Subsurface Alliance

Banded Iron Formations (BIFs) offer a fascinating window into the mechanical behavior of #rocks. Over geologic time scales, rocks can break, stretch or flow! The development of boudins (in picture) highlights differences in rock strength, with quartz-rich layers behaving more rigidly than surrounding layers. Their geometry - whether symmetrical or asymmetrical -provides clues about the stress regime and strain history. #geomechanics #outcrop #stress #strain #rockmechanics #subsurfacealliance

Chapter 8. Induced Seismicity in the Central U.S. - Maximizing the Value of Geomechanics Since 2009, the central U.S. has experienced a significant increase in seismic activity, with earthquakes of magnitude 3.0 or higher becoming far more frequent (Figure courtesy of USGS). While natural tectonic processes play a role, a growing body of research points to deep wastewater disposal and shale development as key contributors to this trend. ?? What's Driving the Surge? ?Wastewater Injection: Large volumes of produced water from oil and gas operations are injected into deep disposal wells, increasing subsurface pore pressures and reactivating pre-existing faults. ?Shale Development: Hydraulic fracturing itself generates low-magnitude seismic events, but in some cases, it can trigger larger quakes when faults are critically stressed. ?Spatial & Temporal Correlations: Studies show a direct link between increased injection rates and seismicity clusters in Oklahoma, Texas, Kansas, and other central U.S. states. ?? A Look at the Data Before 2009, Oklahoma averaged 1-2 M3+ earthquakes per year. By 2015, that number had skyrocketed to over 900. Regulatory adjustments—such as reduced injection volumes—have helped lower seismicity rates, but the issue remains a focal point for industry, regulators, and communities. ?? Mitigation & Risk Management ?Improved seismic monitoring networks ?Adaptive injection rate regulations ?Identification of high-risk faults before operations ?Advances in fluid disposal alternatives #Geomechanics plays a crucial role in understanding and mitigating induced seismicity by assessing subsurface stress conditions, fault stability, and fluid pressure changes. Advanced modeling techniques help predict how wastewater injection and hydraulic fracturing impact fault systems, allowing operators to adjust injection rates, optimize well placement, and implement real-time monitoring strategies. By integrating geomechanical analysis with seismic data, the industry can proactively identify high-risk areas, reduce seismic hazards, and develop safer, more sustainable extraction practices. If you have concerns about the risk for induced seismicity, Subsurface Alliance can help derisk your asset. Let’s discuss. Image courtesy of #USGS (open source) #InducedSeismicity #Geomechanics #OilAndGas #Seismology #Geothermal #SubsurfaceAlliance #usgs

One final post on FAULT STABILITY analysis... Chapter 7: How much do you trust your Mohr Circle analysis? We often put a lot of confidence in the traditional Mohr Circle analysis which is deterministic. It uses single-point values for stresses (e.g., vertical, maximum horizontal, and minimum horizontal stress) and rock properties (like cohesion and friction). This approach assumes that these input values are well-constrained and certain. In reality, subsurface conditions have significant uncertainties. To illustrate the impact of uncertainty, the top figure shown above assumes a 5% uncertainty on each of the principal stresses and in pore pressure. Your Mohr Circle analysis doesn’t seem some reliable anymore, right??Ignoring these uncertainties can lead to overconfidence in stability predictions and underestimation of risks. Subsurface Alliance uses Monte Carlo simulations for a given fault geometry to probabilistically assess uncertainty and generate Cumulative Distribution Functions (lower figure). This approach can help in assessing the probability of fault slip under varying conditions. Relying solely on a Mohr Circle for fault stability analysis without considering the probabilistic approach can lead to: ?? Over-simplification of complex subsurface conditions. ?? Underestimation of risk due to neglected uncertainties. ?? Insufficient guidance for managing induced seismicity risks in subsurface energy projects. In essence, a #probabilistic methodology offers a more robust and realistic assessment of fault stability compared to the traditional Mohr Circle approach. Whether you are in the #oilandgas, #geothermal or #CCS space, if you have concerns about the stability of your faults and risk for induced seismicity, Subsurface Alliance can help derisk your asset. Get in touch now with any of our partners for a #free consultation Ewerton Araujo, PhD, Fermin Fernandez-Iba?ez, PhD, Jorge Santa Cruz Pastor, PhD #inducedseismicity #ccs #ccus #geothermal #waterdisposal #drilling #geomechanics #subsurfacealliance #westtexas #reservoirengineering

