental conditions. Also, till date, no experimental, analytical or modeling study has been carried out in the area of permeability behavior upon injection of liquid and supercritical CO2 in context to Indian Gondwana coals which form a large share of coalfields worldwide, except one recent study on the role of saturation cycles on permeability of coal by this research group recently [36]. In this study, a detailed experimental scheme was followed to study the permeability of coal under deep in-situ conditions at multiple injection pressures and effective stresses. 2. Methodology Bituminous coal samples from Damodar valley coalfields, in the state of Jharkhand, India were used in this study. The samples used for CO2 flow studies were vitrinite and inertinite rich and contained 86.23% by mass of pure coal. The samples were obtained from drill cores and were polished in the laboratory and stored in a humidity chamber to prevent the loss of moisture. As the objective of the study was to study the flow of CO2 through the coal sample under in-situ conditions, a high pressure triaxial setup with the capability of fluid flow through the sample under confinement conditions and axial loading was used. The detailed experimental procedure used in this study is described in the scientific articles published earlier [37]. A simple schematic of the experimental setup is shown in Fig. 1. As two different high pressure phases of CO2 were being used in the experimental procedure, the system was thoroughly examined for safety. High values of confining stresses were used keeping the variations same for tests carried out using both phases of CO2. Constant temperature was maintained at 26
C for liquid CO2 and 34
C for supercritical CO2 flow. During experiments, CO2 was injected through the ports on one end of the sample. The coal specimen was 54 mm in diameter and

Fig. 1. Simple schematic of the triaxial permeameter setup.

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110 mm in length. At each referred condition of CO2 injection, the permeability was estimated using the Darcy’s equation (also applied by [32,20,26,27,38,39]): 2Qpo mL K¼ Aðpi 2 po 2 Þ

ð1Þ

Where, Q is the downstream flow rate, m is the viscosity of injecting fluid, po and pi are the downstream and upstream pressures respectively, L and A are the mean length and cross sectional area of coal specimen respectively. For liquid CO2, the modified Darcy’s law is used since liquids are not compressible like gases and supercritical fluids. Q mL K¼ Aðpi po Þ

ð2Þ

For testing the permeability of CO2 under liquid and supercritical conditions, the undrained flow method was employed. The downstream pressure was monitored and recorded at regular intervals using the data acquisition system. The changes in pressure were obtained and used for calculations of the flow rate using the following equation (also applied by [26,27,32]): Q¼

ð3Þ

Table 1 Test conditions for nitrogen and liquid and supercritical carbon dioxide injection through coal specimen. Confining Pressure Liquid and Supercritical CO2 (MPa) Injection Pressure (MPa) 10 10.5 11 11.5 12 12.5 13

N2 Injection Pressure (MPa) 5, 6 7, 8 9, 10 11, 12

18

10 11 12 13 14 15

–

21

10 11 12 13 14 15 16 17 18

–

10 11 12 13 14 15 16 17 18 19 20

–

24

permeability of coal samples was established at given set of gas injection pressures at 15 MPa fixed confinement. Then liquid CO2 was passed through coal at four different confinement pressures (15, 18, 21 and 24 MPa). N2 gas was again passed through the specimen to estimate the changes in permeability after passage of liquid CO2. A similar series of tests using supercritical CO2 were conducted and finally N2 was passed through the treated coal sample and the permeability changes in each case was estimated. The injection pressure and confining pressure scheme of experiments for liquid and supercritical CO2 and N2 is shown in Table 1. The original N2 permeability was the base value for N2 flow in coal for comparison at different stages of flow during the experiments. It was expected that the passage of liquid and supercritical CO2 through the sample would alter the physical structure of coal and hence, the changes in permeability of N2 was established when tested after each phase of CO2 flow through the coal. The scheme of tests was planned as N2 flow >liquid CO2 flow >N2 flow >supercritical CO2 flow >N2 flow. N2 permeability was tested at initial 15 MPa confinement of coal while the gas injection was varied between 5 MPa through 12 MPa. 3. Results and discussions

dP bV dt

Where, dP is the rate of change of downstream pressure, b is the dt adiabatic compressibility of the fluid phase, V is the downstream volume. In this study, three main schemes of experiments each with N2, liquid CO2 and supercritical CO2 were applied. Initial N2

