In the work environment of continuous improvement as a method to reduce both drilling and production costs, alternative means are required to attain objectives which have previously been deemed unachievable. The introduction of new tools and technologies has enabled increasingly challenging wellpaths to be drilled and completed. Trajectories labeled as undrillable even a few short years ago are now being realized.
The high cost and environmental concerns associated with offshore installations and the resultant requirement for drilling multiple wells from a common facility, has been a significant driver in the field of extended reach drilling to drain reservoirs with significant step-out. Sub-surface geological issues and wells with multiple targets have resulted in complex 3D well trajectories such as ‘pregnant lady’, ‘snake’ and ‘dragon’ wells.
Within the above context, Brunei Shell Petroleum (BSP) has identified that by drilling ‘fishhook’ shape wells, whereby a large section of the wellpath is drilled and completed at an inclination greater than 90º, significant savings in both drilling and production costs can be achieved. This is a case study of such a well drilled in mid 2004, whereby some 875m, or 35% of the total well trajectory was drilled at an inclination greater than 90º. The well also required 7" casing to be run and cemented to well TD, sand control in the form of Frac and Packs, and Expandable Sand Screens (ESS) run across three reservoir zones, with isolation between each zone.
Maximum inclination was 126.5º, and some 366m true vertical depth (TVD) reclaimed.
The Darat asset for BSP is comprised of onshore and near shore hydrocarbon producing fields, principally the Seria field. (Fig.1). It is a very mature field, having produced in excess of 1 billion barrels of oil since the field came on stream in the 1930's, with significant additional development opportunities identified. The near shore target reservoirs exist in extremely compartmentalized, highly stratified blocks. Due to the nature of the faulting in this shallow marine area, many of the blocks dip towards the coast and are comprised of multiple sand-shale sequence reservoirs (Fig.2).
Historically wells targeting these reservoirs have been drilled with offshore units and require the installation of well jackets and subsequent flow lines. By drilling these wells from land, the cost benefits of using a light land rig as compared to a jack up rig, and the associated greatly reduced production costs can be realized. There are also other factors such as environmental issues and HSE exposure to be considered. However, due to the south-hading normal faults (dipping towards the coast) and the relative shallowness of the target reservoirs, in order to intersect the multiple targets contained within each block from a land based surface location, a well trajectory that builds from vertical, through horizontal and continues to build resulting in the ‘fishhook’ shape is required. Essentially it is an upside down fault-scooper.
To clarify the ‘fishhook’ definition, there are many examples of wells drilled worldwide, particularly horizontal geosteered wells, where inclinations of greater than 90º have regularly been achieved to maintain well trajectory within the target reservoir. To be a true ‘fishhook’ well, trajectory inclinations substantially above 90º must be achieved and sustained for a significant interval, resulting in vertical depth reclaimation in excess of 100m. i.e. the vertical depth gain between the deepest point in the well (TVD) and the shallowest point once the well path has passed through horizontal.