Keywords: 
• foam drilling /
• foam drilling fluid /
• circulating equipment /
• calculation method /
• computer program /
• field testing

Abstract: There were two problems exist in conventional foam low-pressure drilling technology. One problem was that the foam can only be used once, this made the foam drilling cost increase greatly, moreover, pollute the environment seriously;Another problem was that parameters like quallty, density of downhole foam and so on changed with the change of surface temperature and pressure because of the compressibility of foam. This made the job design complicated. In order to solve the two problems, the formulation of foam was optimized. The adoption of circularing foam system equipment made foam be used many times but air can be emitted into atmosphere. According to the wellhead injection pressure, minimum flow rate of foam and gas, foam column pressure and so on, 5 factors influenced foam drilling operation were taken into account:1. Taking the gas component of foam as non-ideal gas, the deviation factor Z of gas can be calculated by using Hall-Yarobrough method;2. The friction loss resulted from solid collision and friction;3. Critical submerged speed of detritus;4. The wellhead maximum foam quality was set at 0. 96, the downhole minimum foam quallty was 0. 55;5. Gas/llquid flow rate was determined by submerged speed of detritus. And stipulated that the flowing velocity of foam should be 10% higher than submerged speed at the same depth. 10% was regarded as safety additional factor. Job parameters were calculated systematically and computer software were designed. Six wells were executed on site with all of them succeeded.