Use of natural gas and nitrogen as drilling fluids has been considered as an innovative technique for drilling hard formations, loss of circulation formations, coal bed methane zones, and low-permeability gas reservoirs to improve drilling performance and reduce formation damage. Drilling complications have been reported owing to the use of high-gas injection rate required for hole cleaning. These complications include hole washout, hole deviation, and ice-balling (frozen) of bit. This paper reports a new technique to solve the problems associated with the use of high-gas injection rate. It involves using a new type of flow-diverting joint (FDJ) at the shoulder of drill collar to partially bypass gas flow into the annulus between the drill pipe and open hole. This technique allows for the use of high-gas injection rate to carry drill cuttings while reducing the gas flow rate through the drill bit. As a result, the gas velocity in the drill collar-open hole annulus is maintained at a safe level to avoid hole washout. The reduced flow rate through bit also minimizes wellbore enlargement at bottom hole, which reduces the tendency of hole deviation. In addition, the reduced gas flow rate through bit minimizes the Joule-Thomson cooling effect at bit which is believed to be responsible for ice-balling of bit. This paper presents a method for selecting the optimal FDJ port size based on system configurations. It optimizes the distribution of the injected gas flow between the lower and upper annular sections. Calculation procedure is described and a design example is illustrated with a computer program output. Sensitivity analyses with the computer program show that the FDJ to bit-nozzle area ratio is directly proportional to the annulus area ratio, and the bypassed flow rate fraction remains constant as drilling progresses.
Optimal Use of Flow-Diverting Joint in Underbalanced Gas Drilling
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Guo, Boyun , Zhang, Zongren , and Deli Gao. "Optimal Use of Flow-Diverting Joint in Underbalanced Gas Drilling." Paper presented at the SPE Asia Pacific Oil and Gas Conference and Exhibition, Jakarta, Indonesia, September 2011. doi: https://doi.org/10.2118/143309-MS
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