Abstract:

The design of the appropriate completion technique for a horizontal wellbore in sour gas field is dependent on borehole stability, which is constrained by in-situ stress, rock mechanical parameters and well trajectory. It is feasible to recommend the appropriate completion method for sour gas fields in the northeast Sichuan based on the relationship between in-situ stress and well trajectory. A predictive model was established, which useed the Drucker-Prage criterion and equivalent plastic strain theory. Influence of in-situ stress on horizontal borehole stability with different trajectories and drawdown pressures were numerically simulated, which took the effect of strength reduction and pressure depletion into consideration. Results indicated that rock strength of the northeast Sichuan marine strata is high enough to maintain borehole stability during production. With the increase of drawdown pressure, equivalent plastic strain increased gradually, but the total strain was relatively small. Gas reservoir pressure depletion and strength reduction caused by acidification played an important role in well stability and sand production tendency. Base on the study, appropriate completion method for sour gas fields in the northeast Sichuan was recommended. 1)If well trajectory was coincident with direction of horizontal stress, open-hole completion was recommended,2)If angle between well trajectory and direction of horizontal stress was less than 30°, open-hole completion could be used, but drawdown pressure should be strictly controlled, and 3)if the angle was more than 30°, cased completion should be used. The results have been applied in Puguang for 6 horizontal wells.

Key words: in-situ stress, borehole stability, horizontal wells, well trajectory, well completion, carbonate gas reservoir