Abstract

The Valhall field is an over pressured, undersaturated Upper Cretaceous chalk reservoir located in the Norwegian sector of the North Sea. The reservoirs are consisting of weak chalk. The weak chalk results in significant reservoir compaction, exceeding 10 meters in places, and corresponding seafloor subsidence, exceeding 6 meters above the center of the field. Previous papers have documented the increasing drilling challenges in the shale overburden at Valhall due to 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 is focusing on handling the stress changes occurring in the overburden above a compacting reservoir and not the more traditional situation associated with depleted 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 to GOCAD in order to do wellbore stability calculations using BP's Well Planning Toolkit (WPTK). In this environment one can also easily use supporting data as 4D seismic from the permanent Life of Field Seismic array at Valhall. The paper describes the process used for implementing and verifying the technology. By calculating the operational mud weight window over a high risk interval in the overburden a good correlation to historical non-productive time was found. The methodology is used in detailed well planning where only moving the well 50 meters in one direction can result in problem free drilling or challenges. The paper presents the first application of this technology on a new water injector delivered 60 days ahead of schedule with reduced costs at around 20 million USD and a potential train wreck avoidance of 60 million USD.

Introduction

The Valhall field is an over pressured, undersaturated Upper Cretaceous chalk reservoir located in the North Sea approximately 290 km offshore southern Norway in 69 m of water (Figure 1). Valhall was discovered in 1975 and the field was originally developed to recover 250 MM BO, but has currently produced more than twice this amount, and work is ongoing to recover over 1 Billion BO from the Valhall structure. A three-platform complex with 24 slots was installed in 1981, on the crestal part of the field. The 24 slots were extended to 30 on the original drilling platform around 1990. In 1996 a new 19-slot wellhead platform was installed next to the existing central complex for infill and ERD drilling. Due to severe drilling problems during the ERD drilling a third wellhead platform was installed in the South flank in 2002 and a similar wellhead platform was installed in the North flank in 2003. In 2004 a new drilling and injection platform was installed on the crestal part to accommodate water flooding of the field.

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