Chapter 6. Mohr Circle for Stress-Sensitive Fractured Reservoirs In the past couple of weeks, we have discussed fault stability assessment, and different approaches, one being the use of Mohr circles. Just like faults are treated in the Mohr space, natural fractures can be analyzed in a similar fashion to assess whether your fractured reservoir is stress sensitive. Understanding the interplay between natural fractures and in situ stresses can help unravel unexpected reservoir performance behaviors. When the shear to normal effective stress acting on a fracture surface exceeds that sliding friction of the fracture, the probability of shear failure occurring is high, and then, the fracture is said to be critically stressed. This is especially relevant to carbonate, shale and basement reservoirs (it gets a bit more cumbersome in sandstones). Critically stressed are hydraulically conductive as slippage increases porosity and permeability. In reservoir development terms, they can provide additional permeability resulting in greater than expected injection rates. For instance, figures A and B illustrate the impact of injection on natural fractures interpreted from image logs (figure on the left). The increase in reservoir pressure due to injection results in a larger population of natural fractures being reactivated and potentially an increase in fracture permeability and connected fracture pore volume. It is always recommended that fracture studies go along with a geomechanical analysis to understand whether fractures in the reservoir are sensitive to stress changes during production or injection. The use of Morh circles as a screening tool is adequate, more complex numerical models might be required if the reservoir is especially sensitive to stress changes. Image credit: Yin et al., (2023) - Frontiers in Energy Research,?11, p.1271377. Licensed under CC BY 4.0 DEED #geomechanics #subsurfacealliance #fracturedreservoirs #naturalfractures #criticallystressfractures #faultstability

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Chapter 5: Numerical Models for Fault Stability Assessment The Mohr circle is a valuable tool for quickly assessing the potential for fault slip by examining the current stress state against frictional criteria. In contrast, numerical models provide a comprehensive, dynamic framework that can simulate the evolution of fault behavior under a wide range of conditions. The choice between them depends on whether you need a quick, conceptual check or a detailed, time-evolving analysis of fault stability. Numerical models solve the underlying equations of motion (or quasi-static equilibrium) over time. They can simulate the entire process of stress accumulation, failure, and post-failure evolution, offering insights into the dynamic rupture process. These models can incorporate realistic fault geometries, variable material properties, nonlinear friction laws (such as rate-and-state friction), and interactions with fluids, providing a much more detailed and realistic picture of fault behavior. Numerical models require significant computational resources, especially for high-fidelity, three-dimensional simulations that resolve fine spatial and temporal scales. Key Differences ?? Level of Detail: ?Mohr Circle: Provides a simplified, static view suitable for initial or conceptual analyses. ?Numerical Model: Delivers a detailed, dynamic simulation that can account for complex behaviors and interactions. ?? Time Dependency: ?Mohr Circle: Offers a snapshot of the stress state at a particular moment without temporal evolution. ?Numerical Model: Captures the time-dependent evolution of stresses and fault slip. ?? Assumptions and Limitations: ?Mohr Circle: Based on idealized assumptions (uniform stress field, linear elasticity) and is limited to analyzing potential failure at a point. ?Numerical Model: Can incorporate realistic physics and heterogeneities but at the cost of increased complexity and computational demand. ?? Applications: ?Mohr Circle: Best for preliminary assessments and educational purposes where a quick evaluation is needed. ?Numerical Model: Essential for in-depth research, seismic hazard assessment, and scenarios where the full dynamic behavior of faults is critical. Figure: Model results of stresses, displacements and vertical strains in the formations in a zoomed-in region around the 50m offset fault. After 12.89MPa depletion, at the start of seismic instability: (F) effective vertical stress, (G) vertical strain, (H) horizontal displacements, (I) vertical displacements. Figure after Buijze et al. (2017) – Licensed under CC BY 4.0 #geomechanics #faultstability #subsurfacealliance #inducedseismicity #numericalmodels

Chapter 4 – Fault Stability While Drilling: A Mohr Circle Application The upper bound of any operating mud window might be lowered at the intersection with an optimally oriented fault. Equivalent Circulation Density (ECD) should always be kept below the fault reactivation pressure (or Critical Pore Pressure) once a fault is intersected until the casing shoe is set. The top figure shows a detail of the intersection between a well (Well X) and a 3D fault surface (the inset shows the overall fault surface). Colors on the fault surface map the critical pore pressure (or ECD) required to induce shear along the surface. The colors along the wellbore trajectory correspond with Well X Measure While Drilling (MWD) data showing ECD values. Note how ECD is greater than the calculated Critical Pore Pressure at the well-fault intersection, thus explaining the losses experience in by this well. The bottom figure shows a Mohr diagram illustrating the state of stress when the ECD was greater than the critical pore pressure. The red dot represents the fault plane at the intersection with the well (defined by the normal and shear stresses acting on the fault surface). The shear to normal stress ratio here exceeded the fault strength resulting in fault slippage, breach of the fault seal and subsequent losses due to increased fault permeability. As shown in the top figure, there are strong fault patches (i.e. higher Critical Pore Pressure). These are better candidates for drilling through the fault. In addition to these types of analyses in pre-drill situations, it is always recommended to use MWD technology to monitor bottom hole pressures while drilling through faults. Figure credit: Fernandez-Ibanez et al. (2010) – SPE132826, ATCE 2010 #geomechanics #subsurfacealliance #drilling #faultstability #losses #mwd

