Carbon utilization and storage through groundwater well restoration.

Abstract

This invention relates to opportunities for carbon capture, utilization, and storage (CCUS) through well remediation. Aqua Freed® and Aqua Gard® are well-known technologies for well remediation and preventative well maintenance that utilize liquid CO2 (planned underground injection). The objective of this invention is to quantify the storage capacity of Aqua Freed® and Aqua Gard® and establish the credibility of CCUS.

Description

[Technical Field]

[0001] The present invention relates to the increase in atmospheric carbon dioxide (CO2) concentrations due to anthropogenic activities, which is considered one of the main causes of climate change, and to methods for offsetting carbon footprints. [Background technology]

[0002] The rise in atmospheric carbon dioxide (CO2) concentrations due to human activities is considered one of the main drivers of climate change. 1,2 The need to offset carbon footprints and move to "net-zero" emissions is becoming widely recognized. 3,4 The Intergovernmental Panel on Climate Change (IPCC) has reported that to limit global warming to 1.5°C above pre-industrial temperatures, the world needs to achieve net-zero emissions by 2050. 5 In response, most of the world's largest companies have set "net zero" targets, which means their own timelines for becoming carbon neutral. 6,7 These ambitious goals include: (1) transitioning to low-carbon technologies and "green" energy sources; 8,9 and / or (2) carbon offsetting, i.e., removing an amount of CO2 equivalent to one's carbon footprint. 10 , can be achieved by

[0003] One of the main methods of carbon offsetting is to capture CO2 from industrial waste or directly from the atmosphere and keep it out of the atmosphere through reuse, recycling, storage, and ultimately sequestration. 11 Although there are several natural and technological ways to achieve sequestration (e.g., marine-related, rock weathering, mineralization, geological-related, etc.), of these methods, geological sequestration of CO2 is considered the most promising in terms of storage capacity, timescales, and environmental impacts. 12 .

[0004] It is often more practical and desirable from a commercial perspective to use and recycle CO2 before storing it further away. 13 Carbon capture, utilization, and storage (CCUS) is an umbrella term used to refer to any technological process that captures, utilizes, and stores CO2 away from the atmosphere. 14 The most widely known CCUS technology is CO2-enhanced oil recovery (CO2-EOR), in which CO2 is injected underground to support the tertiary recovery of otherwise difficult-to-produce crude oil. Other known CCUS technologies include CO2-enhanced geothermal systems (which use CO2 instead of water as the working fluid) and CO2-enhanced gas capture (which uses CO2 to enhance the recovery of natural gas from the ground). All of these technologies involve injecting CO2 underground for technical and commercial advantages. In the process, some of the CO2 unintentionally remains in the ground.

[0005] This invention introduces another CCUS possibility: the use of CO2 for groundwater well rehabilitation and preventive maintenance. All groundwater wells accumulate mineral and biological material over time, resulting in a decline in the well's water-producing capacity (specific capacity) and water quality. The traditional approach is for well owners to rehabilitate their wells when their specific capacity has significantly declined. However, research has shown that annual, planned preventive maintenance can extend the life of well components, maintain better water quality, and reduce long-term operating costs. 15 .

[0006] The benefits of using CO2 to improve well performance and aid maintenance have been recognized in the oil and gas industry. For example, Wilson and Bell 16 proposed a method combining liquid CO2, alcohol, and surfactants for in-well and near-well cleaning of gas wells. In addition to well cleaning, CO2 has also been shown to offer benefits when used as a fracturing fluid for stimulation of low-permeability gas reservoirs. 17-19 .

