Extended Reach Drilling with Coiled Tubing: A Case Study on the Alaskan North Slope That Proves the Benefits of Drilling a Straight Hole

Summary A set of five wells were to be drilled with directional coiled tubing drilling (CTD) on the North Slope of Alaska. The particular challenges of these wells were the fact that the desired laterals were targeted to be at least 6,000 ft long, at a shallow depth, almost twice the length of laterals that are regularly drilled at deeper depths. The shallow depth meant that two of the five wells involved a casing exit through three casings, which had never been attempted before. After drilling, the wells were completed with a slotted liner, run on coiled tubing (CT). This required a very smooth and straight wellbore so that the liner could be run as far as the lateral had been drilled. In this paper, we focus on one of the two wells on which triple casing exit was performed. However, the same considerations and results apply to the other wells on which the same technology has been used. Various methods were considered to increase lateral reach, including running an extended reach tool, using a friction reducer, increasing the CT size, and using a drilling bottomhole assembly (BHA) that could drill a very straight well path. All of these options were modeled with tubing forces software, and their relative effectiveness was evaluated. The drilling field results easily exceeded the minimum requirements for success. This project demonstrated record-breaking lateral lengths, a record length of liner run on CT in a single run, and a triple casing exit. The data gained from this project can be used to fine-tune the modeling for future work of a similar nature.

Characterizing Dispersion Effectiveness at Varying Salinities

ABSTRACT Chemical dispersant formulations typically provide maximum oil dispersion in waters between 30–40 ppt (parts per thousand) salt content, which encompasses typical ocean salinity (~34 ppt). As a result, most laboratory studies of oil dispersion effectiveness (DE) are conducted at low to average ocean salinity. Ocean salinity can vary locally from below 20 ppt during ice and snow melt, to extremely high (over 100 ppt) during freeze up periods or within natural brine pools in deeper waters. In this study, the influence of salinity on DE was evaluated using the baffled flask test (BFT) at a dispersant-to-oil ratio (DOR) of 1:25. Benchtop experiments were conducted with Alaskan North Slope (ANS) crude oil in the presence or absence of chemical dispersant at 5 and 25°C and varying salinities (0.2 to 125 ppt). In addition to DE as determined by BFT, oil droplet size distribution (DSD) and fluorescence intensity was measured via a LISST-100X particle size analyzer (Sequoia Scientific, Inc., Bellevue, WA) and ECO fluorometer (Sea Bird - WET Labs, Inc.; Philomath, OR), respectively. Results indicate that in the presence of dispersant, maximum DE occurred at 25ppt, and decreases above and below this salinity. Concentration of small droplets (<10 μm) was twice as high at 35ppt than at the other salinities in the presence of dispersant at 25°C. Treatments without dispersant did not vary significantly as a function of salinity. Flume tank experiments over a range of salinities support the lab scale results of DSD. These results provide a more comprehensive picture pertaining to the influence of salinity on dispersant usage at high salinities.

Underbalanced Drilling with Coiled Tubing: A Case Study on the Alaskan North Slope, Which Proves the Benefits of Drilling a Straight Hole

A set of 5 wells were to be drilled with directional Coiled Tubing Drilling (CTD) on the North Slope of Alaska. The particular challenges of these wells were the fact that the desired laterals were targeted to be at least 6000ft long, at a shallow depth. Almost twice the length of laterals that are regularly drilled at deeper depths. The shallow depth meant that 2 of the 5 wells involved a casing exit through 3 casings which had never been attempted before. After drilling, the wells were completed with a slotted liner, run on coiled tubing. This required a very smooth and straight wellbore so that the liner could be run as far as the lateral had been drilled. Various methods were considered to increase lateral reach, including, running an extended reach tool, using friction reducer, increasing the coiled tubing size and using a drilling Bottom Hole Assembly (BHA) that could drill a very straight well path. All of these options were modelled with tubing forces software, and their relative effectiveness was evaluated. The drilling field results easily exceeded the minimum requirements for success. This project demonstrated record breaking lateral lengths, a record length of liner run on coiled tubing in a single run, and a triple casing exit. The data gained from this project can be used to fine-tune the modelling for future work of a similar nature.