15

237

3.1. Behavior of liquid and supercritical CO2 Liquid CO2 belongs to the subcritical region and has high viscosity. Klinkenberg effect is an important parameter in comprehending the microchannel flow characteristics via the pore radius and the mean free path of fluid flow in porous media [20]. The Klinkenberg effect is higher in liquid CO2 when compared to supercritical CO2 [40,32] that has a liquid-like density and gaslike viscosity [41]. In deep coal seams (>800m), CO2 is present in supercritical state and this facilitates a low volume of CO2 occupation due to the high density nature of supercritical CO2 [42,43]. Hence, the existence of CO2 in coal during geological CO2 storage would be in the liquid and supercritical states. The Langmuir model, which relies on monolayer adsorption, is not expected to be a good indicator of the sorption behavior of supercritical CO2, because only the sorbed molecule-matrix interactions are taken into consideration [44]. Instead, the pore filling concept is a good assumption and thus it is expected that the narrow pore sites would be filled by supercritical CO2 and only partial filling can be attained in the larger pores [45–47]. Also, the supercritical CO2 has the potential to alter the organic presence in the coal seam [48]. At greater depths, the gaseous CO2 will transform to liquid or supercritical phase and the interactions of this type of fluid with the coal matrix vary from that of subcritical interactions. The permeability of coal was significantly lower at similar effective stresses when gaseous CO2 was used in our earlier studies [2,20]. 3.2. Validation of laminar flow of CO2 in coal The experimental scheme to determine the permeability of coal utilized undrained flow conditions as employed by Perera et al. [32]. The reason for the employment of undrained conditions was to maintain the supercritical phase of CO2 throughout the experimental domain which would not be possible with a drained arrangement. The downstream pressure transducers started responding immediately after upstream injection of CO2, but it took about 30 min for the pressure to stabilize. The estimation of permeability was done at 5 min of injection when the (dP ) curves dt were linear. The development of downstream pressure with respect to time was recorded at each combination of confinement and injection pressures. These values were recorded in order to

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Fig. 2. CO2 flow rate vs. injection pressure at four different confining pressures.

estimate the values of (dP ) which in turn is used to calculate the dt flow rate of fluid in the downstream. In an earlier study that utilized gaseous CO2 [2], the downstream flow rate was directly measured by keeping track of the 3.3 mL bubble pulse emitted. In this study, indirect method was implemented by recording the downstream pressures with respect to time. The temperature of the experimental domain was maintained at 26
C for each test using N2 and it was kept constant for liquid CO2 flow through the coal. Nine cases of liquid CO2 injections were carried out at a confinement pressure of 21 MPa while eleven steps of injection were carried out for a 24 MPa confinement pressure. Similar steps were carried out for super-critical CO2 injection. The CO2 phase was controlled by maintaing the fluid temperature to 34
C which is above the critical temperature of CO2, at constant pressure. The flow rate of CO2 behaved in an approximate linear manner with respect to the injection pressure, as shown in Fig. 2. While the flow rate increased from 8.56e–08 m3/s to 1.07e–07 m3/s at 15 MPa of confining pressure, it increased from 3.13e–08 m3/s to 4.07e– 08 m3/s at 21 MPa confining pressure corresponding to the same injection pressures of 10 MPa and 13 MPa. Similar linear variations were observed for supercritical CO2 flow. This characteristic allowed the usage of Darcy’s law for estimation of permeability unlike the results of Jasinge et al. [49] where the flow rate vs. injection pressure curves were nonlinear. The laminar behavior of CO2 flow has been observed in few previous works [2,20,33,50]. At a constant injection pressure, the variation of flow rate was studied as a function of confining pressure and it was observed that the flow rates decreased considerably at higher confining pressures.