[0007] Technologies using CO2 for groundwater well rehabilitation and maintenance include the Aqua Freed® Process and the Aqua Gard® Process 20,21 These processes have been established through the application of CO2 to groundwater well systems throughout the United States and in parts of Europe for over 35 years. The use of CO2 for well remediation has demonstrated advantages over other methods (e.g., conventional chemical and acid treatments, wire charging, fluid shock, etc.) in terms of well performance as well as impact on water quality. 22 The objective of this study is to quantify the carbon utilization and storage potential of CO2-powered well remediation technologies through analysis and modeling of field-collected data, using the examples of Aqua Freed® (AF) and Aqua Gard® (AG). Using field measurements and a statistical modeling approach, we first estimate the fraction of injected CO2 stored in aquifers in a single well based on data from a selected set of wells. We then apply these estimates to a larger database to estimate the total amount of CO2 stored by these technologies over the past decade. Finally, we estimate the storage potential of this CCUS technology based on the market size of private and public groundwater wells in the United States. Summary of the Invention

[0008] We present attractive CCUS opportunities through well remediation. Aqua Freed® and Aqua Gard® are well-known well remediation and well preventative maintenance technologies that utilize liquid CO2 (injected underground) for this purpose. The objective of this invention study is to quantify the storage capacity of Aqua Freed® and Aqua Gard® and establish the reliability of CCUS. Depending on the well being serviced, these technologies can inject up to 40 US tons of CO2 per well. Based on field-collected data and statistical modeling, it was estimated that 82 to 96 percent (median 90%) of the injected CO2 remains underground after injection. Overall, the applicant's results and analysis of the U.S. market suggest that the use of CO2 in well remediation and maintenance could potentially store several megatons of CO2 per year in the United States alone.

[0009] A method for utilizing CO2 for aquifer storage is disclosed, which uses CO2 for well rehabilitation and preventative maintenance to improve well performance and efficiency and reduce the carbon footprint. The method includes sealing a water well and injecting liquid and / or gaseous CO2 into the water well, and unsealing the water well to allow for CO2 storage in the water well aquifer formation.

[0010] The method further includes modifying the Aqua Freed or Aqua Gard technology to achieve and enhance CO2 storage in the well aquifer.

[0011] The method increases CO2 infiltration into the well aquifer by controlling and maintaining a constant CO2 injection pressure to prevent backflow and upward migration of the CO2 / water mixture within the well.

[0012] The process further includes adjusting the CO2 / water mixture to minimize expansion of the CO2 in the water and allow for deeper penetration of the CO2 into the aquifer.

[0013] The process further includes achieving additional CO2 storage by reducing the amount and rate of water flowing from the well after CO2 injection is complete.

[0014] The invention further includes the step of preventing CO2 from escaping from the well into the surrounding atmosphere, thereby allowing the well pressure to dissipate naturally within the well's aquifer. [Brief explanation of the drawings]

[0015] [Figure 1] Figures 1A, 1B, 1C, and 1D are images of the well screen taken during an on-site video inspection conducted before and after treating the well with CO2. [Figure 2] Figures 2A and 2B show the CO2 concentrations observed during flushing for 16 well events (dataset 1). [Figure 3] Figure 3 shows the CO2 concentration profile (mg / l or ppm) over time during flushing for 16 well events (dataset 1). [Figure 4] Figure 4 is a plot showing the fit of the LME model to Dataset 1. [Figure 5] Figure 5 shows a histogram of estimates of the fraction of CO2 stored by individual wells for a total of 31 well events (dataset 1 and dataset 2 combined). [Figure 6] Figures 6A and 6B show the statistical distribution (histograms) of CO2 utilized in STI operations in 2021, based on individual wells. The histogram on the left represents the AF treatment, and the histogram on the right represents the AG treatment. [Figure 7] Figures 7A and 7B are graphs showing the annual distribution of the number of wells serviced by STI and its licensees and the estimated amount of CO2 injected. [Figure 8] Figures 8A and 8B are histograms of the amount of CO2 injected during individual AF treatments performed by STI from 2012 to 2021. Detailed Description of the Invention

[0016] Figures 1A, 1B, 1C, and 1D show in situ photographs of the interior of a groundwater well before and after AF treatment. The photographs clearly show the extent of mineral, biogenic, and iron deposit accumulation. CO2-powered AF remediation has been shown to increase the specific capacity of the well by up to 20-fold. 22 .

[0017] The amount of CO2 injected depends on the size of the well and its depth. The exact amount for a particular well is based on a proprietary formula that takes into account well depth, well diameter and borehole / screen length, but typical values ​​are 2.5-5 short tonnes of CO2, 6-12 short tonnes of CO2 and 20-40 short tonnes of CO2 for small, medium and large wells, respectively.

