Fibrous LCM Sweeps Enhance Hole Cleaning and ROP on Extended Reach Well in Abu Dhabi

Abstract

Abstract Extended reach wells pose some unique problems to the drilling operation in the management of hole cleaning and torque and drag. Cleaning the hole without inducing an excessive equivalent circulating density (ECD), resulting in down-hole losses, can be a major issue when drilling extended reach wells. The reduced mud weight / fracture gradient window can limit pump rate making it difficult to achieve adequate solids transport, particularly in long 12 1/4" tangent sections. Experience on world record extended reach drilling (ERD) operations led to the discovery that certain fibrous lost circulation materials (LCM) have the ability to enhance hole cleaning and dramatically reduce torque and drag in high angle and horizontal well-bores, as well as control losses. Optimised drilling practices, together with the application of engineered hole cleaning sweeps, can greatly improve drilling efficiency in extended reach wells. This paper focuses on the successful application of fibrous LCM sweeps on the first extended reach well to be drilled in Abu Dhabi by the Abu Dhabi Company for Onshore Oil Operations (ADCO). In response to difficulties sliding in 12 1/4" hole, hole cleaning sweeps were routinely pumped from 9,462 feet, in the 63 degree tangent section, to final TD in 8 1/2" hole at 17,525 feet. Coupled with pipe rotation, these sweeps increased the rate of cuttings return to surface by up to 50% and frequently resulted in major improvements in the drilling rate. The effectiveness of these sweeps was monitored on surface at the shakers by visual means and downhole by means of annular pressure monitoring tools in the bottom hole assembly. Introduction This extended reach development well was drilled by ADCO in an environmentally-sensitive shallow marine area offshore Abu Dhabi. A man-made island was constructed to provide a suitable drilling site to access the offshore parts of the field while minimising the environmental impact. For this first well, a profile was selected which would minimise torque and drag, particularly in the 12 1/4" interval. Hole cleaning simulations were run under downhole conditions using in-house software and it was anticipated that hole cleaning would be a major challenge in successfully completing the high angle sections of the well. After drilling the 26" top hole with spud mud, the 17 1/2" hole section was drilled with a non-dispersed polymer seawater-based mud through the Tertiary formations, building angle from around 8 degrees to a maximum of 63 degrees. This interval penetrated the fractured limestones and inter-bedded shales of the Dammam, Umm er Radhuma and Simsima formations and is prone to water flows and lost circulation. Despite lost circulation problems, pump rate was maintained in the 1,000–1,200 gpm range and there was no indication of any hole-cleaning problems, even at relatively high drilling rates. While drilling with seawater, during heavy downhole losses, viscous pills were pumped to clean the hole. The 12 1/4" tangent section was drilled through the Laffan and Nahr Umr shale sections and into the top of the Bab member using a 5 1/2" drill-string and a flow rate in the range 800–900 gpm. Oil-based mud was used to provide lubricity and control the reactive mid-Cretaceous shales. Hole angle was maintained around 63 degrees and hole cleaning fibre sweeps were pumped as necessary to facilitate slide drilling and to clean the hole prior to trips. As a result, the section was drilled and cased without problem. After setting 9 5/8" casing, an 81/2" pilot hole was drilled to evaluate the Lower Cretaceous formations, allowing hole angle to drop off to below 20 degrees. Fibrous LCM sweeps were pumped to help improve weight transfer to the bit and drilling rate, to clean out cuttings beds and control ECD and to condition the hole prior to trips. The 8 1/2" pilot hole target depth was reached in 43 days, 39 days under target. This was followed by two horizontal 81/2"laterals, drilled in the reservoir section. The entire well was drilled and completed trouble-free in 112 days, compared with 173 days planned.