Oil and gas extraction, as well as mining activities, are among the causes of surface deformation. These activities often include pumping groundwater or injecting geothermal fluids, which can lead to gradual subsidence of the ground (slow subsidence of the ground), putting the stability of buildings, infrastructure, or pipelines at risk of landslides or other geological hazards. Monitoring the spatial distribution and temporal variation of land surface deformation is essential for the stability and risk warning of extractive assets. Therefore, the rapid identification and analysis of abnormal deformations can help to warn of damage, collapse and ultimately life-threatening accidents.
Topographic change is an important issue in the onshore oil and gas extraction industry. Oil and gas extraction activities can lead to land subsidence, which usually has large spatial variations, which threatens the stability of the wellbore structure. In the case of fluid injection, which is typically used to improve extraction efficiency, a significant lift is an indication that the process is less efficient. This means that the inspection of surface deformation is not only a safety issue to avoid geological hazards and damage to oil and gas infrastructure, but also for the commercial benefit of more efficient development.
Using traditional field survey methods to monitor large oil and gas fields of hundreds of square kilometers for millimeter-scale surface deformations is time-consuming and expensive. GPS site monitoring accuracy is accurate, but it does not provide highly spatially distributed measurements, as only a limited number of points can be used to obtain a small portion of the area of interest. While these two methods play a key role in monitoring, maintenance, and risk warning, they do not provide true ground displacement time and full space coverage at reasonable operating costs. In addition, site surveys come with a number of safety concerns.
Figure: Automated detection of a well site in an area from high-resolution optical satellite imagery using ENVI deep learning software.
Based on spaceborne
Satellite remote sensing and geospatial technology have evolved rapidly, and today the use of space-based synthetic aperture radar (SAR) provides an efficient, accurate, and low-cost method for risk and stability monitoring in areas such as infrastructure, construction sites, mines, and oil and gas fields. Spaceborne Differential Interferometric SAR (DINSAR) is particularly suitable for measuring abrupt surface deformations at two time points, such as pre- and post-ter. When it comes to slow surface deformation, such as material compaction, ground subsidence, or surface creep, advanced multi-phase interference superposition techniques can be used for greater accuracy.
Generally speaking, there are two types of SAR interference superposition techniques currently in application. The persistent scatterers method is often used to monitor areas with a lot of man-made features, such as buildings, bridges, dams, etc. The Small Baselines (SBAS) method is commonly used to monitor natural features or other types of non-geometric feature objects. Advanced PS and SBAS interferometric superposition techniques analyze time-series SAR images to obtain the spatial distribution and temporal evolution of large-area and long-term slow terrain movements with millimeter sensitivity.
Unlike passive optical remote sensing sensors, which can only work during sunny daylight hours, SAR systems are active sensors, allowing them to collect data day and night, even through clouds. Through the all-weather characteristics of SAR remote sensing technology, the surface deformation monitoring of important infrastructure can be carried out quickly.
Figure: The average annual vertical displacement rate of the oilfield based on SBAS. Positive values (i.e., surface uplifts reddish) are mostly close to the area where the fluid is injected (Source**: sarmap s.).a.)。
Utilization
A recent study (Bayramov et al., 2022) details the use of SAR satellite data and techniques to detect subsidence induced by oil extraction in the Tengiz field in Kazakhstan from 2018-2021. The Tengiz field is located on the coast of the Caspian Sea, covering an area of about 2,500 square kilometers, with a length of 19 kilometers, a width of 21 kilometers, and a thickness of 15 km. It is one of the largest and deepest oil fields in Kazakhstan with more than 100 wells drilled.
The study aims to assess the impact of surface displacement caused by fluid extraction and injection on geohazard risk, while contributing to more efficient mining and improved reservoir modeling. Dense and highly accurate vertical and horizontal displacement measurements obtained by interferometric SAR provide the critical information needed for routine reservoir monitoring, characterization, and geomechanical analysis.
In this study, the temporal evolution of vertical and horizontal surface displacements was obtained based on the high-resolution SAR satellites Cosmo-Skymed, TerraSAR-X and medium-resolution Sentinel-1 by using the SBAS method in SAR interference superposition technology. The high-precision displacement information of the distributed target was obtained.
The authors of the study used Envi Sarscape software for the analysis. The researchers found that the maximum average vertical settlement in the Tenggiz field was 57 mm years. Fifteen wells and three facilities were observed located at - 55Measured settlement range between 6 mm years and - 42 mm years.
This spatial and temporal displacement information can provide early warning signals for decision-makers to help assess the hazard level of priority areas and identify where closer monitoring is needed. However, according to the researchers, there is currently no publicly available data or clear criteria for assessing whether these surface deformation rates should be considered a risk to the safe operation of the field.
The researchers believe that oil recovery and injection technology are the key factors affecting the ground deformation of Tengiz oilfield. However, since the distribution density of the wells is not spatially fully correlated with the detected settlement hotspots, the authors infer that the observed settlement process is not only the result of oil production activities, but also that the second key factor to consider is the natural structure associated with the two ** faults. The third key factor is the direction of the water flow. The proximity of the subsidence hotspot to the Caspian coast is the fourth key factor, as changes in soil structure and moisture content increase the tendency to subside. The researchers concluded that the main causes of the observed surface deformation were caused by both human and natural factors.
Figure: Distribution of Tengiz fields, wells and faults in Kazakhstan (Bayramov et al.).， 2022)。
Figure: Well and reservoir depth 3D model, (a) vertical (lift and sink) displacement velocity and (b) horizontal (east-west) displacement velocity from 2018 to 2020. (bayramov et al.， 2022)。
SAR interferometry provides an efficient and cost-effective solution for monitoring surface deformation in oil and gas fields. Based on the SAR interference superposition technology, the ability of SAR satellites is used to measure the ground displacement evolution and dynamic change in the area of interest. This technology can make high-resolution, millimeter-level observations of slow terrain movements over large areas and long periods of time, providing key information for geological hazard risk assessment and improved production and reservoir dynamic behavior modeling.
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