GLJ’s Erfan Sarvar Amini, Ph.D., P.Eng. will present a paper entitled “Shale fracability in Kaybob-Duvernay: A new approach based on plasticity behavior” at the 55th American Rock Mechanics Association (ARMA) this June 18-25.
Kaybob Duvernay, one of the most prolific source rocks in the Western Canadian Sedimentary Basin, is the focus of this paper — providing new insights on optimization of multi-stage hydraulic frac jobs in the area.
The geomechanical behaviour of the Duvernay source rock is interesting and unusual. It has significant hydrocarbon storage with nano-Darcy scale permeability, and lacks any evidence of widespread micro fracturing when not proximal to a known fault lineament. The lack of micro fracturing, combined with extremely large pore pressures, variability in organic matter, and mineral composition, have resulted in complex mechanical rock-fracturing characteristics.
A novel universal fracability index is developed to quickly identify the best landing intervals for optimization of the frac jobs through creation of a maximum stimulated rock volume (SRV). This new index provides a high-resolution fracturing discriminator to separate the highest geomechanically competent intervals from the lowest, by integrating various parameters: Tectonic in-situ stresses, pore pressure, and geomechanical and petrophysical properties. This approach allows operators to improve the completion quality and well performance at almost no additional cost.
An example of this analysis is presented below for Duvernay Kaybob as a test case. Using the analysis, we show that, unlike commonly held beliefs among the Duvernay operators, the targeted pay (Upper Duvernay) is not geomechanically homogenous. In fact, the rocks become substantially geomechanically competent closer to the bottom of the pay. Currently, the majority of the wells in the Kaybob have landed in low-to-moderate fracable intervals. We determined the best landing interval to be closer to the bottom of the pay (as marked by the red circle). We also showed that highest fracable intervals tend to have higher pore pressure, lower net effective closure stress, higher organic matter, and lower initial frac toughness.
Fracability of well of interest. The fracability index is the lowest (most negative) when rocks are less fracable and highest (less negative) when they are more fracable. The typical and ideal landing intervals are shown on the fracability plot.
- When highest fracable intervals are targeted for multi-stage frac jobs, there are several benefits. This paper presents the development and construction of a novel geomechanics cross plot to split the effects of shear fracturing from tensile fracturing for the Kabob Duvernay source rock as a test case. Using this cross plot, we identified several hydraulic fracturing behaviours, ranging from a simple planar frac to ductile and complex fracturing behaviour — depending on the geomechanical rock characteristics. Using this cross plot, we show how the Duvernay pay favors the creation of fracture complexity, invalidating the planar frac assumptions. The creation of complexity in Duvernay is a necessary condition for economic hydrocarbon recovery from extremely tight Duvernay source rocks. Inherited geomechanical heterogeneity, targeting the highest fracable intervals, improves the chance of creating more complexity.
Kappa-Mu-Yield (KMY) universal cross plot in the Duvernay colored for pore pressure. The main stratigraphic layers are projected on the same plot. The Poisson’s ratio isolines are shown using a dashed line. The tendency of rock to create a hydraulic fracture network, increases with pore pressure.
- Through several simulation case studies using STimsIM (GLJ internal frac simulator), we demonstrated the benefits of targeting higher fracable intervals, particularly for multi-cluster plug and perf frac jobs. We showed four primary benefits of targeting higher fracable intervals:
- Creation of conformity in the distribution of the frac half length during multi cluster plug and perf.
- Improving the chance of frac containments and mitigation of out-of-one fac growth.
- Creating hybrid frac geometries including both vertical and horizontal fracs.
- Creating higher permeability SRVs, less tortuous frac path as well as better chance of proppant transport.
We used two simulation case studies to demonstrate the benefits of targeting higher fracable intervals; (1) when the fracs are completed in low-to-moderate geomechanics intervals (Typical landing), and (2) when the fracs are completed in the highest fracable intervals (Ideal landing). These simulations were done for an example of multi cluster plug-perf frac job assuming 4 clusters at 25 m cluster spacing. For the problem initialization, we used the calibrated geomechancial and geological model as well as an actual frac pumping schedule.
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