Application of a Well Slot Optimization Process to Drilling Large Numbers of Wells in Clusters on Artificial Islands
Authors: Martin W. W. , Walters J. V. , Woodard P. W. 
Published: February 2011

Some new and further developments of offshore fields in Abu Dhabi expect to exploit extended reach drilling (ERD) whereby large numbers of wells are drilled from drilling centers on artificial islands or large platforms. The wells will also have increasingly long reservoir sections (up to 10000 ft), so-called maximum reservoir contact or MRC wells.

Artificial islands may have a much larger well capacity (200 or more wells) and larger slot separations (15–30 ft) than platforms but optimal well allocation and sequencing is far more complex than for single cluster platforms. Large numbers of wells (producers and injectors) require multiple clusters separated by 100–200 ft in order to accommodate rig layouts, allow rig movements and access to all slots. Cluster and slot layouts also impact and are impacted by facilities layouts of manifolds and flow lines.

ERD drilling from islands can require wells with horizontal departures of more than 20000 ft to the landing points and total measured depths of 35000 ft or more. Conceptual ERD well designs have long 12 1/4’" sections landed at 90 degrees in the reservoir. For such long reaches it is beneficial that the 12 section be drilled with oil-based mud, (OBM) due to problematic shale sections that experience borehole instability for well angles of 40 degrees and larger when drilled with water-based mud. On the other hand, the 16 " section must be drilled with water since it crosses formations that frequently experience total losses imposing hole inclination restrictions. In addition, the long maximum reservoir contact (MRC) reservoir sections (typically 10000ft) may require either OBM or WBM depending on completion design and torque, drag, stimulation and mud optimization requirements. Since changing from WBM to OBM and back again requires more rig time than skidding to the next slot, it can be favorable to use batch drilling on a cluster to the extent possible if an acceptable well sequencing can be found that also satisfies reservoir and production target requirements. On islands with large drill pads and multiple clusters it is also highly desirable from a drilling efficiency standpoint that wells are batched drilled in a cluster-by-cluster sequence to minimize major rig moves between clusters.

A process is applied for optimally assigning wells to surface clusters in accordance with the location and orientation of reservoir targets (i.e., horizontal well sections) relative to slot positions in clusters and cluster orientations on the well pads. Wells can be assigned initially to slots within a cluster based on the combined optimization of drilling parameters: Drilled footageDog-leg severity and 3-D complexityWell separation and anti-collision analysis with planned and existing wellsBatch drilling on clusters.

Subsequently further refinement of the slot allocations can be applied by considering multiple objectives of the integrated project including: Required well sequence to reach targeted production build-up to plateau rateTiming of facilities availability for first oil production, separation and gas handling capacityWell pad layout, construction and well hook-upRig usage (number of rigs, rig type and capacity)Reservoir management strategy, numbers of producer and injector wellsFuture well designs and in-fill locations, e.g., WAG or ML wells impact surface facilities differently.

The objective is to find optimal integrated solutions for well slot allocations and drilling sequence under these complex development objectives and constraints for drilling large numbers of ERD-MRC wells from artificial islands. Results are presented showing how the process can be used to assign wells to islands in terms of minimizing total drilling length, optimizing well trajectories (anti-collision perspective) and allocate wells to clusters in a manner that gives flexibility for sequential or batch drilling, achieves production targets, considers SIMOPS issues and constraints as well as catering for future well designs and locations.