多层合采气井产能指示曲线异常的原因与校正方法

An abnormality of productivity indicative curves for multi-layer gas wells: Reason analysis and a correction method

多层合采气井产能试井解释过程中,产能指示曲线容易出现异常情况,有可能导致气井的产能方程和无阻流量无法求取。为了弄清上述异常的产生原因,建立了考虑储层渗流和井筒变质量流耦合的全井段计算模型,用以计算多产层气藏的产气剖面和井筒压力分布;基于对不同产量条件下的气井产气剖面与井筒压力分布特征的分析,剖析了多层合采气井产能指示曲线出现异常的根本原因,提出了校正方法并进行了实例验证。研究结果表明:(1)造成多层合采气井产能指示曲线异常的原因一方面是多层段的井筒中为变质量管流,流压梯度随井深增加逐渐变小,采用压力计所处位置以上的流压梯度折算储层中部的流压高于实际值,另一方面是由于短时关井后多产层段的压力未达到平衡,使测得的静压高于长时间关井各层压力平衡后的静压;(2)多层合采气井产能试井解释的流压宜采用储层顶部以上200 m以内位置处压力计所测得的压力进行折算,并且需采用井筒压力平衡后的静压值;(3)算例和实例均验证了所建模型的可靠性、异常原因分析的合理性和校正方法的正确性。结论认为,该研究成果为多层合采气井的产能评价提供了技术支撑。

An abnormality tend to occur in the productivity indicative curves in the process of productivity test interpretation of multi-lay- er gas wells, resulting in the failure of solutions to their productivity equations and absolute open flow rates. To figure out the reasons for such an abnormality, we established a full-hole calculation model considering the coupling of wellbore variable mass flows and reservoir seepages to calculate a gas production profile and wellbore pressure distribution of a multi-layer productive gas reservoir. Then, based on the analysis of the gas production profile and wellbore pressure distribution characteristics of gas wells at different gas production rates, the root cause of the abnormality in the productivity indicative curves of multi-layer gas wells was analyzed, and a corresponding correc- tion method was proposed and validated based on some examples. And the following research results were obtained. First, there are two reasons for the abnormal productivity indicative curves of multi-layer gas wells. On the one hand, there is a variable mass pipe flow in the wellbore of multi-layer sections and a flowing pressure gradient decreases with the increase of well depth. And the flowing pressure in the middle of the reservoir which is converted based on the flowing pressure gradient above the pressure gauge is higher than the real value. On the other hand, the pressure in the multi-layer producing sections doesn＇t realize a balance after well shutdown for a short time, so the measured static pressure is greater than the one measured when the pressure of each layer gets balanced after well shutdown for a long time. Second, the flowing pressure obtained from the productivity test interpretation of multi-layer gas producer shall be converted based on the pressure measured by the pressure gauge within 200 m above the reservoir top and it is necessary to adopt the static pressure measured after the balance of wellbore pressure. Third, the reliability of the model, the rationality of the abnormality reason analysis and the validity of the correction method are verified based on calculation examples and cases. It is concluded that the research results pro- vide a technical support for the productivity evaluation of multi-layer gas wells.