Synopsis: In many cases, reservoir containment and caprock integrity assessment is considered as a one-time requirement for proving the safety of a subsurface project while it is important to remember that such assessments are dynamic processes that should continue during the entire life of the project and, in some cases, even after its cessation. This article presents a workflow for dynamic reservoir containment assessment (DCAP) that accounts for the dynamic nature of this process.
Underground activities are growing very fast while the technologies used in these activities become more and more aggressive and risky. For instance, these days, industry is using high temperature steam, it is injecting chemical fluids in the rocks and it is intentionally fracturing the rocks. On the other hand, public sensitivity towards environment has been increasing on a daily basis. Economic reasons also play an important role as by controlling the containment of the reservoir we can prevent unwelcomed problems such as wellbore damage or surficial leakage that can be quite costly for any project. Major consequences of containment loss are:
- Leakage of reservoir fluids
- Ground deformation, in general, and ground surface subsidence/heave, specifically
- Well integrity issues
- Induced seismicity (due to induced fracturing and reactivation of faults and existing fractures)
- Inflow of outside fluids into the reservoir
- Heat Loss
All the containment-related geomechanical hazards must be studied under a comprehensive program called caprock integrity or reservoir containment assessment. In many cases, reservoir containment and caprock integrity assessment is considered as a one-time requirement for proving the safety of a project (and receiving operations approval from authorities) while it is important to note that reservoir containment assessment should be considered as a dynamic process for the entire life of the project and that may continue even after ceasing the underground operations. The rest of this article presents Dynamic Containment Assessment Program (DCAP), a generalized workflow with different modules required for containment assessment and caprock integrity analysis. Examples of operations that such workflow can be applied to are:
- Conventional production/water flooding
- Gas sequestration/storage
- Thermal operations
- Unconventional shale gas/oil
- Nuclear waste deposits
- Compressed air storage
- Underground water production
Dynamic Containment Assessment Program (DCAP)
This workflow implements data, tools, and techniques from different disciplines such as geology, petrophysics, geophysics, reservoir engineering, well engineering, hydrogeology, geochemistry, etc. Ideally, all these information resources are integrated in a comprehensive dynamic process that can even continue after cessation of underground operations. Different steps of this workflow are:
- Appraisal data acquisition
- Site characterization
- Data interpretation and modeling
- Feasibility assessment
- Operational criteria and recommendations
- Field monitoring
- Real-time data updating
Appraisal Data Acquisition
In the appraisal phase of geomechanical assessment of reservoir containment, data are collected from several different sources including:
- Geological studies
- Geophysical surveys
- Hydrogeological and geochemical characterizations
- Petrophysical studies
- Production/injection rate histories
- Pressure and temperature histories
- Wireline logs
- Geomechanical lab and field tests
- Leakage evidence
- Well drilling, completion, fracturing and treatment experience
- Ground deformation data
and other sources that may either directly or indirectly help to build an accurate earth model that includes all the sedimentary succession from below the reservoir up to the ground surface. Special attention must be paid to the reservoir and its primary caprock(s) in this process.
After data acquisition, the collected data are used to characterize different properties of rocks in the study area and sedimentary succession of interest. Ideally, a Mechanical Earth Model (MEM) should be constructed based on integrated processing of these data. Such a model usually includes:
- Geological structure
- Sealing mechanisms
- Hydrogeological and fluid flow characteristics
- Petrophysical characteristics
- Geomechanical properties
Site characterization must be seen as an ongoing process during the entire life of the project and, ideally, it should include the monitoring period after cessation of operations.
Data Uncertainty and Ongoing Variations: Data uncertainty is always a main characteristic of data for subsurface studies that usually is addressed in the developed mechanical earth model using geostatistical methods. It is important to note that site characterization is a dynamic process and any additional data that becomes available during the life of the project can improve the quality of characterization. On the other hand, the character of a site may significantly vary with time due to different operations such as hydrocarbon production and fluid/steam injection. Such processes change the fluid content, as well as the pressure and temperature within the reservoir and its surrounding rock and, consequently, can affect the petrophysical and geomechanical properties of the rock.
