Integration of Drilling and Geological Data to Understand Wellbore Instability

Abstract

Abstract Throughout the initial phase of development on the Mungo field in the Central North Sea, wellbore instability and mud losses have caused serious drilling problems and additional expense. Geomechanical modeling around the salt diapir led to refinement of some drilling procedures but the direct cause of the drilling problems still remained poorly understood. A detailed fault mapping study has been carried out from the Paleocene reservoir interval through the overburden. Wellbore instability and mud loss zones displayed along the wellbores were integrated with the 3D fault model and demonstrated that the wellbore instability and mud loss incidents coincide with the location of seismically resolved faults. However, not all faults in the overburden or reservoir were associated with drilling problems. It was found that the intersection angle between the wellbore and the fault surface is critical to determine whether drilling through the fault will induce instability. The Mungo data indicates that small intersection angles (commonly between 0–45°) resulted in drilling problems while larger intersection angles (commonly between 45–90°) were not associated with drilling problems. These results have been incorporated into the current well planning and drilling strategy for Mungo development wells. So far, a sidetrack well and chalk oil appraisal well has been successfully drilled in the overburden. Potential problem zones predicted in advance were mitigated for in the well planning stage and contingencies included in the drilling budget. The overburden drilling risks are understood by visualizing the integrated geology and drilling data in 3D, so it now takes less time, and therefore less cost to drill successfully to the reservoir target. The techniques are also being applied successfully for well planning and drilling on other fields. Introduction As 3D visualization technology advances, it is becoming easier to create and manipulate large field-scale models. These models incorporate reservoir and overburden geological data such as horizons and faults, where geological information is posted along the well trajectories. Commonly these models have not included displays of drilling information so it has not been possible to determine the spatial relationship between drilling problems and geological phenomena. Typical drilling problems include mechanical instability (caused in part by cavings), hole cleaning difficulties, lost circulation, fluid gains, tight hole, stuck pipe, shallow gas and overpressure. These incidents can occur over a range of depths or at a discrete location along the well bore and may result in additional financial costs on top of the approved drilling budget. Multidisciplinary communication between geologists and engineers is becoming easier through the application of 3D visualization technology to build field wide models that incorporate engineering and geology information (Ref. 1). The 3D models are used to assess the impact the overburden and reservoir geology on the drilling success of previous development wells. By understanding the drilling risks, it then becomes easier to plan and drill new well trajectories in three-dimensional space that avoid drilling through any identified high-risk zones. This means that well trajectories may not follow the shortest route between the drilling rig and reservoir target but there is a greater chance that the well reaches the reservoir target without encountering unexpected subsurface drilling problems and additional drilling costs. It is therefore very important in the well planning and implementation stages for geologists and drilling engineers to communicate all knowledge about the geological and mechanical uncertainties in the overburden and reservoir. Effective risk reduction and risk mitigation procedures can then be implemented and the well completed safely, within a cost effective drilling program. The techniques outlined in this paper discuss the impact of integrating overburden and reservoir geology and drilling data during development of the Mungo field.