attributed to the primary swelling of coal matrix in response to CO2 exposure. The coal matrix is fresh and has no molecular presence of CO2 in adsorbed form, thus causing it to swell upon its first contact with a fluid that has affinity to interact with it. Also, at low confinements the matrix would be less restricted to swelling, thus the reduction in permeability. The increase in permeability after the initial reduction is because of the effect of dilation of the flow paths. Fig. 3b–d depict the permeability evolution of liquid CO2 flow through coal for 1 MPa increase in inlet pressure for 18 MPa, 21 MPa and 24 MPa confinement pressures respectively. It can be inferred from the above observations that while higher confining conditions clearly reduced the overall permeability of the sample by tightening the internal cleats, the permeability changes with injection pressures were mostly predictable. About 70% reduction in permeability was observed at confinement pressures from 18 MPa to 21 MPa and about 34% permeability reduction between 21 MPa and 24 MPa at 16 MPa upstream pressures. Another salient observation of the permeability vs inlet pressure graphs is that for consecutive graph pairs, the initial permeability value of the former curve is always greater than the final permeability value of the latter curve. This behavior might be because the permeability changes are mainly controlled by the confinement conditions rather than the injection pressure values. Apart from the first curve whose characteristics can be attributed to the dominant coal swelling and low confinement conditions, the other curves in order of increasing confinement pressures, showed lesser changes in permeability with respect to injection pressures. High values of CO2 permeability in coal at lower confinements may be due to the slippage of fluid at low pressures.

3.3. Coal permeability using liquid CO2 at multiple confining pressures

3.4. Coal permeability using supercritical CO2 at multiple confining pressures

Permeability of coal to liquid CO2 was determined at four different confining pressures as shown in Fig. 3a that represents the changes in permeability for every 0.5 MPa increase in inlet pressure at 15 MPa confinement pressure. The permeability decreased initially from 0.011 mD at 10 MPa to 0.0089 mD at 11.5 MPa before increasing to 0.0103 mD at 13 MPa injection pressure. The initial reduction in the permeability with increasing injection pressure of CO2 during initial confinement conditions is

The same set of confining pressures and CO2 injection pressures were maintained during the tests for supercritical CO2 flow in coal. However, the temperature of the setup was kept constant at a higher value of 34
C. As the downstream pressure developed due to flow of supercritical CO2 was above 7.5 MPa, in each test, it can be inferred that the CO2 phase along the entire sample length remained in the supercritical state. N2 was passed at the end of the

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239

Fig. 3. Permeability of coal to liquid CO2 at four confinements (15 MPa, 18 MPa, 21 MPa, 24 MPa) and different injection pressures in each confining pressure.

experiment to establish the loss in coal permeability due to flow of supercritical phase of CO2. The changes in permeability with respect to injection pressure at varying confinements are shown in Fig. 4. The results for flow of supercritical CO2 at initial confinement revealed a reduction in permeability of flow by nearly 60% as compared to liquid CO2. A part of this reduction may be due to the adsorption of liquid CO2 onto the coal structure and its consequences and changes in coal confinement from 15 MPa to 24 MPa in the previous set of tests. At 15 MPa injection pressure, the permeability reduced from 0.00326 mD to 0.00154 mD i.e. reduced to less than half its value from 18 MPa to 24 MPa confinement conditions, in that order. This confirms that supercritical CO2 would significantly alter the permeability characteristics of coal. The reason for this pattern could be that the permeability reduction due to the swelling is compensated for by the dilation due to increase in inlet pressure, unlike in the case of liquid CO2. The same trend follows till the last stage of the test (i.e. at 24 MPa confinements) which indicates that the scope of swelling still persists under the supercritical state of CO2 flow. Another salient observation is that the permeability of CO2 in its supercritical state is nearly half of the value in liquid state. A part of this reduction is attributed to the changes in the structure of coal due to previous flow of liquid CO2. However, a magnitude of the change was partly recovered due to the flow of N2 through the sample. High affinity of supercritical CO2 to coal for adsorption is the reason behind this phenomenon. Thus, large volumes of CO2 get preferentially adsorbed on to the fractured and micro porous surfaces of coal. Perera et al. [32] found that the adsorption of supercritical CO2 in coal leads to a higher percentage of swelling

