CO 2 Effect on Coal Strength, Implications for the Field, and Basis for Further Work

Transcription

1 CO 2 Effect on Coal Strength, Implications for the Field, and Basis for Further Work Harpalani s results at Coal-Seq V in November 2006 (ref 1) showed that when Illinois basin coal core was saturated with methane in a triaxial cell, and followed by CO 2 injection, while holding the sample under uniaxial strain conditions, the core failed. The experiment was repeated several times, with only minor differences. In a followup presentation, Palmer (ref 2) showed that the failure was quite unexpected, and speculated that the core was weakened by the CO 2 swelling. To confirm this, it was decided to measure unconfined compressive strength (UCS) in two sister coal cores, one of which would be saturated with CO 2, the other without. Two sister cores from a block of coal taken from the HVB rank Herrin seam, Illinois basin, were prepared. One was exposed to CO 2 at 500 psi for 96 hours. The two cores were sent to Terra Tek, where UCS tests were conducted on each core, anticipating that the CO 2 core would be weaker. The results of the first UCS testing are given in Table 1, where UCS is called effective compressive strength. The CO 2 core was actually stronger, but the clean sample (ie, without CO 2 ) was compromised by a weak plane that led to early failure. Nevertheless, before that failure, Youngs modulus was lower than in the CO 2 core, and UCS and modulus generally trend together, which suggests that the CO 2 core was indeed stronger. Table 1: First UCS tests on cores. Sample ID Diameter (in) Length (in) Bulk Density (g/cm 3 ) Effective Compressive Strength (psi) Young s Modulus (psi) Poisson s Ratio Sample A (clean) Sample B (CO2) 241, , , In the second testing, the coal was moisture saturated prior to exposure to CO 2. As Table 2 shows, the CO 2 core was definitely stronger, and substantially so. Youngs modulus is also larger, which supports this observation. Images of the cores are given in Appendix A. 1

2 Table 2: Second UCS tests on cores. Sample ID Sample A (CO 2 ) Sample B (Clean) Diameter (in) Length (in) Bulk Density (g/cm 3 ) Effective Compressive Strength (psi) Young s Modulus (psi) Poisson s Ratio , , Possible explanations for the unexpected UCS results at TerraTek: 1. The CO 2 has to remain in the core to experience the core weakening: in this case the core was shipped to TerraTek, and then delayed at least several days before testing, so the CO 2 would have escaped from the core. Based on the Thermal Cracking section below, we do not think this is the answer. 2. Plastification of coal occurred during the CO 2 saturation, and therefore weakened. We do not think this happened, because lab work indicates no plastification up to a temperature of 80F, and very high pressures (ref 3). 3. The CO 2 injection for 96 hours would have dried out the coal, and less water can mean stronger coal, since water can have the effect of lubricating the cleats and allowing them to slip more easily. In one lab measurement (ref 4), the UCS of coal was reduced from 2425 psi to 693 psi, and cleat lubrication was invoked as one explanation. Note: an opposite view has also been suggested, that removal of water causes weakening of coal as the macrostructure collapses (ref 3). This illustrates the complexity and uncertainty of this work, because it is so new. Despite this, we favor explanation 3. as the answer to the unexpected UCS results at TerraTek. That is, coal that has been saturated with CO 2 in the lab (but then the CO 2 removed) becomes stronger because it has been dried, and the removal of the water lubrication effect dominates the CO 2 weakening due to differential expansion (which is discussed below). Implications for double-u coal strength guide: The two clean samples of HVB coal rank (without CO 2 ) gave UCS values of 876 and 2550 psi. However, the 876 psi value appears to have been compromised by a preexisting plane of weakness. The higher UCS = 2550 psi falls below that predicted by the double-u curve (see Figure 1), where UCSmin = 4800 psi. The double-u curves provide a guide to coal strength versus rank when no core tests are available. This result suggests the double-u curve can sometimes be an overestimate of UCS in cores. 2

3 HVB Figure 1: Double-U curve for predicting UCS of coal. Explanation for coal failure in the uniaxial strain tests at SIU: Harpalani and Palmer (refs 1, 2) describe the details of these tests. When CO 2 is adsorbed, differential expansion occurs on a local scale due to the heterogeneous and anisotropic nature of coal. This can cause localized failure. If enough localized failures occur, they can link up and we can get gross failure, and the coal pops. The uniaxial strain condition may also be a key, for when CO 2 is adsorbed under constant stress, the core expands but without apparent failure. We suggest that the localized failures are induced to link up more quickly under uniaxial strain, because the confining stress acting on the core plug has to be increased to maintain this condition. Differential expansion: A good example of this is provided in Figure 2 (ref 5), which shows expansion of different coal particles as they adsorb a liquid solvent, and swell in size. The amount of swelling was measured, and has been plotted in Figure 3, where it can be seen that the swelling differential can reach a factor of 10. 3

