Study On Pressure Fluctuation Mechanism Of Gas Well Drilling

Because of the advantages such as improving rate of penetration, overcoming leakage, lowering cost and protecting reservoir, the gas drilling technique is regarded as an effective approach to exploit unconventional oil and gas resources, and it has been applied in Sichuan basin and Tarim basin several times with good results. However, because of the unstable high-speed flow of wellbore gas caused by reservoir gas output while gas drilling gas reservoir, a series of safety accidents may happen. At present, wellbore pressure fluctuating law while gas drilling gas reservoir isn’t studied clearly, and quantitative calculation model is lacking, as a result, drilling risks can’t be assessed accurately and control methods can’t be proposed effectively. In this thesis, the wellbore pressure fluctuating mechanism while gas drilling gas reservoir was studied comprehensively, and the main contents and results of the study are shown as below:(l)Governing equations of wellbore gas flow were built, which were solved by characteristics method and finite difference method. Based on the calculating method, wellbore transient flow mechanism and its influencing factors were analyzed.(2)Based on the actually measuring variation characteristics of methane concentration, the transient seepage models for matrix reservoir, fracture group reservoir and single fracture reservoir were built, and the seepage characteristics and their influencing factors of different types of reservoirs were studied.(3)An experimental device of one-dimensional core transient seepage was designed, by which the experiments for one-dimensional core transient seepage under constant-pressure boundary and closed boundary were performed, and the experimental data from the device was close to the theoretical computed results, which proved the accuracy of the formation transient seepage models established in the study.(4)Based on the coupling solution method of wellbore transient flow and reservoir transient seepage, the wellbore pressure fluctuating law while gas drilling fracture group reservoir was analyzed combing with the field case. The analysis shows that at the moment while fracture group reservoir is penetrated by gas drilling, the bottom pressure increases rapidly, and the bottom flow rate surge to peak, then decrease gradually. When bottom pressure disturbance propagates to the wellhead, the standpipe pressure and the flow rate of annulus exit will increase, which helps to predict gas reservoir with high gas production.(5)The wellbore pressure fluctuating law while gas drilling single fracture reservoir was analyzed, which indicates that the obvious pressure disturbance is generated while gas drilling single fracture reservoir, the bottom flow rate and the bottom pressure both surge to peak and then decrease gradually. However, when bottom pressure disturbance propagates to the wellhead, the pressure disturbance decreases greatly.The study in this thesis reveals the mechanism of wellbore pressure fluctuation while gas drilling gas reservoir, which is important for wellbore safety risk control.