Effect of Carbon Sequestration and Reservoir Conditions on Brittle Creep in Etna Basalt
Effect of Carbon Sequestration and Reservoir Conditions on Brittle Creep in Etna Basalt
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碳封存和储层条件对埃特纳火山玄武岩脆性蠕变的影响
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发表时间：
2022
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通讯作者：
E. Mansbach
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作者：
E. Mansbach
帮助应对大气中二氧化碳浓度不断增加的一种工具是地质碳封存(GCS)，即二氧化碳溶解在水中或加压成超临界流体，然后注入地表以下。如果注入部位含有含有二价阳离子的矿物，酸性流体可以通过矿物溶解释放这些原子，并沉淀碳酸盐矿物，将二氧化碳困在矿物结构中。这个过程已经在冰岛的CarbFix计划中发生了，在那里，水中溶解的二氧化碳被隔离在玄武岩中。然而，注入过程和矿物的溶解可能会削弱岩石，这可能导致诱发地震活动。在地壳中，当岩石在很长一段时间内被加载到低于峰值强度的应力时，就会发生随时间变化的脆性变形，这一现象最近引起了科学家的兴趣，可能是地震前的一种机制。以前的研究表明，岩石会经历亚临界裂纹扩展，导致一种称为脆性蠕变的现象，可分为三个阶段：(1)初级减速蠕变；(2)次生恒速蠕变；(3)第三级，加速蠕变。在这个项目中，我对埃特纳玄武岩样品在不同有效压力和不同孔隙流体下变形，以确定固碳和储层条件对脆性蠕变的影响。我们的结果表明，埃特纳玄武岩的应变率不会随着额外的载荷而显著增加，直到样品达到体积膨胀。这与以前只在高应力下进行的实验形成了鲜明对比，以前的实验表明应力和应变率之间存在高度相关性，而冰岛玄武岩上的实验即使在低应力下也表现出应力敏感行为。此外，孔隙流体中二氧化碳的存在和有效压力的变化并没有导致应变率的显著变化。我们认为，应力不敏感可能是由于低应力下的小裂纹密度，在样品积累更大的应变水平之前，不会引发裂纹-裂纹相互作用和更高的应变率。我们的结果还质疑了加载历史对应变率的作用，以及高应力下的脆性蠕变实验是否允许样品在附加加载之前达到稳态蠕变。
One tool to help combat the increasing concentration of carbon dioxide in the atmosphere isgeological carbon sequestration (GCS), in which carbon dioxide is dissolved in water orpressurized to a supercritical fluid and injected below the surface. If the injection site containsminerals with divalent cations, the acidic fluid can liberate these atoms through mineral dissolutionand precipitate carbonate minerals, trapping carbon dioxide in the mineral structure. This processis already occurring at the CarbFix program in Iceland, where dissolved carbon dioxide in wateris being sequestered into basalt. The injection process and dissolution of minerals, however, mayweaken the rock, which can lead to induced seismicity. In the Earth’s crust, time dependent brittledeformation transpires when rocks are loaded to stresses below their peak strength for extendedperiods of time, a phenomenon that has recently become of interest to scientists as a possible preearthquake mechanism. Previous studies have shown that rocks can undergo sub-critical crackgrowth leading to a phenomenon known as brittle creep, which can be divided into three stages:(1) primary, decelerating creep; (2) secondary, constant creep; and (3) tertiary, accelerating creep.In this project, I deform samples of Etna Basalt at different effective pressures and with varyingpore fluids to determine the effect of carbon sequestration and reservoir conditions on brittle creep.Our results show that Etna Basalt strain rates do not significantly increase with additional loadinguntil the samples reach bulk dilation. This is in stark contrast to previous experiments conductedonly at high stresses, which show a high correlation between stress and strain rate, and work by onIceland Basalts, which show stress sensitive behavior even at low stresses. Additionally, thepresence of carbon dioxide in the pore fluid and changes in effective pressure did not lead tosignificant alterations in the strain rates. We propose that the stress-insensitivity may be due tosmall crack densities at low stresses that will not instigate crack-crack interactions and higherstrain rates until the sample accumulates larger levels of strain. Our results also call into questionthe role of loading history on strain rates and whether brittle-creep experiments at high stressesallow samples to reach a steady-state creep prior to additional loading.