[0018] Both AF and AG follow the process described above. The main difference between the two is that AG is a preventative care program for wells after AF treatment, so it requires less CO2 (typically about one-fifth) for the same well compared to AF.

[0019] Figures 1A, 1B, 1C, and 1D are images of the well screen taken during an on-site video inspection conducted before and after treating the well with CO2.

[0020] <Approach to estimating storage potential> Since the amount of CO2 injected into the well is known, the most reliable way to quantify how much CO2 has been stored is to estimate the amount of CO2 lost at each step of the AF / AG process described above. By subtracting the total amount of CO2 lost throughout the process from the amount injected, we can directly derive how much CO2 remains in the ground after the process is complete and the well is returned to its owner.

[0021] After a detailed review of the process, it was concluded that CO2 losses can occur during the venting, flushing, and post-injection pump test stages. Usually, flushing is performed immediately after the post-injection pump test. Therefore, the loss during post-injection pumping can be added to the loss during flushing. Therefore, the mass balance equation for CO2 in the AF / AG process is: Stored CO2 x Injected CO2 - {CO2 Loss During Venting x CO2 Loss During Flashing}.

[0022] <Result> (Estimated loss amount) Vent Data during the venting process were collected using flow meters in two separate wells undergoing AG treatment. The venting time for the first well was 1 minute, with a flow rate of 100 ft for the first 30 seconds. 3 □ minutes, 25ft for the remaining 30 seconds 3 The density of CO2 was 1.87 kg / m 3 Converting the volume of CO2 to mass, assuming a pressure of ~15°C and 1 atmosphere, the mass of CO2 lost during venting was estimated to be 7.2 lbs. Since the mass of CO2 in this well was 2000 lbs, the percentage of CO2 lost during venting was estimated to be 0.36%. Following the same procedure, the percentage of CO2 lost during venting in the second well was estimated to be 0.39%. Venting timing in other wells was observed to be in a similar range. Therefore, venting losses were expected to be less than 1% in all cases and were assumed to be negligible in storage volume estimates.

[0023] Flushing (including post-treatment pump test) As part of the present study, CO2 concentrations and pumping rates during flushing were recorded for 16 wells undergoing the AF / AG process (referred to as Dataset 1). Additionally, historical data was obtained from STI records for 15 additional wells with pumping rate data but no concentration measurements (referred to as Dataset 2). Dataset 1 (with two or more concentration data points per well) was used to model the trend of concentration decline over time. Visual inspection of concentration versus time plots showed an exponential trend for most wells. Therefore, the natural logarithm of these values ​​is expected to follow a linear trend. The distribution of concentration data collected during flushing for the 16 well events (Dataset 1) is shown in Figures 2A and 2B.

[0024] Figures 2A and 2B show the CO2 concentrations observed during flushing for 16 well events (dataset 1).

[0025] Linear mixed effects (LME) regression was used to determine the overall trend and rate of concentration decline during the flushing process. 23 We chose this approach because it is widely used in multilevel or hierarchical modeling, where both population-wide trends and subgroup-level trends are important. 24,25In this case, the LME model takes into account both trends in individual wells (called "random" effects) and trends across the population (called "fixed" effects). A detailed discussion of model selection and cross-validation is provided in the Methods section and Supplementary Information. Relevant statistics for the LME regression model fitted to Dataset 1 are listed in Table 1. Figure 3 shows the resulting regression fits for the individual wells (random) and the population fits (fixed), which convert the natural logarithms back to the actual concentrations. The goodness of fit is graphically depicted in Figure 4. The plot of fits versus observations shows that the points are evenly spaced along the diagonal (y = x), indicating that the model predictions closely match the observed data. The residual plot shows that the points are randomly scattered around the residual = 0 line without any particular pattern, indicating that the LME model is the right choice for this dataset.

[0026] Table 1. Results of a linear mixed-effects (LME) model fitted to trends in CO2 concentrations during flushing for 16 individual wells (Dataset 1). [Table 1]

[0027] Using the individual concentration trends from Dataset 1, the population concentration trends from Dataset 2, and the known volumetric flow rates during flushing for these wells, the mass of CO2 lost over time is simply calculated as the mathematical product of concentration (mass per volume), flow rate (volume per time), and flushing time. This gives the total mass of CO2 lost during flushing, which, when compared to the mass of CO2 injected, yields the effective fraction of CO2 stored underground for that well. Estimates of the fraction of CO2 stored in each of the 31 wells from Dataset 1 and Dataset 2 are shown in Table 2.

