Experimental and Numerical Assessments of CO₂ Injectivity and Flow Assurance during Storage in Depleted Hydrocarbon Reservoirs

Background
Reusing old (e.g., mature, depleted and/or decommissioned) oil and gas reservoirs for permanent geologic carbon sequestration (GCS) may have several benefits such as the good knowledge of the reservoir, proven storage potential and seal, and possible reuse of existing infrastructure. In addition, there may be a potential for enhanced oil and gas recovery from CO₂ injection in these reservoirs. Consequently, CO₂ storage in old oil and gas fields can be a more favourable alternative than finding and developing new reservoirs, and thus, reducing the barriers to accelerate GCS on a larger scale.

Critical to the efficient and safe implementation of GCS is a detailed understanding of the flow and transport processes in boreholes and the coupling with the reservoir. Wellbores in general have been identified as the most likely conduit for CO₂ and brine leakage from GCS sites, especially those in sedimentary basins with historical hydrocarbon (oil and gas) production. On the other side, near-wellbore effects and phenomena, like salt precipitation, rock dissolution, Joule-Thomson cooling effect and hydrate formation can impair CO₂ injectivity and impede CO₂ injections at planned rates. Predicting and controlling the above-mentioned issues are important for handling CO₂ flow assurance problems.

The INJECTWELL project will investigate these challenges in more detail during CO₂ injection into depleted hydrocarbon reservoirs (DHR) with emphasis on cyclic injection mode and high aquifer impact. There are several examples of such cases in the Dutch-Danish and Norway offshore sectors (e.g., project Greensand, project Porthos, NPD Storage Atlas Norwegian North Sea), and better knowledge to link wellbore integrity and near-wellbore effects are thus required, to reduce the risk, environmental impacts, and costs of such processes in DHR but also GCS in general.

Project description
The current proposal is a technological development study with focus on experimental and numerical assessments of actual flow assurance aspects and challenges in DHRs. Available commercial simulators will be tested and compared for simulating the coupled dynamics and effects from well to reservoir, considering the near wellbore region. The simulators will be validated-calibrated with the experimental results derived and compared in terms of reliability and practicality. The goal is to provide a recommended simulation tool for coupled wellbore and reservoir model and a state-of-the-art understanding of critical parameters important to further developments, to secure well integrity and CO₂ injection capacity during storage in DHRs.

Handling the coupled and combined CO₂ flow assurance aspects, which both affect well integrity and near-wellbore injectivity, is the main driver of this proposal. This is, in our opinion, necessary before basin-scale simulations and storage capacity can be estimated more accurately.

Project potential
There are several depleted oil and gas reservoirs in the North Sea, both on the Norwegian and British sectors, as well as several other places in Europe. Many fields are also in their final years before decommissioning. The results from this project could have a direct transfer value to these reservoirs, as well as provide useful learning in the planning phase of future CO₂ storage projects in DHR but also GCS in general.

Project goal The main goal is to accelerate GCS on a larger scale. To contribute to this goal, we are in this project focused to:

Overall project objectives:
– Identify the impact and interplay of fluctuating wellbore dynamics on well materials and well integrity – and vice versa.

– Identify the most critical parameters for injectivity impairment in the near-wellbore reservoir zone. Emphasis on cyclic CO₂ injection schemes, water restoration and interplay of combined effects (i.e., J-T, hydrate, impurities (CH4) and salinities).

– Validate commercial numerical flow simulators with project experimental data and determine their reliability, flexibility and practical applicability towards CO₂ storage projects in DHRs. Emphasis would be given to the coupled/integrated processes and effects (i.e., wellbore + near-wellbore reservoir) that can occur during cyclic CO₂ injection mode.

Key results/goals:
– Recommendations on optimal operational windows for cyclic CO₂ injection in terms of well integrity, injectivity and energy use for selected conditions and cases based on calibrated modelling results and experimental data.

– Guidelines and improvement-suggestions on the selection of simulation tools in terms of reliability, flexibility and practical applicability.