Chapter 3. One Circle to Rule them All: The Mohr Circle The Mohr Circle is a graphical representation used in geomechanics to describe stress states at a point. It is particularly useful for visualizing normal and shear stresses acting on different planes (faults or fractures) within a material. Any point on the circle represents the normal and shear stress on a plane at a given orientation. ?Mohr circles are extensively used in fault mechanics, rock failure analysis, and earthquake studies. ?? Determining Critical Conditions for Different Fault Types - If stress conditions favor failure along pre-existing weaknesses, a fault can slip. Mohr’s Circle also helps predict normal, thrust, or strike-slip faults. ?? Shear Failure and Earthquakes - Earthquakes occur when shear stress on a fault exceeds the rock’s shear strength. The Mohr-Coulomb failure criterion helps determine the stress conditions required for earthquake generation. ?? Pore Pressure Effects - High fluid pressure reduces effective stress, shifting Mohr’s Circle closer to the failure envelope, making fault slip more likely. This, in part, explains induced seismicity in fluid injection scenarios (e.g., fracking, geothermal energy). ?? Stress Drop in Earthquakes - The Mohr Circle helps visualize the change in stress before and after an earthquake (stress drop). Limitations of Using Mohr's Circle - While Mohr’s Circle is a valuable tool in geomechanics, rock mechanics, and fault analysis, it has several limitations, particularly when applied to complex geological settings. Stay tuned to our next posts if you want to learn more. #geomechanics #subsurfacealliance #faultstability #earthquakes #inducedseismicity

Chapter 2. How to Approach Fault Stability: Main Factors, Risks and Methods Fault stability refers to the condition of fault in which it remains in equilibrium without slipping or reactivating under existing stress conditions. It is a key concept in geomechanics, petroleum engineering, and earthquake science, as fault reactivation can lead to seismic events, wellbore instability, and reservoir integrity issues. 1. Main factors affecting fault stability are: ?? Stress State: Fault stability is typically address on a first pass using Mohr-Coulomb failure criteria. A fault becomes unstable when the shear stress exceeds the shear strength ?? Pore Pressure Changes: An increase in pore pressure reduces the effective normal stress, making it easier for the fault to slip. This is critical in petroleum operations, especially during fluid injection (e.g., waterflooding, CO? sequestration, or hydraulic fracturing). ?? Fault and Stress Orientation: Faults optimally oriented relative to the in-situ stress field are more likely to slip. The fault stability is often assessed using the Slip Tendency (ratio of shear stress to normal stress) and Dilation Tendency (propensity to open). ?? Mechanical Properties of the Fault Zone: Fault gouge, lithology, and clay content affect frictional strength. Weak fault materials (e.g., smectite-rich gouges) tend to reduce stability. ?? Reservoir Depletion and Compaction: Pressure depletion can induce differential compaction and stress redistribution, sometimes leading to fault reactivation and even induced seismicity. 2. In the energy sector, fault stability becomes relevant for: ?? Well Integrity Risks: Reactivation of faults can lead to casing deformation or loss of well control. ?? Reservoir Management: Stress changes from production or injection can trigger slip, affecting permeability and fluid flow. ?? Induced Seismicity: Injection-induced pressure changes may destabilize faults, causing small- to moderate-magnitude earthquakes. 3. Common Methods for Assessing Fault Stability ?? Slip Tendency Analysis: Determines fault slip potential based on stress conditions and fault orientation. ?? Coulomb Failure Criterion: Used in geomechanical modeling to predict failure under different stress regimes. ?? Numerical Modeling (e.g., Finite Element Analysis): Simulates stress evolution and fault behavior over time. Figure depicts common risks associated to CO2 injection, this includes fault instability and induced seismicity #geomechanics #subsurfacealliance #faultstability #inducedseismicity #geoscience #drilling

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