Geomechanical Optimization of Mud Weight Design and Fracture Risk Prediction in D1 and X1 Fields, Central Niger Delta

Abstract

This study employs an integrated geomechanical approach using exclusively secondary data to evaluate safe mud weight windows and fracture risk in two Niger Delta fields—D1 and X1. Secondary datasets, including drilling reports, wireline logs, leak-off tests, and formation pressure measurements, were analyzed to estimate key geomechanical parameters: pore pressure, minimum horizontal stress, overburden stress, and fracture gradients. Results indicate that both fields exhibit a normal faulting stress regime, with D1 displaying a wider mud weight window (8.65–13.5 ppg) compared to X1 (8.27–11.35 ppg). Wellbore stability analysis showed that instances of mud losses and kicks corresponded with periods when actual mud weight exceeded or fell below the predicted safe window, validating the model. A comparative analysis revealed that X1 poses a higher drilling risk due to overpressure and narrower margins. This study demonstrates that secondary data, when properly calibrated, can effectively guide mud weight design and mitigate wellbore instability in mature fields.

Introduction

Optimizing mud weight and predicting fracture risks are crucial in geomechanical planning for drilling in complex, overpressured basins like the Niger Delta. Incorrect mud weight can cause wellbore collapse or fracturing, leading to costly non-productive time. This study applies an integrated geomechanical workflow—using stress modeling, pore pressure analysis, borehole imaging, and pressure tests—to two Niger Delta fields, D1 and X1, which, despite close proximity, exhibit distinct geomechanical behaviors. D1 shows stable conditions and higher fracture resistance, while X1 faces overpressure, narrow mud weight windows, and frequent wellbore instability.

Key data from drilling reports, wireline logs, and pressure tests inform calibrated models to define optimal mud weight windows and fracture thresholds. The study confirms both fields have a normal faulting stress regime but finds that X1 has higher pore pressures, lower fracture gradients, and narrower safe mud weight windows, increasing drilling risks such as lost circulation and kicks.

Comparisons reveal wells staying within planned mud weight windows avoid losses, whereas exceeding these limits, especially near fracture pressures, causes significant fluid losses. A geomechanical well plot highlights an incident where the equivalent circulating density exceeded fracture gradient, causing loss, validating the model's predictive power.

The study underscores the necessity of precise, field-specific mud weight management informed by real-time data and detailed geomechanical modeling to ensure wellbore stability and minimize operational hazards in the Niger Delta’s challenging drilling environment.

Conclusion

This comparative geomechanical study of Niger Delta fields demonstrates that precise mud weight management, grounded in accurate pore pressure and stress gradient evaluation, is critical for minimizing drilling hazards. The D1 field exhibits relatively stable and narrower pore pressure ranges (0.42–0.46 psi/ft) and higher fracture gradients (0.80–0.81 psi/ft), affording a broader mud weight window and lower incidence of wellbore losses when controls are properly maintained. In contrast, the X1 field shows elevated and more variable pore pressures—particularly in sands (0.55–0.60 psi/ft)—as well as lower fracture gradients (0.59–0.69 psi/ft). These parameters yield a narrower safe mud weight window, making X1 more susceptible to kicks and lost circulation if mud weight overshoots occur. Field incidents from Table 1 and the wellbore stability plot (Figure 4) confirm that exceeding the upper mud weight limit—approaching or surpassing the fracture pressure—directly correlates with formation breakdown and fluid losses. Overall, the study underscores that local geo mechanical variability dictates drilling risk: fields with higher overpressures and lower fracture thresholds require more conservative drilling practices than those with more benign stress regimes.

References

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Copyright

Copyright © 2025 Adiele O, Okengwu K, Ugwueze C. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.