The Situation

Downhole loss of drilling mud is one of the biggest economic challenges while drilling, especially in the Gulf of Mexico deepwater. Losses occur when the drilling fluid weight requirement to overcome the pore pressure needed to stabilize the wellbore exceeds the fracture gradient of the formations. During one such drilling project, mud losses and wellbore stability while drilling offset wells were identified as major challenges to drill a sand section above salt, as well as the salt itself, the “rubble zone” below the salt, and perhaps zones deeper than that.

The Solution

To ensure success in this high-risk drilling scenario, M-I SWACO engineers recommended wellbore strengthening via I-BOSS* in an interval above the salt section, an interval within the salt, and three intervals below the base of the salt. The technique of choice in all five intervals was circumferential stress enhancement, or stress caging, which is attained by inducing shallow fractures in a formation using elevated wellbore pressure and simultaneously forcing Wellbore Strengthening Materials (WSM) into the fractures to keep them propped and in a stressed state. Wet sieve analysis was used to maintain the correct concentration and sizes of the Wellbore Strengthening Materials. For all five intervals treated, blends of sized SAFE-CARB* and G-SEAL PLUS* products were used as the Wellbore Strengthening Materials to treat the whole mud system. Each interval required a different Wellbore Strengthening Materials blend, which was determined through application of software that determined the widths to which fractures needed to be opened in order to achieve the required wellbore stress and degree of strengthening and also the optimum Planned Shut-Down (PSD) of the Wellbore Strengthening Materials blend.

The Results

There were zero mud losses in this well while drilling the permeable zones of interest using the continuous application of Wellbore Strengthening Materials. More than 19,000 ft (5,790 m) was drilled with this technique using 20 mesh screens, which greatly reduced cost in materials and rig time. A total cost for the Wellbore Strengthening Materials in the sections that were strengthened was approximately $700,000. Downhole mud losses in those sections on previous wells amounted to approximately $2,650,000, for a net cost savings of $1,950,000 in materials alone.

The 2,600+ ft (792+ m), 18 1/8 in. x 21 in. interval above the salt section was drilled with no mud losses or downtime related to hole stability. Screens of 14 mesh over 20 mesh were used on six shakers with one shaker dressed with 175 mesh screens on the top deck in order to catch cuttings samples. The intention was to omit the last screen on the top deck of one shaker to allow the fluid and Wellbore Strengthening Materials to pass through the 20 mesh screens and be retained in the system. Using this method, cuttings integrity was improved compared to two offset wells using conventional 110–140 mesh screens. An 18 in. liner was set, and the shakers were dressed with 120–140 mesh screens while drilling 7,000+ ft (2,134+ m) of salt without incident, and 13 in. casing was run. The 12 1/4 in. x 15 in. salt exit was drilled (4,300+ ft / 1,310+ m) without incident and an 11 7/8 in. liner was run, though the liner hanger failed to set. A second hanger failure required a clamp tool be run below, and a successful cement squeeze was performed to seal the 300 ft liner lap.

The low-pressure sand section below the salt was drilled without losses, followed by a shale section with an expected pressure regression. No Wellbore Strengthening Materials were used in the shale section. A 75 bbl (12 m3) kick, which was circulated out, was deemed to require a 0.4 lb/gal (0.05 s.g.) increase in mud density, but the additional Equivalent Circulating Density (ECD) generated exceeded the maximum ECD design of the I-BOSS plan. The shale fractured, and severe mud losses were encountered. An expandable liner was successfully run to seal off the fractured zone. The rig had a power drive failure, which was initially thought to have been caused by the high concentration of Wellbore Strengthening Materials in the mud. Upon inspection of the tool, cement pieces as large as 1/2 in. were discovered, and it was determined that the cement was the primary cause of the failure. There were two other mechanical failures of downhole tools while drilling this well, but there was no evidence that the Wellbore Strengthening Materials caused the failures.

Questions? We’ll be glad to answer them.

If you’d like to know more about the I-BOSS Technology Suite and how it is performing for our other customers, please call the M-I SWACO office nearest you.