Case History: Single-String Horizontal Well Design for Shallow, Unconsolidated Sand

Most conventional horizontal well designs employ an intermediate casing string through the build section to stabilize the wellbore, isolate the production interval, and provide a mechanical transition for change over to drill-in fluid. On shallow wells, capital savings of up to 25%-35% may be obtained if the intermediate string is omitted and a single-string of pipe serves the functions of production casing and slotted or pre-drilled completion. This technique raises special concerns, however, in unconsolidated sands where poor hole stability is a factor. The following paper describes the successful planning and implementation of such a design for eight wells in the unconsolidated Tulare heavy oil sand in the San Joaquin Valley, CA. An understanding of the stress regime and its impact on wellbore stability, combined with an assessment of the economic risk, made this design breakthrough possible. Lessons learned in this pilot program will have application to broader issues surrounding shallow horizontal drilling and extended reach wells. Time and cost analyses are included with comparisons to the benchmark conventional horizontal well. The wells were drilled as a single hole size from surface to TD. A clay-based, fresh water mud system was used for the build section, with an open-hole change over to a potassium chloride/biopolymer drill-in fluid made prior to drilling the lateral completion interval. A single-string of tubulars consisting of a slotted liner tied back to production casing was run to TD. The upper section of the string was isolated from the completion interval with an external casing packer, then cemented in place via a port collar or stage tool. Field results showed the design to be a success. By the end of the program, the wells achieved the targeted 25% cost reduction relative to the benchmark conventional horizontal well. Based on projected activity levels, this will result in a annual savings of at least $1.0 MM for the field development.