Title : A novel geomechanical workflow for well design optimization through effective stress coefficient calibration and friction coefficient estimation, a case study from WDDM concession, Mediterranean sea, Egypt
Abstract:
1. Introduction:
Accurate prediction of fracture pressure and minimum horizontal stress (Shmin) is fundamental for establishing safe drilling mud-weight windows and reducing drilling risks. Errors in stress prediction can lead to severe operational problems such as lost circulation, wellbore instability, differential sticking, sidetracking, and increased non-productive time.
The West Delta Deep Marine (WDDM) Concession, located offshore Egypt in the Mediterranean Sea, contains numerous gas-bearing Pliocene deep-marine turbidite reservoirs. While many successful wells have been drilled in the concession, several wells experienced drilling challenges that highlighted the importance of accurate geomechanical characterization during well planning.
This study presents a new innovation field-based methodology that derived an equivalent a friction coefficient (µ) estimation inspired from the Jaeger and Cook concept which estimated the friction coefficient (µ) from the slope of Mohr envelope failure line. In this approach, the overburden stress is considered as the normal vertical stress in a triaxial compressive stress lab test, while Leak-Off Test (LOT) measurements are used as the shear stress. By rearranging the stress relationships, 20 wells used to draw a chart between OBG at Y-axis and LOT at X-axis and draw the best fit line, assumed this line is equivalent to the Mohr envelope failure line and used the slope of this line to estimate the friction coefficient and use it in (Jaeger & Cook) minimum horizontal stress equation.
Estimated the effective stress coefficient (K˳) at the LOT depths for the same 20 wells, plotted the ko results against the depth (TVD) and determined the lower bound of K˳ and use it in (Mathews & Kelly) minimum horizontal stress equation. Applying more different methods for the minimum horizontal stress prediction; Horsrud, Hubbert & Willis, and showed the Eaton fracture gradient prediction while drilling. The methodologies evaluated against actual drilling events from the SSc-Da well to assess its effectiveness in improving well design and reducing drilling uncertainty.
2. Well Story:
SSc-Da well original hole was drilled with MW 13.5ppg, total losses, pack-off then plug and side track the well! Side Track-1 which was drilled with MW 13.5ppg, but again several tight spots encountered and differential stuck, then side track-1 was plugged back and drill Side Track-2 which was wisely managed and drilled with the previous MW 13.5ppg till certain depth and then set a casing and decrease MW to 11.5ppg and continue drilling until the target was safely reached
3. Objectives
The objective of this study is to improve well design reliability through better calibration of geomechanically
parameters controlling minimum horizontal stress prediction.
The specific objectives are:
- To evaluate different industry methods for minimum horizontal stress estimation.
- To calibrate the effective stress coefficient using available field data and observed drilling performance.
- To develop a new innovation field-based methodology for estimating an equivalent a friction coefficient estimation inspired from the Jaeger and Cook concept which estimated the friction coefficient from the slope of Mohr envelope failure line.
- To investigate the impact of the calculated friction coefficient on minimum horizontal stress prediction.
- To validate the methodology against actual drilling events encountered in the SSc-Da well.
- To identify the most representative workflow for future well planning and mud-weight window design within the WDDM concession
4. Conclusion & Recommendation:
The study demonstrates that accurate calibration of geomechanically input parameters has a significant impact on minimum horizontal stress prediction and, consequently, on well design quality. The Matthews and Kelly methodology, when calibrated using the appropriate effective stress coefficient trend, provided the most reliable method to produce different cases (Low-Mid-High) for minimum horizontal stress prediction and showed the best agreement with observed drilling events and formation behavior.
Provided a new innovation methodology in the workflow of minimum horizontal stress prediction that derived an equivalent a friction coefficient estimation inspired from the Jaeger and Cook concept which estimated the friction coefficient from the slope of Mohr envelope failure line. In this approach, the overburden stress is considered as the normal vertical stress in a triaxial compressive stress lab test, while Leak-Off Test (LOT) measurements are used as the shear stress. By rearranging the stress relationships, 20 wells used to draw a chart between OBG at Y-axis and LOT at X-axis and draw the best fit line, assumed this line is equivalent to the Mohr envelope failure line and used the slope of this line to estimate the friction coefficient and use it in (Jaeger & Cook) minimum horizontal stress equation.
The study concludes that applying both of calibrated effective stress coefficients with field-derived friction coefficients provides a practical and effective workflow for improving fracture pressure prediction, optimizing mud-weight windows, reducing uncertainty during drilling operations, and enhancing future well designs within the WDDM concession.
