The Subsurface Stress & Induced Seismicity Pillar focuses on the interrelated topics of subsurface stress and induced seismicity (IS). In addition to addressing the characterization and manipulation of stress and IS, the pillar includes elements focused on their use to assess and modify permeability and on risk analysis for subsurface processes (including induced seismicity).
Research in the Subsurface Stress & Induced Seismicity Pillar is associated with four different topics, called elements, including:
- State of Stress (measurement and manipulation)
- Induced Seismicity (measurement and manipulation)
- Relate Stress and IS to Permeability
- Applied Risk Analysis to Assess Impact of Subsurface Manipulation
State of Stress (measurement and manipulation): The high-level 10-year goal of this element is to advance stress monitoring methodologies for automated inversion of the stress tensor.
Knowledge of the subsurface stress state is important for many subsurface activities related to energy production and storage (or disposal) of waste, including prediction and control of hydraulically induced fractures and the activation and/or re-opening of faults. The induced seismicity associated with subsurface operations is a related topic, but one of singular concern given the current increase in mid-continent seismicity. Current capabilities to directly measure or infer the in situ stress are woefully inadequate, especially away from boreholes. This limitation leads to significant uncertainties and the loss of opportunities to take advantage of the subsurface for energy production and waste storage, as well as public distrust in the subsurface energy sector.
Induced Seismicity (measurement and manipulation): The high-level 10-year goal of this element is to identify critically stressed faults, pre-injection and/or before unwanted seismicity, and to demonstrate forecast and management mechanisms that will decrease the likelihood of M2-3 by ten times within a defined time period.
Anticipating and mitigating unwanted induced seismicity is crucial to many subsurface energy issues. Current attempts to control subsurface fluids can lead to unexpected and uncontrolled induced seismicity. Our current knowledge of induced seismicity comes mainly from “accidental” seismicity associated with subsurface engineering (such as water disposal wells or enhanced geothermal energy projects), and our ability to predict the magnitude of these events is woefully inadequate. Current approaches to interrogate the subsurface using seismicity either provide information about only a very small fraction of the relevant subsurface and/or provide an unacceptably fuzzy interpretation of in situ conditions (e.g., inversion of induced micro-earthquake data). For example, microseismic events often are located as a “cloud” of events, but with improved measurement and/or analysis the cloud collapses to more of a plane.
To make use of induced seismicity as a tool, while minimizing or eliminating unwanted seismicity, we need to increase our knowledge and control of seismicity associated with subsurface engineering. We first need to improve our measurement technology. Following improvement in stress measurement (the primary uncertainty), the seismic events themselves can be better measured, and thus rock/fluid properties better inferred.
Relate Stress and IS to Permeability: The 10-year goal of this element is to develop the ability to characterize induced flow paths in and around faults, characterizing fault permeability and the geometry of flow zone impacted (e.g., size, shape) with an order of magnitude improvement.
The crucial elements of stress and induced seismicity need to have their research crosscut with the impact on permeability. Understanding stress state and seismicity are not independent goals; rather, they work together toward achieving control of subsurface permeability through fracturing. Current understanding often allows only empirical links to permeability, and experiments often are independent. This element is needed to develop the theoretical and experimental links between stress/IS and permeability.
Applied Risk Analysis to Assess Impact of Subsurface Manipulation: The 10-year goal of this element is to develop risk-driven adaptive controls on operational envelopes (injection rates, volumes, pressures, well locations) for subsurface engineering activities, as well as to develop a total system performance assessment (risk assessment) for SubTER activities.
It is of primary importance to establish a framework and protocol for assessing key risk drivers associated with subsurface engineering activities. Similar in flavor to the U.S. DOE NRAP Program tasked with identifying risks associated with geologic carbon sequestration, a systematic framework is needed to identify the risk factors associated with all engineered subsurface activity. Examples of risk contributors could potentially include the natural geological fabric and stress state, existing fracture and fault networks, and wellbore and natural subsurface fastpaths that serve as conduits to environmental systems.