Research And Design Of Safety Monitoring System For Bottom Hole Pressure Of Coalbed Methane Cave Gas Injection Underbalanced Drilling

Currently,the majority of exploration and development of coalbed methane(CBM)in China has adopted the cavity aerated underbalanced drilling technology realized by pumping down air through the cavity well during underbalanced drilling.Due to the existence of annular gas-liquid-solid three phase fluid in the process,bottom hole pressure occurs in varying degrees of fluctuation with the changes in site conditions,easing program and annulus fluid flow patterns.Once the bottom hole pressure exceeds the given safety range,it is likely to result in a variety of underground complicated conditions that in severe cases can even give rise to some complex accidents which threaten lives and property.Thus,keeping bottom hole pressure stable and reasonable in real time has a great significance for ensuring the safety of CBM cavity aerated underbalanced drilling.At present,for exploration and development of cxoalbed methane,the pressure monitoring systems and bottom hole pressure measurement tools applied in current conventional drilling and underbalanced drilling operations still have large limitations.Therefore,this paper proposes a set of bottom hole pressure safety monitoring techniques based on ground survey data for the CBM cavity aerated underbalanced drilling,and has obtained the following results:1.Through the experimental application of two wells,the bottom hole pressure safety monitoring system designed and developed in this paper achieves accurate real-time bottom hole pressure monitoring during the cavity aerated underbalanced drilling,and is able to conduct timely and reliable control analysis on potential underground complicated conditions,which provides reliable data support and decision-making basis for field technicians.2.The bottom hole pressure safety monitoring system designed and developed in this paper integrates real-time data collection,real-time monitoring of engineering parameters and bottom hole pressure with real-time control analysis of bottom hole pressure as a whole,and realizes real-time dynamic monitoring of bottom hole pressure during drilling operations,which effectively guarantees the stability of bottom hole pressure and the security of drilling operations.3.Based on theoretical models of multiphase flow and single-phase flow,and the characteristics of the CBM cavity aerated underbalanced drilling technology,a wellbore flow analysis model is established,which provides theoretical basis to the design and development of the bottom hole pressure safety monitoring system.The application practice verifies the accuracy and adaptability of the model,and proves that the wellbore flow analysis model established in this paper can satisfy the requirements of site construction.4.Based on real-time drilling data,the bottom hole pressure control analysis system designed and developed in this paper can conduct real-time dynamic analysis on wellhead back pressure,bottom hole pressure and each controllable parameters that affect bottom hole pressure,and provides data basis to the key parameter design and real-time control of bottom hole pressure.The bottom hole pressure safety monitoring system for the CBM cavity aerated underbalanced drilling designed and developed in this paper has practical and promotional values that can meet the low cost requirements of coalbed methane development in China,realizes accurate real.time monitoring of bottom hole pressure,reduces and even avoids the happening of underground complex accidents effectively,and enhances the security of cavity aerated underbalanced drilling operations.