华北ND1区块气体钻井地层适应性评价

华北地区ND1区块ND101井中深部古近系沙河街组首次采用气体钻井提速，因井壁失稳、地层流体产出及井下燃爆等一系列的难点，严重阻碍了该区块气体钻井的实施。为此，基于气体钻井理论并与现场实践相结合，建立了一套有效的气体钻井适应性评价分析体系，包括气体钻地层井壁稳定评价和产出地层流体影响评价。通过评价产出地层流体对气体钻井壁坍塌、井下燃爆的影响，得到了3个方面重要认识：①快速产出的储层气体降低了近井壁地层孔隙压力，对气体钻井壁稳定更有利，因而，适当提高注气量可保障沙河街组氮气钻井有效进行；②修正的气体钻井井壁稳定分析模型提高了地层产水条件下坍塌压力当量密度预测的精度，可作为合理筛选该地区中下部地层气体钻井井段的依据；③采用邻井测井资料估算地层出水量并计算不同产水量下立管压力的方法，可确定维持气体钻井在一定湿度范围内的相应注气量参数，实钻中可根据立管压力、排砂管线湿度变化判断井下地层出水、出油及井壁干燥情况，及时调整或改变钻井措施。

Assessment of formation adaptability in gas drilling in Block ND1.North China

The gas drilling aimed to ROP improvement was first applied in the medium and deep Paleogene Shahejie Formation in Well ND101, Block ND1, North China. However, the drilling encountered a series of impediments including wellbore instability, formation fluid output and borehole burst. In view of this, an effective assessment and analysis system for gas drilling adaptability was established by combining the gas drilling theory with field practices. This system can assess the wellbore stability of formation in gas drilling and the impact of formation fluid output. Through the assessment of the impact of formation fluid output on the wellbore collapse and borehole burst during gas drilling, the results are concluded in the following three aspects. First, the reservoir gas at a high output speed lowers down the formation pore pressure near the wellbore, which benefits the wellbore stability in gas drilling; therefore, the gas injection volume can be improved appropriately to guarantee the effective nitrogen drilling in the Shahejie Formation. Second, the model analyzing the wellbore stability in gas drilling is modified with higher prediction precision of the acceptable range of density (in terms of equivalent mud weight) for maintaining wellbore stability under formation fluid output; this model can be taken as the basis for reasonably selecting the drilling sections of gas drilling in the middle and lower formations of this area. Third, the neighboring well logging data is used to estimate the water volume and calculate the standpipe pressure under different water volumes, and to further define the gas injection volume within a certain humidity range, and moreover, the liquid and oil output of downhole formation as well as the wellbore dryness in the actual drilling can be determined by the standpipe pressure and the humidity change of sand discharge lines, so proper drilling measures can be adjusted or altered in time.