than subcritical CO2. They showed that nearly two times higher swelling was induced in coal. The highlight of this study is that, a naturally fractured coal, already swollen under subcritical conditions could still swell significantly more under supercritical CO2. This explains the drastic reduction in permeability in coal when CO2 was injected in various pilot projects. Tao et al. [35] carried out supercritical CO2 based coalbed methane extraction experiments and observed that the CH4 displacement efficiency reached upto 77.8% and proved that production of CBM can be achieved efficiently using supercritical CO2. Kiyama et al. [51] performed laboratory tests to better comprehend the CO2 injectivity in Yubari fields. They observed that the permeability decreased after liquid/ supercritical CO2 was injected and inferred that their experimental conditions favored adsorptions induced swelling, the culprit behind the decrease in permeability. Supercritical CO2 can also dissolve the organic matter in the coal seam, creating deposits in the flow path and reducing permeability [52]. The reduction of flow permeability can also be because of the pore filling in coal by supercritical CO2. 3.5. Evolution of N2 permeability of coal Nitrogen was used as a relatively less sorptive medium as compared to CO2 in permeability tests in coal in order to compare the reduction in flow due to different phases of CO2. The permeability reduction versus N2 inlet pressure is plotted in Fig. 5 which shows that the higher reduction in N2 permeability takes place due to supercritical CO2 flow, even though there is already some loss in permeability during liquid CO2 flow. It is

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Fig. 4. Variation of permeability of coal to supercritical CO2 at different confinements (15 MPa, 18 MPa, 21 MPa, 24 MPa) and different injection pressures in each confining pressure.

evident from the above discussion that the original permeability is the highest with an overall reduction due to flow of CO2. However, under any circumstance, the N2 permeability of coal increases with increase in injection pressure, which reflects that minimal or no swelling is induced in coal due to N2 flow, as also mentioned by White et al. [10]. However, there is not much difference in permeability before liquid CO2 injection and post liquid CO2 injection as compared to that of post supercritical CO2 injection.

Fig. 5. Variation of permeability of coal sample to N2 at three different cycles: base value of permeability, that after liquid CO2 and supercritical CO2 flow.

Thus, supercritical CO2 has a much higher potential to reduce the permeability due to higher adsorption of CO2 in coal and resultant matrix deformation in terms of swelling. Fig. 6 shows the reduction in permeability (%) with respect to N2 injection pressures at the start and end phase of each of the three stages in injection: preliquid CO2, post-liquid CO2 and post-supercritical CO2. The highest reduction in permeability as seen from the curves was for the preliquid and post-supercritical CO2 flow. The reduction in permeability was at a maximum of 50% between 5 MPa and 6 MPa N2 inlet pressure but, the values continued to hover a little below 50% post 6 MPa till 12 MPa N2 inlet pressures. As mentioned earlier, this effect is due to the supercritical CO2’s capability to alter the permeability of coal and its high affinity for adsorption into the coal microstructure, thus resulting in significantly higher extents of swelling than that expected from the subcritical CO2. As expected, for the post-liquid and post-supercritical CO2 flow, the reduction in permeability was between 35% and 42%. Similar to the earlier case, a reduction in the permeability of coal to N2 was observed after 6 MPa injection pressure. Then value is seen to rise a little after 9 MPa before stabilizing till 12 MPa. The effect of dilation of flow regime can be made responsible or is attributed to this for this behavior. The last curve i.e. the pre-liquid and post-liquid CO2 flow showed unique pattern in its reduction in permeability with respect to N2 injection pressure. The reduction in value was between 12% and 18% with an initial rise in value till 18% at 8 MPa and a brief fall before stabilizing again. It can be reasoned that the CO2 molecule has a relatively smaller kinetic diameter as compared to that of N2, thus facilitating micropore adsorption alongside flow through macropores and has nearly twice its diffusivity in coal as compared to its diffusivity in N2. Thus, N2 flow predominantly

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241

Fig. 8. Variation of supercritical CO2 permeability of coal with effective stresses.