4 FIGURE 2: Eight NMR vials of Waterberg arranged by increasing colour with corresponding relative swelling extent (courtesy ref 5) Waterberg Highveld FIGURE 3: Single-particle swelling results of Waterberg (top) Highveld coal (bottom) (courtesy ref 5) 4

5 Coal is a very heterogeneous material, and when CO 2 is adsorbed differential swelling will occur between different macerals: eg, a particle of vitrinite will expand much more than a particle of inertinite. The differential expansions will cause interparticle stresses to be set up, and these can lead to localized failure. Studies have been made of coal samples, kept under constant effective stress, and subject to CO 2 sorption, and examined under X-ray CT (refs 12, 13). The swelling and CO 2 sorption in coal are heterogeneous processes. For example, vitrite showed the highest degree of swelling, while the clay (kaolinite) and inertinite region was compressed in response. The volumetric strains associated with swelling and compression were between ±15%. Although the effective stress on the sample was constant, it varied within the sample as a result of the internal stresses created by structural changes induced by CO 2 sorption. Figure 4 is a visual picture that shows a situation where coal and non-coal units are adjacent. This sample was held under uniaxial strain during the CO 2 injection test. The image shows cracks developed between coal and non-coal units. No preexisting fractures were seen in separate high resolution (pixel= 6-7 micron) micro-tomography image, from the sample under uniaxial stress. It appears that hairline cracks appear in the coal units after CO 2 cycles. A hairline crack is all that is needed at a localized failure site, before many such sites are linked up by stress increase to maintain a uniaxial strain condition. FIGURE 4: Visual of polished surface showing cracks induced by differential swelling as CO 2 is adsorbed in heterogeneous coal (courtesy ref. 6). 5

6 Two Australian papers have addressed the weakening of weak lignite coal (UCS <170 psi) by CO 2 (refs 14,15). They collect evidence from the literature for such weakening. In their own experiments, they find that UCS is lower by 13%, and Youngs modulus is lower by 26%. The explanation proposed is that microcracks, through their individual tensile strength, determine the bulk strength. When CO 2 is adsorbed in coal, surface energy is reduced, and this reduces the tensile strength of the microcracks, and therefore the bulk strength. However, under a confining pressure of 1450 psi and an internal pore pressure of 290 psi, there is no measured difference in triaxial strength at failure, nor Youngs modulus. In the first paper (ref 14), various explanations are given for the lack of a CO 2 effect when the coal is under an effective confining pressure as large as this (1160 psi). Note for comparison, the confining pressure was 650 psi in Harpalani s tests (refs 1,2) which failed the coal, with a pore pressure of 500 psi, so the effective confining pressure is only 150 psi, and a lot lower than 1160 psi in the Australian tests. Note, however that the UCS of Harpalani s samples was ~2550 psi, which is vastly stronger than the lignite samples (<170 psi). In the second paper (ref 15), an explanation is offered for the lack of CO 2 -induced weakening under high confining pressures. This is in terms of a decrease in surface film confinement (a kind of surface tension effect), which is difficult to understand. The paper concludes that weakening by CO 2, failure by in-situ stresses, and permeability increases will not be an issue for sufficiently deep coal seams. But this is based on lab tests of very weak lignite, and its risky to apply the same conclusion to relatively strong bituminous coals (such as the HVB coal used in Harpalani s tests). However, the paper states that a decrease in Youngs modulus is still likely, no matter what the effective confining pressure, and that this would benefit retention of sequestered CO 2. Finally, the permeability was measured during one triaxial test, and it spiked at failure when the perm enhancement was >4.5. But as axial stress increased further, the perm decreased fairly rapidly. These results can have important implications for CO 2 sequestration, and so further study of how CO 2 weakening depends on coal rank and insitu stress may be quite important. Thermal cracking: Thermally cycling a rock (or other heterogeneous collection of anisotropic minerals) weakens it - even under hydrostatic stress (ref 7). This is especially true if it is a highly anisotropic phenomenon (eg. like the swelling of montmorillonite clays due to adsorbed water, or thermal expansion of calcite). But it won t normally fail under hydrostatic stress, unless you reach the crush pressure which is very high in coal. However, thermal cycling under uniaxial strain will similarly weaken the rock, but now you can observe the failure at the same time because you are loading the core differentially (ie, axial stress different from confining pressure). It is not the stress state itself that weakens the rock, but the thermal activation (or CO 2 adsorption) does that via the heterogeneous/anisotropic response at the mineral/maceral level. 6