Characterizing Sealing Mechanisms: Initial sealing mechanisms can be identified by studying the geological structure of the field and its constituent faults and fractures, their mechanical and hydraulic properties, hydrogeological information, and in-situ pore pressure, temperature and stresses. The pressure history of the reservoir and the records of well testing are also very important in this process. Any evidence of reservoir fluid leakage is also very useful to identify sealing mechanisms and their potential alteration during the production life of the reservoir.
Data Interpretation and Modeling
Geomechanical models are constructed based on the collected data and site characterization. These models are calibrated using historical data and utilized to identify the potential geomechanical issues in the past history and the future life of the reservoir. Different types of geomechanical modeling tools may be used to studying these issues. These tools cover a broad band from simpler analytical and semi-analytical models to more complicated numerical models.
Different Geomechanical Models: Analytical and semi-analytical models are usually constrained by simplifying assumptions regarding the geometry, mechanical properties, and fluid flow characteristics of the system. To consider more details for the problem (e.g., more realistic geometry and material properties), using numerical models is essential. However, more detailed data are required for more complex models. In an ideal case, the geomechanical models are fully coupled with fluid flow models but, in reality, the degree of coupling might be looser due to different issues such as time, cost, and computational power.
Modeling Process: The developed models, along with historical production and injection rates, pressure and temperature history of the field can be used to study the geomechanical response of the reservoir during its production life. These studies are capable of identifying induced fractures and reactivation of existing fractures and faults, and their effect on the sealing mechanisms of the field during this period. The validity of the results of modelling can be evaluated using historical geomechanical data such as recorded wellbore instabilities, seismic activities, and ground deformations. History of reservoir treatment activities such as hydraulic fracturing may have significant effects on the hydraulic integrity and must be considered during these studies. In addition, the developed models are used to predict the geomechanical response of the field to future developments such as injection and production during operations.
It should be noted that besides numerical modeling, it is also necessary to experimentally test caprock sealing properties such as capillary entry pressure, to ensure the capability of the caprock for preventing capillary leakage. Another important issue in modeling of these operations is accounting for the hysteresis behaviour of the reservoir and its surrounding rock when they become subjects of repeating cycles of injection and production during operations. After starting the operations, the developed models must be updated during the operations and calibrated with the real-time data.
Geomechanical feasibility is evaluated based on the collected data and modeling results. The major issues considered for this assessment include minimizing the potential for leakage, wellbore stability concerns, induced seismicity, and ground deformation. This process may also include geomechanical assessment of injectivity enhancement potential such as hydraulic fracturing. The feasibility assessment must be studied in the context of provincial/state and federal regulations. Feasibility assessments may lead to different conclusions: In cases which potential risks are not tolerable and cannot be mitigated or controlled, the project may be disqualified. In other cases, limitations and operational criteria may be defined and recommended to minimize the potential risks for the project.
Operational Criteria and Recommendations
If the reservoir is qualified for underground operations, some criteria are usually defined to ensure the safety of operation. These criteria are applied to limit the injection rates, fluid pressures, fluid temperatures, ground deformations, etc. In addition, instructions are given for wellbore (re-)design and treatment. These criteria may be a direct result of modeling or imposed by regulations and standards. The initial criteria and limitations may change during the life of the project when the new data and observations become available for updating the feasibility assessment results.
Field monitoring is an important part of any subsurface project that is designed to record the potential changes induced by the field operations. The results from monitoring are employed to evaluate the field performance and identify the changes in the field condition during and after operations. Some of the monitoring techniques used for this purpose are: seismic surveys, microseismic monitoring, well logging and monitoring, groundwater sampling, soil contamination measurement, tilt meters, satellite monitoring of surficial deformation.
The acquired data from the reverse analysis of monitoring results can be very valuable for understanding and predicting the geomechanical behaviour of the field. Such analyses provide information about fluid flow within the reservoir, potential leakage, ground deformation, and location and characteristics of faults and fractures and rock properties.
Real-time Data Updating
The collected initial data mentioned in the appraisal stage must be updated and modified by using the newly acquired information from different sources that become available during the reservoir’s life. As mentioned, one important source for such data is field monitoring. Other sources include new geological, geophysical, petrophysical, geochemical, and hydrogeological studies. In addition, new logs, lab tests, and field tests and data from wellbore stability studies can be very useful. These real-time data will be implemented to update the site characterization for the field and, subsequently, for updating the geomechanical, fluid flow and other models. The results of such analyses are used to re-define and modify the operational criteria and recommendations for continuation of the operation.