and Wellbore Stability Analysis for Deepwater Drilling Operations
All too frequently deepwater drilling operations are hampered by formation fluid influx (kick), drilling fluid loss (hydraulic fracture) and wellbore instability due to shear failure. These events are estimated to cost the industry almost $8 billion every year, and this number has not decreased despite advances in pre-drill modeling. Indeed, a GOM operator study cited 44% of their non-productive time (NPT) was associated with improper management of annular pressures leading to geopressure and wellbore instability related delays in well construction. These unscheduled events result when annular pressures stray from inside the safe operating envelope defined by shear failure pressure, pore pressure and fracture pressure. The integration of pre-drill geopressure and geomechanics analyses with real-time analysis has been a consistently effective strategy for avoiding or mitigating pre-drill uncertainties and subsequently improving well construction and evaluation efficiency.
Mud losses are a frequent problem when drilling in deep water. Maintaining wellbore stability is also quite challenging in the deep water environment. Induced mud losses occur when the mud weight, required for well control and to maintain a stable wellbore, exceeds the fracture resistance of the formations. In deep water drilling the safe operating envelope for mud weight, i.e., the difference in pressure between the pore pressure and fracture gradient, is often quite narrow. This means that the calculation of pore pressure and fracture gradient must be extremely accurate; hence the need for a system that can reliably predict pore pressure and fracture gradient with the ability to be updated while drilling.
Here is an example of the safe operating pressure envelope. Drilling annular pressure must be greater than the pore pressure and shear failure pressure, but less than the fracture pressure.
Landmark’s DecisionSpace® Desktop software, in which geopressure, earth stress and wellbore stability analyses are fully integrated with the earth model, seismic and geologic data in addition to well planning scenarios. This allows data and expertise to move from well planning through well construction and completion seamlessly and without information loss. It also helps ensure consistency from pre-drill prognoses in the well planning stages through well construction. However, the key to avoiding or at least mitigating unscheduled well events lie in the ability to update the pre-drill and well plan in real time.
Logging data streamed via WITSML in real time can be used at the bit for analyses and updated ahead of the bit forecasts for geopressure, earth stress and wellbore stability. The results of these analyses are an envelope of safe operating pressures which are used to constrain the annular pressures while drilling to prevent formation fluid influx, drilling fluid loss, or wellbore instability.
Drilling can cause the stresses around a wellbore to redistribute as the excavated rocks are replaced by drilling fluid. This can lead to either shear failure (wellbore instability) or tensile failure (fracturing.) Wellbore instability caused by a mud weight that is less than the shear failure pressure (outside the safe operating envelope) can result in borehole breakouts, hole closure, pack-off or hole collapse. Wellbore stability analysis is critically coupled to geopressure and earth stress analysis. Thus, it is critical that wellbore stability modeling be integrated with geopressure and earth stress modeling.
Determining the safe mud weight operating envelope is of critical importance for improving well planning, preventing wellbore stability problems, and reducing borehole drilling trouble. Having regional geology data, formation pressure measurement and well log data fully integrated with the earth model and well planning allows data and expertise to move from well planning through drilling and completion seamlessly and without information loss. It also helps ensure consistency from pre-drill prognoses in the well planning stages through well construction. The use of real time analysis performed while drilling to update the pre-drill model is the key to reducing uncertainty, avoiding drilling incidents, and increase drilling efficiency.
[toggle title=”Learn More About Cary Purdy”]
Manager – Geomechanics and Petrophysics
Cary Purdy has more than 30 years experience in the oil and gas industry. He is currently Manager of Geomechanics and Petrophysics for Halliburton’s Landmark Software and Services business line. Prior to Halliburton’s acquisition of Knowledge Systems, Cary was that company’s Senior Vice President of Software. Cary has also worked as General Manager for Petrotechnical Open Software Corporation (now Energistics) and Mobil Oil, where he spent the majority of his career in various technical and managerial roles. Cary has degrees in Geology and Physics from Bowling Green State University.
For more information, please feel free to contact me.