7 Steve Bauer at Sandia did a thermal cracking PhD dissertation. This indicates for granites that thermal cycling (unconfined, to high temperatures in the range C) reduces the UCS by about 50%, due to microcracking caused by differential thermal expansion at the grain scale. However, he notes that a confining pressure of 2900 psi or more is sufficient to eliminate this effect of the microcracks on strength. For comparison, the mean effective stress in San Juan basin is about 1100 psi, which is a lot smaller then 2900 psi. One might think that it might take less stress to eliminate microcracks in softer coal, but the crack inducement may be a lot stronger in coal because it is more heterogeneous. So more lab testing needs to be done to see how CO 2 -induced cracking depends on in-situ stresses acting on coal. This information may be used by analogy, because the effect of a large temperature change (600 C) on strong rock (granite UCS ~30,000 psi) would likely be similar to a much smaller temperature change on a much weaker rock (coal). In the CO 2 lab test at SIU we are not dealing with temperature change of course, but rather with swelling induced by CO 2, and we would have to convert this to an equivalent temperature change to take this approach any further. The important thing is that cracking by thermal cycling in granite, and weakening of this very strong rock, is an analog to the situation of CO 2 adsorption/cycling in coal and the weakening of coal. Implications for the field: This has important implications for CO 2 sequestration in the field, since most fields are likely to be under uniaxial strain or close to it. If coal fails easily and early when CO 2 is injected, it is likely to affect the permeability of the formation encountered by the CO 2, and the injectivity at the well. For example, shear failure in brittle rock is generally accompanied by dilatancy, when the permeability of the rock is increased (although this may be mitigated by fines creation and plugging). A net permeability increase would counteract the swelling of the coal by CO 2 adsorption, and the injectivity at some point may increase rather than decrease. This could be very desirable. If failure of coal is induced by CO 2 injection, we will need to improve the current reservoir models for perm change and injectivity, both of which are important to describe correctly. The current reservoir modeling of CO 2 sequestration, which has reached a high level of sophistication, may be inadequate. Role of huff-and-puff: It has been reported (refs 8, 9) that huff-and-puff cycles of CO 2 injection caused coal cores to fail in the lab, and that more cycles gave more breakage (each cycle was a few days). Some experiments were done to explore if CO 2 huff-n-puff could be used to enhance near-wellbore permeability, by inducing coal to fracture. Some coals are fragile enough that the swelling and shrinking of coal by CO 2 adsorption and desorption can induce sufficient stress to cause the coal to fall apart. Each cycle was a few days: injection followed by time to reach equilibrium. The failure seemed to be progressive, 7

8 and became more apparent after several cycles. None of this was published, but the idea was transmuted into a patent, by Raj Puri, as described in Appendix B. This information supports the thermal cracking analog discussed above. Each cycle of CO 2 injection would be accompanied by differential swelling, and induce microcracks, but possibly in different local sites than the time before. That is, the extent of failure is likely to be increased during each cycle of huff-and-puff. In a micropilot in China, a well produced better after a huff-and-puff treatment with CO 2, and this may indicate failure of coal (ref 10). In a CO 2 sequestration project in Japan, there is evidence for an injectivity increase over time (ref 11). Further study should be directed at finding the best procedure for sequestering CO 2 in coal, eg. it might be a series of huff-and-puff injections, optimally timed to increase the coal failure, and boost the injectivity. Conclusions: 1. We have offered explanations for the behavior of coal under CO 2 saturation. These explanations need to be confirmed by further lab testing which is carefully designed. 2. The double-u curves of UCS versus rank provide a guide to coal strength when no core tests are available. The lab UCS measurements in this report suggest the double-u curve can sometimes be an overestimate of UCS in cores. 3. Coal that has been saturated with CO 2 in the lab (but then the CO 2 removed) becomes stronger (UCS measurement) because it has dried, and the removal of the water lubrication effect dominates the CO 2 weakening due to differential expansion. 4. Coal fails very easily and very early in the lab when CO 2 is adsorbed under uniaxial strain conditions. Differential expansion occurs on a local scale due to the heterogeneous and anisotropic nature of coal. This can cause localized failure. If enough localized failures occur, they can link up and cause gross failure. 5. Studies have been made of coal samples subject to CO 2 sorption, and examined under X-ray CT. Cracks developed between coal and non-coal units, and hairline cracks appear in the coal units after CO 2 cycles. A hairline crack is all that is needed at a localized failure site, before many such sites are linked up by stress increase to maintain a uniaxial strain condition. 6. Two Australian papers give interesting results on CO 2 -induced failure and permeability changes. The paper concludes that weakening by CO 2, failure by insitu stresses, and permeability increases will not be an issue for sufficiently deep coal seams. However, this is based on lab tests of very weak lignite, and its risky to apply the same conclusion to much stronger bituminous coals, such as HVB, HVA, or MV. Further study of how CO 2 weakening depends on coal rank and insitu stress will be important. 7. Cracking by thermal cycling in granite, and weakening of this very strong rock, is an analog to the situation of CO 2 adsorption/cycling in coal and the weakening of coal. 8

