The Optimal Range of the Nitrogen-Injection Rate in Shale-Gas Well Drilling

摘要

Given the downturn of natural-gas prices in North America, it is vitally important to reduce the cost of drilling shale-gas wells. Gas-percussion drilling with nitrogen gas recently has been used in shale-gas field development. It has increased footage capacity by approximately 60%. Because the rate of penetration (ROP) is fast in gas-percussion drilling, it requires injection of an adequate quantity of nitrogen gas to clean the hole. However, a high rate of gas injection often causes wellbore erosion and deviation problems. It is imperative to determine the optimal range of the nitrogen-gas-injection rate that can mitigate the problem. This paper presents an analytical method for predicting the optimal range of the nitrogen-gas-injection rate required to balance the borehole-cleaning and -integrity issues. The optimal gas-injection rate was sought between the minimal value required for hole cleaning and the maximal permissible value to avoid hole erosion. The minimal value was determined on the basis of Angel's kinetic-energy theory for cuttings removal modified with Charles' theory of particle-grinding energy. The maximum required value was estimated by use of the sonic flow criterion at the bit. Good consistency was observed between the predicted optimal range of gas-injection rate and field problem-free nitrogen-gas-injection rates. This paper provides drilling engineers a practical tool for designing the optimal range of nitrogen-gas-injection rate to cut shale-gas drilling cost.