Process zone stress (PZS) has been found to correlate to poor stimulation efficiency and low production. Several models for estimating PZS have been developed by correlating to petrophysical logs (e.g., bulk density (RhoB), porosity (PhiE) and shale volume (Vsh)). Common practice, which does not capture lateral reservoir heterogeneity, uses logs in vertical wells to build a layer-cake model. This study uses geomechanical data acquired while drilling a horizontal well to build a calibrated petrophysical interpretation of bulk density, porosity and shale volume in order to estimate PZS. This workflow results in a stage-by-stage prediction of possible operational issues, which can improve operational efficiency and maximize the effectively-stimulated lateral length.
Geomechanical data were acquired in both a pilot and horizontal well in the Lower Lance Pool (LLP), Green River Basin, Wyoming. The geomechanical properties (Young's Modulus, Poisson's Ratio and VTI anisotropy) were calibrated with wireline in the pilot to calculate RhoB, PhiE, and Vsh. The calibrated petrophysical model was then applied to the mechanical data in the lateral wellbore, providing the inputs necessary for the PZS calculation.
Three PZS models were built in a 3D multi-well finite difference simulator using each of the three petrophysical inputs. Scalars for the models were calibrated to offset DFIT data. The resulting models were compared to determine the optimal model. Pre-stimulation instantaneous shut in pressures (ISIP) was evaluated in the horizontal well for each stage. High pre-job ISIP values are an indicator of stages that may be difficult to break down because of the apparent increase in stress associated with initiating and propagating the fracture. The ISIP analyses were compared to the prediction based on the synthetic PZS models to validate the result.
The three PZS models were evaluated for consistent, predictive behavior in the LLP. Results indicate that the RhoB and PhiE models were more consistent than the Vsh model. Additionally, the RhoB model benefits from more widely available calibration to offset triple-combo data, allowing it to be used with greater confidence throughout the basin.
A predicted ISIP was calculated (closure pressure + PZS) and compared to the pre-job ISIP analyses. The model predicted an average ISIP around 2% or less across the lateral. In contrast, a layer-cake model using the pilot data workflow predicted ISIP around 9% of actual. This variability may be caused by lateral changes in reservoir quality which the layer-cake model does not account for.
This workflow provides a calibrated method for incorporating PZS into a horizontal well using geomechanical data. The application accounts for the reservoir's heterogeneity, where the layer-cake approach to applying PZS is insufficient. The integration of the petro-mechanical and completion methodologies provides a unique opportunity to optimize completions in horizontal wells.