Geomechanical analysis is critical to understanding reservoir thickness changes that occur due to production and injection. With an accurate understanding of subsurface mechanical responses, geologists and engineers gain greater confidence in planning and operational decisions. Leveraging a geomechanics workflow for reservoir analysis can lead to a more efficient way to guide all phases of a reservoir’s development and production.
The Benefits of Leveraging a Geomechanics Workflow
Geomechanics modeling can involve significant volumes of input data and often requires many iterations, varying parameters with each iteration, before reaching a conclusion. CoViz 4D, an industry-leading software package for dynamic reservoir visualization and analysis offers an optional 4D geomechanics workflow module that simplifies and accelerates the reservoir analysis by providing a formal, consistent method for:
- inputting the subsurface data sources used to create the geomechanical models
- selecting the attributes of the cellular grid used to calculate volume change
- defining the parameters relating to deformation, strain, and stress changes
- controlling aspects of the seismic velocity change and time-shifts
- performing the calculations and visualizing the results in 3D or 4D (time-series)
Throughout the life of a reservoir, geomechanics workflows can easily incorporate new data into the analysis to provide a more accurate model.
A More Efficient Reservoir Analysis
The relationship between overburden deformation and reservoir thickness change can be complex. It depends on a multitude of parameters—geometric, lithologic, and geophysical. Finite element modeling is used to understand how changes in the reservoir thickness might impact the overburden and surrounding areas, which in turn has direct impacts on well integrity. However, running a full finite element analysis is time-intensive and costly.
CoViz 4D provides an alternative to finite element modeling, using the Geertsma (1973) subsidence equation to compute reservoir volume change, overburden deformation, changes in uniaxial thickness, overburden stress changes, and seismic time-shifts based on reservoir simulation model inputs.
Using a first-order computation of deformation based on parameterized inputs, geologists and reservoir engineers can quickly gain an understanding of structures. This insight can either confirm their predictions to guide immediate planning and operations or determine that finite element analysis is indeed required to evaluate shear or wellbore integrity risks in greater detail.
The CoViz 4D geomechanics workflow consists of four steps. Here’s a brief description of each:
- Indicate the types of calculations (deformation, strain & stress changes and ⁄ or velocity change & time-shift) and inputs (reservoir simulation cellular grid with attributes such as dynamic pressure, net-to-gross ratio, porosity, and pore volume multiplier, and initial velocity grid to sample the volume change and compute the deformation).
- Select the attributes of the cellular grid, time steps, and effective stress coefficient (α) to calculate volume change.
- Define the parameters related to deformation, strain and stress change (Poisson’s ratio, overburden Young’s modulus value, displacement grid vertical spacing).
- Determine aspects of the seismic velocity change and time-shifts (method to define the R-factor).
The output of the geomechanics workflow varies, based on the input data and parameters provided and may include any of the following:
vertical stress change
|change in seismic interval velocity,
new seismic interval velocity,
truncated initial interval velocity
|horizontal displacement (in x, in y),
horizontal strain (εxx and εyy),
horizontal effective stress change (in x, in y)
|uniaxial thickness change,
change in two-way travel time,
|shear effective stress change (Δτxy, Δτyz, Δτzx)||shear strain (εxy or εyx, εyz or εzy, εzx or εxz)|
The results are easily visualized in 3D and 4D to understand the displacement, strain, thickness, and other changes in, above and below the reservoir. CoViz 4D enables reservoir teams to view results in context of existing wells and horizons to understand how deformations impact reservoir performance. Multiple time-step outputs can be viewed using CoViz 4D’s temporal animations capability.
CoViz 4D offers a rapid, easy to use, first-look geomechanical analysis for assessing reservoir thickness changes that could lead to well integrity issues.
The geomechanical workflow is easy to set up and delivers results in significantly less time than a finite element analysis approach (minutes to hours vs. days to weeks). Computation time can be further reduced on computers that have a GPU.
Customized Scripts for Geomechanics Workflow Efficiency
Scripts containing all the function calls to reproduce the geomechanics operation—including all the parameter settings—can be saved. Scripts can save significant time when iterating on parameters, performing sensitivity or uncertainty analysis, or similar repetitive geomechanics tasks. Experienced geophysicists also have the option of using Python and Jupyter Notebooks to write customized geomechanics workflow scripts to automate the analysis. This is the optimal technique when iterative sensitivity analysis is required to change parameters and evaluate the effect. As new data are acquired, saved geomechanics workflows are easily updated and run to calculate new models.
Accurate geomechanical models are critical to making well-informed planning and operational decisions. CoViz 4D facilitates rapid, cost-effective assessment of geomechanical dynamics by reservoir teams. Using an intuitive geomechanics workflow user interface or custom-developed scripts, reservoir teams easily combine data sources, select parameters, run calculations, and visualize results to quickly gain a better understanding of reservoir dynamics.