Sixty Days Ahead of Schedule: Reducing Drilling Risk at Valhall Using Computational Geomechanics

Abstract

Summary The Valhall field is an overpressured, undersaturated Upper Cretaceous chalk reservoir located in the Norwegian sector of the North Sea. The reservoirs consist of weak chalk. The weak chalk results in significant reservoir compaction, exceeding 10 m in places, and corresponding seafloor subsidence, exceeding 6 m above the center of the field. Previous papers have documented the increasing drilling challenges in the shale overburden at Valhall because of the subsidence. This paper presents the computational-geomechanics technology developed and implemented to assist drilling into the highly depleted and compacted crest from the new water-injection platform at Valhall. The work focuses on handling the stress changes occurring in the overburden above a compacting reservoir and not the more traditional situation associated with depletion-induced fracture-gradient changes in the reservoir itself. The technology is based on a history-matched full-field finite-element-based geomechanics model to calculate stresses, strains, and displacements. The results are exported from the finite-element model (FEM) to a geological modeling software to perform wellbore-stability calculations using BP best practices for prediction of wellbore stability. In this environment, one can also easily use as supporting data 4D seismic from the permanent life-of-field seismic array at Valhall. By calculating the operational mud-weight window over a high risk interval in the overburden, a good correlation to historical nonproductive time (NPT) was found. The methodology is used in detailed well planning where moving the well 50 m in one direction is the difference between problem-free drilling or huge drilling challenges. The paper presents the first application of this technology on a new water injector delivered 60 days ahead of schedule with reduced costs of approximately USD 20 million and with avoidance of a potential "train wreck" that could have cost USD 60 million.