[0028] (Estimation of storage volume) Estimated storage volume per well The general statistical distribution across the entire database of well events is shown in Figure 5 and Table 3. Based on the combined set of 31 wells, the median estimate of the fraction of CO2 remaining or stored underground after AF or AG processing is approximately 90%, with an interquartile range of 82% to 96%. This means that in at least three-quarters of groundwater well treatments with CO2 via AF / AG processes, ~82 percent (typically ~90%) of the injected CO2 mass can be stored underground after the treatment process is complete.

[0029] Figure 3. CO2 concentration profiles (in mg / L or ppm) over time during flushing for 16 well events (Dataset 1). Colored circles represent actual data points for each well, and colored lines represent regression fits to these data points using an LME model (converted from natural log to real values). Thick black lines represent "fixed" effects or population trends, while colored dashed lines represent "random" effects or individual well trends.

[0030] Estimated storage volume for 2021 The total amount of CO2 utilized (injected underground) by well rehabilitation and maintenance services completed by STI in 2021 was 700,600 pounds (350.3 US tonnes). The statistical distribution based on individual wells serviced with AF and AG treatment in 2021 is shown in Figure 6. Based on the interquartile range of individual well storage rates calculated above, the estimated mass of CO2 stored by STI's operations in 2021 is 287-336 US tonnes. These estimates represent STI's operations only and do not include CO2-powered well treatments performed by STI's licensees.

[0031] Estimated storage volume for the 10-year period from 2012 to 2021 An estimate of the total amount of CO2 utilized in CO2-powered well rehabilitation and maintenance treatments (AF and AG) over the past 10 years (2012-2021) was made for all wells serviced directly by STI or by its licensees. To the applicant's knowledge, no other entities operate with this technology for groundwater well rehabilitation. The annual breakdown of the number of wells serviced and the tons of CO2 utilized is shown in Table 4 and graphically in Figure 7.

[0032] While the exact amount of CO2 injected by STI into each well over the past 10 years was known, information about AF licensees was unavailable beyond the number of wells they serviced. However, we can assume that the median and interquartile range of CO2 injection by AF licensees are comparable to those for AF treatments conducted directly by STI. Figure 8 shows the statistical distribution of CO2 amounts utilized in the 477 AF treatments conducted by STI over the past 10 years. The median of the distribution was 2 tonnes, with an interquartile range of 0.65–3.5 tonnes of CO2. The estimates in Table 4 for the tonnage of CO2 utilized by AF licensees were calculated based on a median of 2 tonnes. The total range of CO2 tonnage (based on an interquartile range of 0.65–3.5 tonnes) was estimated to be 2,761–5,668 tonnes of CO2. Therefore, assuming a median storage amount of 90%, the estimated cumulative amount of CO2 stored by STI and its licensees over the past 10 years (2012-2021) ranges from 2,485 to 5,101 short tonnes (median 3,724 short tonnes).

[0033] Future predictions According to the National Ground Water Association (NGWA), there are approximately 16 million wells in total in the United States. 26 Approximately 13 million of these wells are residential, and the remainder are non-residential (industrial, public water, agricultural, etc.). Based on STI's experience, residential wells are typically small wells, and collectively, AF treatment requires approximately 1,000 pounds of CO2. Non-residential wells are divided into three categories based on size and capacity: small, medium, and large.

[0034] 4A and 4B are plots showing the fit of the LME model to Dataset 1.

[0035] Figure 5 shows a histogram of the estimated fraction of CO2 stored in individual wells for a total of 31 well events (dataset 1 and dataset 2 combined).

[0036] The amount of CO2 required for AF treatment is 2.5-5 tonnes, 6-12 tonnes, and 20-40 tonnes for small, medium, and large wells, respectively. Subsequent annual maintenance AG treatment typically requires about one-fifth that amount.

