Since the advent of steerable motors in the mid 80's, the drilling of complex horizontal 3D wells has become a standard in development drilling. To further extend the envelope and optimally position the wellbore to improve drilling efficiency and optimize production more sophisticated systems were required. Now two decades on, the industry has developed intelligent Rotary Steerable Systems (RSS) and sophisticated Logging While Drilling technologies that have both lowered well cost through NPT reduction and given the operator the opportunity to position the well path optimally in the eservoir, based on real time logging data. This environmenthas placed an increased reliance on Logging-While-Drilling LWD) measurements. In this paper we will review wells rilled and logged on a North Sea field and how the pplication of a number of advanced LWD technologies have aximized answers through acquiring full formation valuation in a single pass. We will discuss the importance of aving an integrated LWD system engineered for diverse rilling applications and also designed as a compact, modular ystem with sensors close to bit with increased flexibility and wide range of measurements. The realtime aspects of LWD re important in delivering valuable answers. Examples will e illustrated, which demonstrate the value of these echnologies in accurate estimating reserves, well positioning, on-productive time (NPT) reduction and drilling hazard itigation. With such a complex, integrated system, a horough approach to pre-job planning, realtime follow-up and ost-well analysis is a key factor in achieving full formation valuation data acquisition in parallel with improved drilling erformance.
The Grane field is located in the Norwegian North Sea (Fig. 1), and the reservoir consists of massive, predominantly fineto medium-grained, moderate-to-well sorted, turbidite sandstones of the Heimdal Formation of Palaeocene age, between Lista Formation shales located approximately 1700 meters below sea level. The sandstones are very friable, slightly quartz cemented and show excellent reservoir properties with permeabilities commonly in the 5-10 Darcy range and an average porosity of 33%. The Grane field was first discovered in 1991 and in early wells, internal shale layers were rarely encountered. Into production, shale sections were more frequently found closer to the reservoir boundaries. Logging-While-Drillling (LWD) imaging data reveal that these shales often have steep boundaries - suggesting a deformational origin associated with injections, folds and faults. The base and top reservoir surfaces both display rugged topographies. Despite the shallow depth, the seismic definition and interpretation of the reservoir boundaries are problematic, and several of the encountered shale intervals are difficult to detect on seismic data (Fjellanger 2006)
Fig. 1 - Grane Field location map, Norwegian North Sea.(available in full paper)
Helgesen et al. 2005 describes the background for well placement on Grane. The small density difference between oil and formation water created a long transition zone above the Oil-Water Contact (OWC) (Fig. 2). Drainage strategy nessessitated support by gas injectors and later water injectors and a dense well pattern of long horizontal producers were planned. Where a water leg existed the wells were to be placed 9m above the OWC and the reservoir base was above OWC, the wells were to be placed close to the base.