Maersk Olie og Gas AS as operator for the Danish Underground Consortium (DUC) has successfully planned and delivered an Observation and Monitoring well in the Halfdan field located in the southern part of the Danish North. Although not entirely unique to the industry (for further examples see Richardson, 1977 1 ; Widmyer, 1987 2 ;Wannell & Ezekwe, 1992 3 ) this will be the first well of its kind for Maersk Oil and the DUC placed in a chalk reservoir. This paper describes the planning and execution phases of the monitoring and observation well legs, summarizing the formation evaluation results primarily related to remaining oil saturations. The data derived from the evaluation program enables an evaluation of the success of the novel wells pattern design in the Halfdan field, enabling optimization of the reservoir recovery, in addition to confirming the vertical extent of the hydrocarbon column.As oil and gas fields mature, the monitoring of production-induced changes becomes crucial to sustain, optimize, and improve production levels. Enhanced recovery techniques are applied to extend the field life, as a result reservoir behavior, including vertical and lateral sweep, becomes more complex and challenging to model. Water injection is a common practice used to maintain the reservoir pressure and enhance oil sweep; yet sweep efficiency is not always equal, with water tending to move heterogeneously through the reservoir seeking higher permeability pathways and leaving trapped/un-swept oil behind. The fluid movement and distribution within the reservoir characterises the efficiency of the production system. Such inherently complex and capital intensive nature of understanding and optimising the recovery mechanism behoves the developer to acquire information to evaluate and enhance the recovery mechanism targeting maximising returns.Monitoring and Observation wells allow the detection of in-situ fluids, enabling modification and enhancement of the dynamic modelling, assist with evaluation of the applied IOR technique, and lay the foundation for potential future EOR opportunities. The two-pronged well provides an early indication of the recovery mechanism success in terms of sweep efficiency, and is a guide to further performance optimisation; additionally it is an opportunity to identify and develop any un-swept volume.The Halfdan field is situated in the Danish North Sea Central Graben approximately 250 kilometers off the West coast of Denmark, and is located between the Dan and Skjold fields. The Halfdan reservoir is Maastrichtian and Danian aged chalk characterised with relatively high porosity (25-35%) and low permeability (0.5-2 mD). Halfdan was discovered in 1998 with a 30,000 ft long horizontal well drilled from the Dan field. The first vertical well was completed in 1999. First production from Halfdan was obtained in late 1999.A slant observation and monitoring well on the Halfdan field was drilled between neighbouring injector and producer horizontal wells respectively, the first such well in the Danish Nor...

Well engineers are often presented with a number of potential solutions to drilling challenges as they develop a plan to safely and efficiently execute the drilling program to meet the overall well objectives. The solutions will be evaluated and the option that presents the lowest risk while providing the maximum benefit is normally selected as the preferred solution.The scope of this paper is to review the challenges faced by a North Sea Operator after identifying the likely presence of high pressure gas stringers in a carbonate formation on a recent brown-field datagathering well, and the subsequent deployment of solid expandable technology to provide a safe and effective solution. Solid expandable tubulars have demonstrated value as a reliable alternative solution to conventional liners in providing zonal isolation of troublesome formations while maximising drill-through inside diameter (ID). The improved technology and increased reliability in the systems has provided greater confidence that the systems can be deployed successfully with an acceptable level of risk.The Operator required the isolation of a high pressure gas-bearing zone located below the 9-5/8Љ casing shoe to enable the safe drilling and logging of the reservoir section. The use of conventional liners was evaluated but discounted due to the reduction in internal diameter that would prevent subsequent logging and running of the planned liner in the reservoir section. Deepening the 9-5/8Љ shoe to cover the high pressure stringers presented higher risks associated with drilling into the anticipated depleted reservoir. After evaluation of the various options, solid expandable technology was selected as the best option to meet the well objectives.A 7-5/8Љ 29.7 lb/ft solid expandable liner system was successfully installed to isolate the high pressure gas stringers located below the 9-5/8Љ shoe. The system included an expandable liner hanger set in open hole to provide zonal isolation in place of cement. In addition, the liner lap was extended back into the 9-5/8Љ casing to isolate a leaking stage tool. The application of both repairing a casing leak and isolating a high pressure open hole zone using an uncemented liner was a first for the Operator. This allowed the Operator to drill the reservoir section with a lower mud weight while minimizing the risk of differential sticking and losses, the logging program to be conducted as desired and the 5-1/2Љ fibreglass liner to be installed as planned. The safe and successful drilling of the well, coupled with the capture of desired information met the Operator's well objectives, adding value to future field development.While this was the first solid expandable installation in this size for the Operator in the area, their confidence in the technology from previous installations allowed them to adapt the system to provide the required open hole zonal isolation whilst also reinstating integrity to the 9-5/8Љ casing.

Tracers in drilling mud have been used during coring operations as a tool to quantify the degree of mud filtrate-invasion in the core. This paper describes the application of a deuterium oxide water-based mud tracer that allows for real-time analysis of the tracer concentration while the drilling operation is ongoing. This ensures that a uniform mud tracer concentration can be maintained during coring operations. By using a Cavity Ring-Down Spectroscopy (CRDS) technique, the tracer concentration offshore can be assessed within 30 minutes of sampling. The combination of deuterium oxide tracer and CRDS analysis constitutes the first HSE-compliant real-time tracing for offshore coring operations. This method was applied successfully on a Maersk Oil exploration well drilled in the North Sea. The mud tracer concentration was monitored throughout the coring operation and subsequently compared to the tracer concentrations found in the core material. It was concluded that mud filtrate invasion ranged from 26% to 62%.