9 8. This has important implications for the field, which is likely to be under uniaxial strain: if coal fails easily and early when CO 2 is injected, it is likely to affect the permeability of the formation encountered by the CO 2, and the injectivity at the well. The injectivity may increase rather than decrease. 9. The reservoir modeling of CO 2 sequestration, which has reached a high level of sophistication, may be inadequate, and may need to be corrected. 10. A basic theoretical/modeling study needs to be done to understand coal failure, and to predict the effect of permeability changes due to coal failure induced by CO 2 injection (as well as perm changes due to swelling, which are fairly well understood). This will lead to improved reservoir modeling of CO 2 injectivity, which is of paramount importance to the economic success of CO 2 sequestration. 11. The study should also be directed at finding the best (optimal) procedure for sequestering CO 2 in coal, eg. it might be a series of huff-and-puff injections that increase the coal failure, and boost the injectivity. 12. A program of study needs to be defined and funded to study the many aspects of this whole issue. References: 1. Harpalani, S. Core flooding experiments, Presented at Coal-Seq V, Houston, November, Palmer, I. Geomechanical analysis of core flood experiments, Presented at Coal-Seq V, Houston, November, Keleman, S., private communication, Zheng, Khodaverdian, and McLennan, Static and dynamic testing of coal specimens, SCA Conf, Paper 9120, Van Niekerk and Mathews, Solvent swelling of similar rank South African vitrinite-rich and inertinite-rich coals: swelling extent and maceral influence, to be published, Karacan, O., private communication, Higgs, N., private communication, Unpublished Amoco work 9. Puri, R., private communication, Gunter et al, Report on CO 2 micropilot in China injectivity may increase 2004 or 2005? 11. Fujioka, M. Yubari project, Coal-Seq V, Houston, November, Karacan, O. Heterogeneous sorption and swelling in a confined and stressed coal during CO 2 injection, Energy and Fuels, 17, , Karacan, O. Swelling-induced volumetric strains internal to a stressed coal associated with CO 2 sorption, Intl J. Coal Geology, in press, Viete, D.R. and Ranjith, P.G. The effect of CO 2 on the geomechanical and permeability behavior of brown coal: implications for coal seam CO 2 sequestration, Int J. of Coal Geology, 66, , Viete, D.R. and Ranjith, P.G. The mechanical behavior of coal with respect to CO 2 sequestration in deep coal seams, Fuel, in press,

10 Acknowledgements: We have benefited from discussions on the subject of coal failure with Zissis Moschovidis (who first suggested the CO 2 weakening), Nigel Higgs (who suggested the thermal cracking idea), Jonathan Matthews (who provided the solvent swelling information), Ozgen Karacan (who provided the 2D pictures of core cracking), John Olson, John McLennan, and Wes Martin. Ian Palmer (Higgs-Palmer Technologies) 23 August 2007 On behalf of Ian Palmer (Higgs-Palmer Technologies) and Satya Harpalani (Southern Illinois University) Appendix A: Details of second set of UCS results by TerraTek. The figure shows X-ray images of the whole core, and cross-sections. The CO 2 core appears to have more cleats, although most of these are calcite-filled. This core is also a little longer, and a bit less dense (see Table 1). All these effects would tend to make the CO 2 core weaker, but in fact it is stronger, so we can rule out these effects as contributors to the strength discrepancy. Appendix B: Extracts from patent number 5,014,788 10

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