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

An abnormality tends to occur in the productivity indicative curves in the process of productivity test interpretation of multi-layer 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 for the abnormality in the productivity indicative curves of multi-layer gas wells was analyzed, and a corresponding correction 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 provide a technical support for the productivity evaluation of multi-layer gas wells. Keywords: Multi-layer gas reservoir, Commingling, Productivity test, Indicative curve, Variable mass flow, Abnormality correction, Flowing pressure, Static pressure