[0037] Based on CO2 requirements and STI's database of treated wells over the past 10 years (see Figure 8), the ratios for small, medium, and large wells are 89%, 8%, and 3%. These ratios are considered conservative because they include both residential wells (which are typically smaller and more numerous) and non-residential wells. However, due to a lack of information, we assume the same ratios apply to non-residential wells across the United States.

[0038] Based on the number of wells in the United States and the typical amount of CO2 required for rehabilitation and annual maintenance, it is estimated that implementing CO2-powered well rehabilitation and maintenance technologies could potentially harness and store approximately 20 million short tons of CO2 every 15 years (the average rehabilitation cycle for a water well) (see Table 5). Annual maintenance programs like AG could support the storage of an additional 4 million short tons per year. These are estimates of the utilization potential for a single country.

[0039] <Discussion> Storage vs. Sequestration While the terms storage and sequestration are often used interchangeably, it is important to note that there are subtle differences between them. Storage is a broad term, while sequestration is a subset of storage, usually referring to long-term, "permanent" storage (typically hundreds to thousands of years into the future). 27,28The focus of this study was on quantifying CO2 storage (what remains underground after the process is complete and the well is returned to the owner). Investigating the long-term fate and migration of injected CO2 is a more complex area of ​​research and would require multiphase geochemical reactive transport modeling of site-specific case studies (similar examples can be found in the literature). 29-31 Subsequent studies may aim to address these research directions.

[0040] Long-term fate of CO2 Typically, once CO2 is injected, it remains in the ground either as a separated gas / supercritical phase, a dissolved water phase, or a precipitated mineral phase.

[0041] Table 2. The database of 31 AF / AG events used to calculate CO2 loss during flushing, along with an estimate of the percentage of CO2 stored during each event, is shown in this table. The first 16 well events considered Dataset 1 (rows marked "yellow") were conducted as part of this study, and CO2 concentration measurements over time and flow information were available. The next 15 well events (rows marked "blue") were taken from STI records prior to this study, and only flow information was available for these events. [Table 2]

[0042] A common mode of underground CO2 trapping is geological or structural gas trapping (gas trapping between strategic geological structures such as faults, seals, and caprocks). 32,33 , residual gas trapping (residual gas that cannot be removed is trapped in the pores by capillary forces) 34,35 , dissolution trapping (trapped in solution as bicarbonate) 36,37 , and mineral trapping (converted to carbonate mineral phases) 38,39The relative proportions of CO2 stored by each of these individual mechanisms may depend on the exact site-specific hydrogeological conditions. In this application, the relatively small amount of CO2 injected compared to CO2-EOR or deep brine sequestration is expected to result in a relatively low structural gas trapping capacity. Nearly all of the stored CO2 will be converted relatively instantaneously to soluble CO2. The increased acidity caused by this soluble trapping is geochemically buffered by in situ water-rock reactions, which consume the dissolved CO2 and may convert some of it to secondary precipitated minerals. Initially, when fluid pressures are high, especially near the well, CO2 can dissolve in water to form bicarbonates. As pressure decreases, especially at near-surface depths, the bicarbonates may revert to gaseous CO2. 40 Therefore, more sequestration is expected in deep wells and aquifers confined by seal layers compared to shallow, unconfined ones. Depending on the mineralogy of the aquifer, dissolution of minerals (especially carbonates) in the region near the well may lead to wormhole formation. 41 may be created or preferential flow paths such as faults and fractures may be enhanced. 42 On the other hand, some of the dissolved CO2 is trapped in faults, cracks, and deep underground. 43,44 It may precipitate as carbonates in the atmosphere, as evidenced by many outcrops. 45,46 Some of the gaseous CO2 may infiltrate the vadose zone and soil. Depending on the level of vegetation in the area, some of the CO2 may be consumed by plants. Thus, the net sequestration rate from the amount of CO2 initially stored may vary from site to site. Several case studies conducted under different hydrogeological conditions may help to draw generalizations.

