深水钻井最大允许气侵溢流量的计算方法

深水钻井对井控提出了更高的要求,气侵的早期监测便成为深水井控研究的热点。由于溢流量预警值为经验值,使得现有泥浆池增量法在水深较深、原始地层压力较高的井中应用的普适性和可靠性较差。为此,通过引入最大允许关井套压和环空气体上升位置,计算不同溢流量条件下的压井风险,建立了基于气侵的溢流量预警值反算方法,以此来保证在现有井控设备、施工参数和地层参数等确定的条件下进行更为精确可靠的气侵溢流监测。同时引入含可信度地层压力预测方法对溢流量预警值反算方法进行改进,最大限度地降低了因地层压力预测不准而导致的气侵监测误差。实例计算结果表明,该方法充分考虑了气侵后气体上升位置和压井风险,能够大大提高深水钻井气侵早期监测的准确率和可靠性,对于实现因井而异的溢流量预警值设计和降低气侵监测成本有实际意义。同时提出推荐采用精度更高更稳定的进出口流量计对泥浆池增量进行监测,以满足半潜式钻井平台的测量精度。

A calculation method of the maximum allowable gas invasion overflow in deepwater drilling

Deepwater drilling requires more rigorous well control. Accordingly, proactive gas invasion detection is a hot topic in research of deepwater well control. The warning overflow value is an empiric value, so the existing pit-gain method is less applicable and reliable for the wells with a high initial reservoir pressure in deep water. In this paper, the well-killing risks under different overflow conditions were calculated with consideration to the maximum allowable shut-in casing pressure and the annulus gas-ascending position. Then, a back calculation method was developed on the basis of the warning overflow value of gas invasion so as to monitor gas invasion overflow more accurately and reliably with the existing well control equipment, construction parameters and formation parameters. Furthermore, this method was improved by integrating the credibility formation pressure prediction method to minimize the gas invasion monitoring errors caused by inaccurate formation pressure prediction. Practical calculation shows that this method takes the annulus gas-ascending positions and well-killing risks into full account, so the accuracy and reliability of proactive gas invasion detection in deepwater drilling are improved greatly. This method plays a practical role in designing well-specific warning overflow value and reducing gas invasion monitoring costs. Finally, it was recommended to monitor the pit gain by using the import and export flow meters of higher accuracy and stability so as to satisfy the measurement accuracy of semi-submersible drilling platforms.