Production Decline Analysis Of Tight Gas Wells With Time-dependent Mechanisms

Yan'an Gas Field locates in the southern Ordos Basin.With annual capacity of 5billion cubic natural gas,it is playing a positive role in ensuring China's energy supply.Production decline is an important process that gas reservoirs must go through.Currently,production decline has appeared in some gas wells in Yan'an Gas Field,and it is difficult to forecast their production performance during decline period.Therefore,it is necessary to establish appropriate methods to predict the production decline,so as to provide theoretical and technical support for gas field management.Based on the geological characteristics and development methods of Yan'an Gas Field,the time-dependent mechanisms of reservoir parameters are revealed.Mechanism analysis and core displacement experiment were carried out to reveal the mechanism of bound water production and the time-dependent characteristics of relative permeability curve.The matrix stress sensitivity and time-varying fracture conductivity in Yan'an Gas Field were studied and characterized mathematically.Then,numerical simulations were adopted to study the influence of time-dependent mechanisms on the production decline.The results show that,time-dependent mechanisms would reduce the gas well production capacity.For gas wells which produce at a constant rate at first and turn to a constant BHP at late stage,the time-dependent mechanisms will shorten the stable production period and make the production decline period longer and decline speed slower.For gas wells with a constant BHP,time-dependent mechanisms will lead to lower gas rate and more rapid production decline.Further analysis of the production decline curve show that decline curves have time-dependent shapes due to time-dependent mechanisms and the changes of flow pattern.Four typical empirical decline models(Arps,SEPD,Power Law and Duong model)were evaluated based on synthetic and practical well production data of Yan'an Gas Field.Then,three methods were proposed to better forecast well performance: a combined production decline model based on the combinatorial prediction theory;an extended Duong decline model and an extended Arps decline model considering the time-dependent shapes of decline curves.The results show that the combined decline model and two improved model are more accurate than each of the four models,the extended Duong decline model is more suitable for gas wells with constant BHP,the extended Arps decline model is more suitable for gas wells which produce at a constant rate at first and turn to constant BHP at late stage.Finally,the novel decline models were applied to analyze production data of different types of gas wells in Yan'an Gas Field.Additionally,the decline characteristics were summarized,and the production management suggestions were put forward.