[0043] Table 3. Statistics of estimates of fraction of CO2 stored from the database of individual wells. The median estimate of storage across the set of 31 wells is ~90%, with an interquartile range of ~82% to 96%. [Table 3]

[0044] Figure 6A and Figure 6B. Statistical distribution (histograms) of CO2 based on individual wells utilized in STI operations in 2021. The histogram on the left represents the AF treatment, and the histogram on the right represents the AG treatment.

[0045] Impacts on Groundwater Quality. Undisturbed, naturally occurring groundwater is usually found to be in chemical equilibrium with the solid phase (rock minerals) of the aquifer. However, when CO2 is added to an aquifer, it dissolves in the groundwater, lowering the water's pH (increasing its acidity), and disturbing geochemical equilibrium. The resulting geochemical reactions (called buffering) between this acidified groundwater and the aquifer minerals can then result in the migration of certain trace metals (including lead and arsenic). Studies have shown that concentrations of these migrated trace metals never reach or exceed maximum allowable levels, except in rare cases where the aquifer is naturally rich in these minerals and concentrations in the groundwater are already high. 47-49 .

[0046] Use of recycled CO2. To have a greater impact on climate change, it is crucial to obtain injected CO2 by "capturing" it from anthropogenic sources (such as coal-fired power plants and other industries). CO2 capture technologies have been around for decades, but economic challenges (high costs) have been the main obstacle preventing widespread commercial adoption of these technologies. 50 However, the global drive towards carbon neutrality has significantly driven research and development into these technologies, with some early-stage commercial-scale efforts underway. 51 Direct air capture of CO2, which addresses capturing CO2 directly from the atmosphere rather than from a central source such as a chemical plant, is a challenging but important area of ​​future technology. The use of recycled CO2 is essential to achieving net-zero emissions, and techno-economically efficient CO2 capture methods are a first step in that direction.

[0047] <Conclusion> An analysis and review of the CO2 utilization and storage potential of Aqua Freed® and Aqua Gard® technologies demonstrated the benefits of using CO2 for well remediation and preventive maintenance to improve well performance and efficiency and reduce carbon footprint. Thus, injecting CO2 underground for well remediation is another proven method for achieving CCUS and moving closer to net-zero emissions. It is typically estimated that approximately 90% of injected CO2 remains underground. If CO2-powered well remediation were to become widely adopted as a CCUS technology, it could potentially store several megatons of CO2 annually.

[0048] Table 4. Annual distribution of CO2-powered well rehabilitation and maintenance activities over the past 10 years (2012–2021). An estimated 4,100 tonnes of CO2 were utilized (injected underground). [Table 4]

[0049] Figures 7A and 7B graphically represent the annual distribution of the number of wells serviced by STI and its licensees and the estimated amount of CO2 injected.

[0050] <Method> (Field data collection) Losses during venting were estimated by placing a flow meter in the vent during the venting process. To estimate losses during flushing, CO2 concentrations were measured over time in the field as groundwater was flushed during AF / AG treatment in 12 different wells treated during the study period. The procedure for measuring CO2 concentrations in the field was as follows:

[0051] Measure 100 mL of water using a graduated measuring cylinder and transfer it to the flask.

[0052] Add one packet of phenolphthalein indicator powder and allow to dissolve.

[0053] Use a handheld digital titrator equipped with a 3.636N cartridge.

[0054] Add sodium hydroxide a few drops at a time and stir until the water turns pale pink.

[0055] Figures 8A and 8B are histograms of the amount of CO2 injected during individual AF treatments performed by STI from 2012 to 2021.

[0056] Table 5. Estimated national capacity for CCUS through water well rehabilitation and maintenance. [Table 5]

[0057] · Read the digital titrator reading and multiply by the appropriate multiplier depending on the amount of water measured and the desired range.

[0058] In addition to CO2 concentrations, the drainage rate was also recorded when the well was flushed.

[0059] (Statistical Data Modeling) To model the trends in CO2 concentrations during the flushing phase of AF / AG well events, we use a linear mixed-effects regression approach. The linear mixed-effects (LME) model is a simple linear regression (SLR) model. 25 The adoption of the LME approach has proven useful when collected data can be divided into meaningful subgroups and has been widely applied in biological, social, and health care research. 24 In this study, the subgroups are individual well data. The LME model estimates well-specific regression coefficients in conjunction with population-level regression coefficients, thus deriving not only overall trends (across all well events) but also well-specific trends (Table 1).

