Once a geological site has been highly graded for its storage potential, a project developer must perform detailed reservoir characterization to determine whether it’s truly suitable for long-term carbon storage. This evaluation—meant to reassure with greater granularity a site’s feasibility—focuses on verifying the selected site’s capacity versus its intended storage, its integrity to contain the carbon dioxide (CO2), and the injection rate it can sustain (to ensure it’s high enough for long-term economic viability).
You might be thinking that all these steps require additional data, and you’d be right. Detailed reservoir characterization requires further investment in data acquisition: Either you acquire new seismic data and process it, or you drill characterization wells and integrate that new data into your geological models. Either way, it’s an upfront investment that doesn’t necessarily guarantee the site will prove itself fully suitable for carbon storage, but it will provide very much needed support in deciding whether a project should proceed into development or not.
And the assessment is not done there. Your storage modeling must be combined with a holistic measurement, monitoring, and verification (MMV) plan, developed to explore major risks and provide an adapted program for verifying CO2 movement and the integrity of the injection complex. Between this and the additional data analysis required, you can safely and securely move forward with choosing the best carbon storage site for your unique conditions, resources, and goals.
Capacity, injectivity, and containment—the winning combination for a carbon storage site
One of the first activities in a storage site evaluation is to complete a performance assessment. During this process, the site’s technical suitability is fully evaluated against three key performance criteria:
- Capacity—the volume of CO2 that can be injected into the site
- Containment—the caprock's ability to provide an adequate barrier, along with the confirmation that faults and wells within the site do not provide any significant leakage risk
- Injectivity—the estimated injection rates that can be achieved in the planned wells.
Understanding these three criteria allows you and the rest of your project development team to properly assess the economic feasibility of a carbon storage site, after which you must also ascertain if the associated risks are acceptable or manageable during subsequent development and injection operations. For this, you must adhere to proven workflows that provide traceable records—key to streamlining the performance assessment process and ensuring that all potential risks have been fully vetted.
These workflows combine various sources of acquired data (e.g., seismic and well data) into the construction of detailed geologic, geomechanical, reservoir flow, and flow assurance models. Such dynamic modeling can leverage industry simulators originally developed for the exploration and production of hydrocarbons from subsurface reservoirs, but to architect a truly reliable carbon capture and storage migration model, you must introduce several nuances into the methodology.
Subsurface knowledge is directly applicable to the simulation of carbon capture and storage pros and cons.
For example, consider the impact of geomechanical stress alterations during the injection phase. As CO2 propagates through the injection layer, changes in ambient pressure impact the local geomechanical stress. This can, in turn, lead to undesired effects like fault reactivation or stress-induced weakening of the caprock. Therefore, a sound approach during the evaluation process can be to assess the impact of injected CO2 on geomechanical stresses by linking the plume migration model with the geomechanical stress model (coupling).
Beyond these modeling deliverables, a performance assessment should also include the foundational work for developing an MMV plan. An MMV plan is a risk management plan for the storage site, from its initial development all the way through the project's life. Not only is an MMV plan required by regulators, but more importantly, it is critical for managing a project’s risks. This includes things like having a good understanding of how the injected CO2 migrates through the geological layer and frequently monitoring risk areas where potential leaks could occur to demonstrate integrity.
A systematic approach to designing measurement, monitoring, and verification plans
To outline a basic MMV plan, a project developer must: (1) identify high-risk areas and (2) predict the response to measurement technologies in these areas. This is important because the correlated analysis allows developers to select specific measurement technologies that cost effectively satisfy monitoring and measurement objectives.
The measurements in an MMV plan can be divided into four groups:
- Shallow depth and surface environment—the sampling and geochemical analysis of groundwater, soil, and air for evidence of non-naturally occurring CO2 content.
- Injection integrity and well monitoring—continuous pressure observation and verification through corrosion logs, pressure tests, etc.
- Operational injection parameters monitoring—continuous pressure, rate, and composition measurement.
- Plume migration and pressure front monitoring—the more complex category as it depends mostly on a combination of indirect measurements (e.g., seismic), computational modeling, and ad hoc direct measurements (e.g., observation well fluid sampling).
Regulators such as the Environmental Protection Agency in the US encourage project operators to develop a risk-based approach and plan that uses the appropriate technologies. Given the wide spectrum of technologies available today and the large variation in cost and level of information each type provides, developing a truly risk-based strategy is a complex optimization problem.
Prior experience with carbon sequestration operations helps in providing insight into the efficacy of different measurement technologies in specific environments, along with how different types may be used in combination to paint as comprehensive a picture as possible. Digital modeling, for example, can be utilized to “test” the effectiveness of different measurement technologies—with artificial intelligence, machine learning, and high-performance computing providing the highly efficient processing of multiple realizations and outcomes. Testing various MMV strategies allows you to select the most effective plan, reduce overall risk, and determine overall project economics.
Optimizing storage to maximize project economics
Assuming its capacity, injectivity, and containment are deemed favorable, a site’s overall economics will be determined by the project operator’s ability to effectively fill all the pore space within the site (especially if the site is a multiwell injection hub).
Given its buoyancy at the level of the geological injection layer, CO2 will migrate to the caprock where the plume will start to dissipate. When pressure fronts between wells oppose each other, this may limit CO2 injection rates. As a result, the pore space between wells can remain unfilled, meaning the site does not reach its full capacity. Accessing unfilled, available pore space requires drilling infill wells or developing additional acreage or injection layers, options that add to a project’s capital expenditures and deteriorate its economics.
The oil and gas industry already has the tech and workflows necessary to properly evaluate a carbon storage site.
Building accurate geologic models to simulate and assess storage efficiency, along with well placements and the selection of a good completion strategy, is key. Accurate modeling extends into injection operations to verify injection conformance and the optimal use of storage capacity. In addition, through measurement and monitoring, early observation of any nonconformance of CO2 fill can be mitigated by changing the injection strategy. The rapid integration of acquired and live-streamed data through cloud-based computing and machine learning-based proxy models allows for continuous testing of potential anomalies in the plume migration model. Early warnings can then lead to more focused measurements for confirmation and adjustments made to the injection strategy to optimize fill, all while mitigating capex.
In other words, not only does the oil and gas industry have the technologies for properly evaluating carbon storage sites from a technical and economic perspective, but it has also developed specific workflows for deploying them. The detailed performance assessment of the geological site—as well as the development of MMV strategies that are cost effective, reduce risk, are compliant with regulatory requirements, and provide assurance of CO2 sequestration—can all be completed through workflows that combine digital tech with subsurface and surface experience. Meaning there’s nothing stopping us from using this to accelerate our collective path to net zero.