Fig. 6. Reduction in permeability of coal sample to N2 as compared to different cycles of liquid CO2 and supercritical CO2 flow.

exists in the fractures within the sample with most resistance from the already adsorbed CO2 molecule in the micropores. The rise-fallstabilize and the fall-rise-stabilize trends as observed in the previous two cases of permeability reduction (post-liquid and post-supercritical CO2 flow, pre-liquid and post-liquid CO2 flow respectively) with respect to N2 inlet can be imputed to the preceding characteristics. N2 also would have restored some amount of permeability in coal samples, as also observed by Perera et al. [33]. 3.6. Role of effective stresses Effective stress is an important parameter in judging a coal sample for its potential for sequestration because it directly affects the permeability of fluid flow in the seam and was calculated using the formula below (also used by [2]):

s e ¼Pc

PiþPo 2

ð4Þ

Where, Pc is confining pressure and Pi and Po are upstream and downstream pressures respectively. The deep coal seams have very high confinement conditions and the required amount of injectivity of fluid (CO2) is attained by creating high values of upstream pressures but not so high as to compromise the mechanical integrity of the seam. The pore

Fig. 7. Variation of liquid CO2 permeability of coal with effective stresses.

pressure, which is computed to be the average of the upstream and downstream pressure, develops positively upon high injection pressures. This reduces the effective stress at a constant confinement. Figs. 7 and 8 depict the permeability vs effective stresses curves for liquid CO2 and supercritical CO2 respectively. As expected, a decrease in permeability is seen in both the curves for every increase in effective stress values. For liquid CO2 and supercritical CO2, the permeability values reduced by 91% and 82% respectively between 10 MPa and 20 MPa effective stresses. There were many values scattered in both the cases but an exponentially decreasing curve was fitted that gave the equation for permeability k¼0:1085e0:243s e for liquid CO2 and k¼0:0265e0:169s e for supercritical CO2 where k is the permeability and se is the effective stress. These results will help us to arrive at the boundary limits for the injection pressures to facilitate better permeability at confining conditions. 4. Conclusions In many large scale field injections of CO2 for geological storage worldwide, the intensive properties of CO2 fall in the liquid and supercritical state. CO2 is mostly stored in the supercritical state as its liquid-like density confines it to its storage domain and gas-like viscosity facilitates required levels of injectivity. In this study, CO2 permeability in coal was estimated at varying confinements i.e. in simulated environment corresponding to different depths. Supercritical CO2 created the highest reduction in the permeability of nearly 60%, owing to its higher affinity to coal matrix as compared to liquid CO2. Also, the initial permeability of coal at 10 MPa injection pressure reduced from 0.011 mD at 15 MPa confinements to 0.0004 mD at 24 MPa confinements. Thus, confinement condition played an important role in determining the injectivity of coal because of its effect of compression of coal seam cleats that restricts swelling beyond a certain level. From the experiments carried out, the bituminous coal has shown potential for injection of liquid/supercritical CO2 upon increase in injection pressure in an otherwise constrained confinement. The permeability improved by nearly 16% for liquid CO2 and 8% for supercritical CO2 with respect to increase in upstream pressure at a confinement of 15 MPa. Different empirical equations were suggested for coal permeability with respect to the effective stresses for both liquid and supercritical phases of CO2. This can be used for future studies and close to real estimations of CO2 permeability. These results have shown promising potential for CO2 injection and subsequent storage in Indian bituminous coal seams at greater depths. These results can also act as a precursor for advanced studies of core flooding experiments and modeling studies coupling the physical mechanisms ranging from adsorption, diffusion and laminar flow

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