[0060] It is important to note that the LME approach is significantly different from approaches that estimate well-specific trends individually and then estimate the overall trend separately. The LME model estimates the "distribution" of well-specific random effects relative to population-level fixed effects. Therefore, the fixed effect parameters (slope and intercept) estimated by LME may differ from the slope and intercept values ​​obtained by the SLR model.

[0061] For data that can be aggregated by group, some of the advantages of using LME over SLR are:

[0062] · LME uses data more efficiently and is more scalable to a larger number of wells.

[0063] These features allow it to work well with unbalanced or small datasets (by avoiding convergence issues) and to accommodate missing observations (e.g., missing concentration measurements at specific time points).

[0064] LME jointly estimates population and subject eigenvalues, resulting in a more accurate (and easier to interpret) representation of repeated measurements data. Correlations between repeated scans from a subject are naturally taken into account. This naturally prevents overfitting.

[0065] Because the concentration data for well events from Dataset 1 are exponential, the regression model used the natural logarithm of the concentrations (which are linear). Four regression models (three different LME combinations and one SLR for comparison) were tested and cross-validated to identify the model that best represented the data.

[0066] Model selection: 1. LME: Both the slope and intercept vary across both fixed and random effects 2. LME: Fixed effects vary both the slope and intercept, while random effects vary only the intercept. 3. LME: Fixed effects vary both the slope and intercept, while random effects vary only the slope 4. SLR: Apply simple linear regression to the concentration data of all well events from the combined dataset 1 (for comparison with LME)

[0067] Cross-validation is one of the most common statistical methods widely used in model validation. 52,53 Essentially, cross-validation can indicate how well a selected model can generalize to "unknown" data. Details and results of model selection and cross-validation are provided in the Supplementary Information. Model 2 was found to have the smallest "test" error (meaning it generalized the data best) and was selected for further analysis in the Results section.

[0068] All modeling was done using MATLAB software. 54 The code for this study is based on original code developed by Sharma (see github). Table 2. This table shows the database of 31 AF / AG events used to calculate CO2 loss during flushing, along with an estimate of the percentage of CO2 stored during each event. The first 16 well events, considered Dataset 1 (rows marked "yellow"), were conducted as part of this study, for which CO2 concentration measurements over time and flow information were available. The next 15 well events (rows marked "blue") were taken from STI records prior to this study, for which only flow information was available.

[0069] Some wells have been decommissioned due to low water levels, sand buildup, poor water quality, and contamination. Decommissioned water wells can be used for carbon storage. This includes water supply wells, environmental restoration and purification wells, injection wells, and ASR wells. Using this technology on decommissioned wells would eliminate the need for venting, flushing, and purging, and could store nearly 100% of the injected CO2.

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Claims

1. CO in the aquifer ２ 1. A method for storing a liquid containing -CO for well rehabilitation and preventive maintenance ２ improving the performance and efficiency of the well and reducing its carbon footprint using - Sealing the water well and adding liquid or gaseous CO ２ and injecting the CO into the water well aquifer formation by opening the water well. ２ enabling storage of the A method comprising:

2. The Aqua Freed or Aqua Gard technology may be modified to reduce CO2 in the well aquifer. ２ The method of claim 1 including the step of achieving and enhancing retention.

3. CO ２ The injection pressure is controlled to maintain a constant value, and the CO ２ Preventing backflow and upward movement of the CO / water mixture into the well, thereby preventing CO ２ The method of claim 2, further comprising the step of increasing the penetration of

4. CO ２ The CO in water mixture was adjusted ２ Minimizing the expansion of CO2 and increasing the amount of CO2 released into deeper aquifers ２ The method of claim 1 further comprising the step of allowing permeation.

5. CO ２ Additional CO2 can be prevented by reducing the amount and rate of water flowing from the well after injection is complete. ２ The method of claim 1 further comprising achieving pooling.

6. CO ２ 6. The method of claim 5, further comprising the step of: preventing well pressure from escaping from the well into the ambient atmosphere, thereby allowing well pressure to naturally dissipate within the well aquifer.

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