Summary Ultra-high-pressure high-temperature (uHPHT) reservoirs undergo extreme pressure depletion during their production life cycle. This results in significant reservoir compaction and consequent overburden subsidence with major consequences for wellbore mechanical integrity, safety, and field economics. However, the use of underdetermined geomechanical models to accurately predict compaction-induced stress/strain changes on wellbores and its consequences during production time results in significant residual uncertainty. One method of measuring compaction-induced stress/strain changes in wellbore is by the emplacement and measurement of radioactive markers. Although it is long established in normal pressure reservoirs, it is rare in uHPHT projects. The Culzean uHPHT gas-condensate field is located in the UK Central North Sea. To constrain geomechanical model compaction uncertainty, radioactive markers were deployed. The objective was to accurately acquire preproduction baseline measurements and subsequent changes through periodic measurements during production life. These accurate wellbore measurements would then be compared with the geomechanical model to help calibrate predicted to actual compaction. By doing so, the objective is to enable better informed decisions regarding well and field management. The Culzean uHPHT radioactive marker project comprised a planning phase and a preproduction safe deployment including a baseline survey phase. Subsequent repeat measurements are planned during field production life. The emplacement and surveying of the subsurface radioactive markers for compaction monitoring in uHPHT reservoirs is a high value but nontrivial operation. In addition, much knowledge and experience of the methodology has been lost. This paper contributes to published literature by describing the successful emplacement and monitoring of subsurface radioactive markers on Culzean and aims to capture learnings and knowledge for future workers. Early detailed planning coupled with extensive testing is key to successful deployment. Timely engagement of all stakeholders and ensuring all decisions are aligned with safety and environmental considerations also contribute to realization of the project aims.
The Donan Field is a mature asset in the final phase of production, following the redevelopment facilitated by various advances in technology and subsurface understanding. The original development utilized an ingenious single-well oil production system vessel which made small hydrocarbon accumulations economic, while the use of a floating production, storage and offloading vessel to redevelop the Donan Field as the ‘Dumbarton Project’ allowed the previously stranded Lochranza and undiscovered Balloch fields to be developed.Donan and Lochranza are typical Paleocene oilfields with excellent water drive from a large regional aquifer. Balloch is an Upper Jurassic oilfield of equivalent size to Lochranza supported by a large regional aquifer but has a considerably higher recovery factor on account of excellent reservoir properties combined with a more optimal geometry to effectively sweep the reservoir. Most of the fields have exceeded pre-development expectations, particularly Balloch on account of it being developed whilst considerable subsurface uncertainties remained.Recognition of a seismic amplitude v. offset response across the Donan Field was key to redevelopment, significantly increasing the oil in place and guiding the locations of development wells. This was supplemented by the ability to geosteer the horizontal development wells in the shallowest possible reservoir sand to maximize the recoverable resources. The use of horizontal development wells facilitated the development of short, areally extensive, oil columns; while the design of the production facilities and wells to include permanent artificial lift and capacity to process large water volumes was essential.
The Culzean field is an ultra-High Pressure, High Temperature (uHPHT) gas-condensate accumulation and is one of the largest development projects of recent years in the UKCS. From experience in analogue fields, compaction is likely to occur and negatively impact the deliverability of wells. With accurate geomechanical modelling such issues can be assessed and proactively mitigated. Measurement of changes in distance between radioactive markers is a recognised technique for compaction monitoring. The measurement is undertaken throughout the life of the well and is used in the calibration of field geomechanical models. Markers were emplaced in two wells in Culzean. To ensure safe and successful emplacement of the markers a real-time estimation of rock strength was undertaken using logging-while-drilling (LWD) sonic data. Key to success was obtaining the correct data for timely decisions on the depth of radioactive markers. The paper describes the process used to validate sonic data from an LWD sonic tool by comparison with a wireline acquisition and its later application in two additional wells where markers will be emplaced. A unipolar LWD sensor was included in the drilling assembly to transmit semblance image in real time, which was manually picked on surface for compressional and shear slowness. Rather than automatic downhole picking, this approach allowed immediate offset wells comparison, improved reliability and quality checking which increased confidence in the final product. In the initial well, the resulting logs were compared with data acquired from a wireline dipole sonic tool showing a very good match and proving "equivalent-to-wireline" quality of the measurement. In two subsequent wells once total depth was reached, raw memory and diagnostic data were downloaded from the tool for functional and quality checks and processed to generate semblance images. After undergoing quality check, evaluation and post processing, when needed for improvement, the acoustic data was used to calculate Unconfined Compressive Strength (UCS) estimates. This then guided the selection of depths for the radioactive markers. The use of a LWD sonic tool, providing reliable acoustic data to generate a UCS, allows a quick turnaround for mobilisation of equipment (i.e. markers) and enabled the operator to acquire data in an uHPHT environment.
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