Emphasis on safety, risk reduction, environmental concerns and cost reduction has led to a growing focus on geomechanics in recent years. As such, attention has shifted to large operational safety risks, most of which are traced back to unmonitored reservoir volumes or stress changes. The consequences of these risks can be serious, costly and lead to numerous problems such as collapsed overburden, uplift, bedding plane slip, fault reactivation and induced seismicity. Disregarding a fully integrated subsurface geomechanical study may lead to breach or casing shear, rendering existing production and future wells useless. The following are some of the most serious documented examples of the impact of geomechanics throughout the asset life cycle.

In 2017 the Groningen Field was shut in following significant production-induced seismicity, giving rise to earthquakes of up to 3.4 magnitude. Not only did this national crisis impact the delivery of gas to 90% of Dutch homes but resulting dangerous flow paths led to serious well-control incidents and danger to life. In another example, the Elgin-Frankin fields suffered a major gas leak in 2012. This was partly caused by significant production/depletion in the deeper target reservoirs leading to geomechanical effects in the overburden (Hod Formation). The field restarted production one year later at approximately 50% of the original production rate.

More recently, carbon capture and storage also has caused geomechanical issues. The In Salah CO2 storage project in Algeria, for example, observed overburden uplifts of up to 2.5 cu. m (88 cf) in 2013 because the CO2 injection pressure was significantly higher than the formation pressure. Additionally, fracturing, human-induced seismicity, natural geohazards and associated environmental issues have caused significant controversy. Some countries are completely opposed to these drilling practices; most notably, fracturing was banned in France in 2017. With the global increase in onshore shale gas exploration, care must be taken to ensure that such geomechanical impacts are closely monitored.

Performing a geomechanical study, however, can be relatively complex. Pre-existing datasets from a number of different disciplines (petrophysics, geophysics, geology, geophysics, drilling, production and reservoir engineering) are required. Regardless of the model complexity, which can be 1-D, 2-D, 3-D or even 4-D, much of the data are fundamental to many of the studies already being performed as part of standard industry workflows. A 1-D geomechanical model would require well logs (VP, VS, Rho, GR, resistivity) as input data, whereas a 3-D model would be constructed from seismic inversion cubes (AI, VP / VS, SI, VP, VS, Rho) calibrated to lithologies, porosities, core data and drilling histories.

Despite the industrywide acknowledgement of the importance of understanding and integrating geomechanics into subsurface workflows, a problem remains. The study of geomechanics is largely used in a reactive rather than predictive sense to drilling or field development problems.

Even within geomechanics, there are subdivisions such as wellbore geomechanics or reservoir geomechanics. Often these specialists work within isolated teams that do not necessarily communicate enough with each other. Wellbore geomechanicists work on optimizing well designs, well trajectories and wellbore stability both predrill and while drilling. Furthermore, they provide useful input into the designing of well completion programs for incorporation into associated drilling operations and engineering departments.

By contrast, reservoir geomechanicists focus on the understanding of large-scale geological deformations over production timescales, changes in fluid pressures and injection leading to permeability changes and impacting reservoir performance. Meanwhile, modeling safe operating conditions for pressure/temperature (and other parameters) to avoid subsidence, fault reactivation and well integrity issues are treated by reservoir and drilling teams independently even though they play a critical role in quantifying risk, drilling safety/efficiency and maximizing economic recovery.

A comparison of a cross-plot (left) of P-wave velocity versus inclination angle through the overburden formation for a group of deviated wells across a field appraisal area are shown. The well panel (right) shows results of the anisotropic correction of well logs and the impact upon the well tie. Tracks 1-3 show GR/caliper, VClay and porosity, respectively. Tracks 4-8 show elastic logs VP, VS, Rhob/CNL, AI and VP/VS for the measured (black) and corrected (red) cases. Track 9 shows the wellbore inclination. Tracks 10 and 14 show the seismic to well tie before (left) and after (right) correction for anisotropy. Tracks 11 and 13 show prestack synthetics before (left) and after (right) anisotropic correction compared to prestack seismic data at the well location (Track 12). Without incorporating anisotropy into the workflow, it is not possible to calibrate the seismic to the well. (Source: Ikon Science Ltd.)

Collaboration and integration

As discussed, geomechanics should be part of every subsurface discussion from early opportunities in data gathering (e.g., acquisition of logs, quality control and analysis of laboratory core samples) through field development and production monitoring, where associated stress changes lead to complex and potentially costly consequences. These might include casing deformation and long-term wellbore integrity, “narrow” mudweight windows, production, overburden deformation, seal integrity issues and fault reactivation).

As such, a fully integrated geomechanics software package that can link all workflows and best practices throughout the stages of the field life cycle provides a practical solution to these issues. Such a software package should allow the managing and interpreting of data from different disciplines to effectively and efficiently propagate and communicate risk within full 1-D to 4-D workflows for geology, geophysics and drilling. Ikon Science provides this with RokDoc, an integrated geomechanical risk modeling solution of cross-disciplinary workflows, leveraging the interaction between geopressure, quantitative interpretation rock properties and drilling as well as allowing intelligent extrapolation of geomechanical properties and more robust subsurface models. The single-platform workflow allows all data to be captured and multiple models to be tested quickly to effectively evaluate any uncertainty, facilitating clear and consistent communication of risk.

Case study on integration

A short case study associated with anisotropy highlights the importance of an integrated, multidisciplinary workflow.

Most rocks are anisotropic to some degree. Many geological processes result in the preferred horizontal orientation of minerals, mineral assemblages, lithologies and sequences, which in addition to the intrinsic anisotropy of many minerals means that vertical transverse isotropy (VTI) is very common. It also has been determined that the magnitude of such anisotropy can be large over a wide range of measurement scales. Mitigation for VTI (and other forms of anisotropy) is now common during seismic processing and correction of elastic measurements in deviated wells. All of this has had a positive impact on seismic reservoir characterization, pore pressure prediction, and geomechanical analysis and modeling. A key component to the success of subsurface projects relies on the awareness of the team and the specialists within it to know where and how the data are used, and their derivatives can form useful inputs and calibration for other important workflows. For example, bedding planes, fracture planes and borehole breakouts interpreted from image logs can be utilized in workflows to correct or model sonic logs during anisotropic analysis and modeling workflows, improving seismic-well calibration and characterization of oil and gas fields. Vertical seismic profile data provide independent meso-scale data that can be useful in upscaling well-based measurements to seismic measurements. All of the anisotropic parameters derived through these analyses can be combined and fed into geomechanical models to improve their accuracy and robustness, ultimately leading to improved well design and productivity.

Conclusion

Geomechanical issues affect all stages of a field’s development from the initial exploration stage through the appraisal and development of the field and finally to full field abandonment. To improve geomechanical models, considerations of the subsurface through integrated cross-disciplinary techniques must be utilized. Two parts can enable improvements: first, having a software platform that allows integrated functionality and the ability to capture all data types, and secondly, the awareness of teams and specialists to know where and how their data can be used and how their derivatives can form useful inputs and calibrations to other important workflows.

References available.

Contact Rhonda Duey at rduey@hartenergy.com for more information.

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