Paper: Underbalanced Drilling Review

The MSDU 1 (Mary Scharbauer Devonian Unit No. 1) was completed in December 1998. It was Mobil Drilling Midland's first attempt to drill a horizontal lateral while underbalanced. The Mary Scharbauer experience provided valuable data on the benefits and drawbacks of UBD (Underbalanced Drilling) in the extended reach environment. Increasing ROP in order to reduce cost was the primary goal of the MSDUl project. Increases in ROP from UBD were apparent when optimal bottom hole pressure was achieved. Maintaining optimal bottom hole pressure was impossible do to the limitations of conventional MWD (Measurement While Drilling) equipment. Extensive drill stem deterioration caused project costs to skyrocket and placed any future UBD projects on hold until a solution to the problem could be developed. Investigation of the drill stem corrosion showed that the most severe pitting occurred on the drill pipe in the lateral section of the well. Analysis of the drill fluid as well as formation fluids showed that the most reasonable explanation for the deep pitting observed on the drill pipe was "Oxygen Erosion Corrosion." Oxygen erosion corrosion was then simulated in the laboratory in an apparatus that produced conditions like those found in the lateral section of the MSDUl well. Many corrosion tests were completed in an effort to find inhibitors that would function in a deep, hot, oxygenated, and abrasive well bore environment like that of the MSDUl well. While this was being done 99.9+ % oxygen free UBD methods were evaluated. Eventually a corrosion inhibition system that worked under conditions like those of the MSDUl was located and proven on Mobil Midland's second UBD extended reach well the PFU 23-24 (Parks Field Unit 23-24). Conventional MWD equipment required that the drill pipe be loaded with water in order to generate a readable mud pulse. When surveys were needed underbalanced conditions were eliminated and ROP on the MSDUl dropped from 26ft/hr to 8ft/hr. Optimum underbalanced conditions were recovered after the well bore could be re-gasified. It was estimated that using a MWD system that did not require the drill pipe to be loaded would cut the cost of drilling a MSDUl like lateral by $60,000. Midland Drilling located an EM MWD (Electromagnetic Measurement While Drilling) company that stated their equipment would work for drilling a Devonian lateral underbalanced. When the equipment was evaluated on the PFU 23-24 it performed acceptably but learning were documented that will improve future operations. Mobil Drilling Midland continues to develop technology to improve bottom line profitability. Currently work is being done to determine the feasibility of drilling extended reach horizontal laterals with air compressors and a water/foam drilling fluid. If feasible this method will be half as expensive as our current lowest cost UBD option. The drilling curve will continually shift to the left as key learnings are implemented on fume projects.