The Study On The Production Dynamic Performance Of Multi-layer Unconsolidated Sand Gas Reservoirs

Multi-layer unconsolidated sand gas reservoir is considered to have good physical properties and dramatic pressure variation.However,interference is obvious between wells and interlayer in the reservoir.Each layer’s pressure and production rate is changing dynamically,which increases the evaluation difficulty of gas well dynamic reserves.S gas field is a typical multi-layer unconsolidated sand gas field in western China.Based on it,this thesis researches intensively the production dynamics of gas wells,main control factors influencing dynamic reserves and methods of dynamic reserves evaluation by physical simulation,numerical simulation and theory of reservoir engineering.Through the study of this thesis,the conclusions are listed as follows:1.The evaluation methods of dynamic reserves are summarized through literature research.The adaptability,advantages and disadvantages of different methods are cleared including Arps decline analysis method,material balance method,modern rate decline analysis method,numerical simulation method and power-law exponential rate decline method,etc.2.The physical simulation experiment of multi-layer unconsolidated sand gas reservoir is done to evaluate the influences,caused by different permeability ratio,porosity,initial interlayer pressure,production proration and plane heterogeneity,to gas wells’ production and pressure.And it also reveals the change rule of gas well’s production dynamics.The result shows that:(1)Each layer’s production transforms automatically.The production of the early stage is dominated by the high permeability layer.And the production of the late stage is dominated by the low permeability layer;(2)The contribution of the high permeability layer during the stable production period is greater than that of the low permeability layer;(3)The backflow may occur in commingled production of multilayers with different pressure;(4)Production proration mainly affects the production of the low permeability layer;(5)In the model with high permeability near the wellbore and low permeability far away from the wellbore,the stable production period is greatly influenced by the proration.3.Through the numerical simulation research,dynamic reserves evaluation methods are discussed combining Arps decline analysis method and material balance method.The result shows that:(1)Permeability ratio increase will exacerbate the difference of gas supply capacity between low and high permeability layers during the early and late stage of development;(2)The increase of the thickness ratio between low and high permeability can shorten the stable production period and make the low permeability layer dominate the gas well’s production earlier;(3)The decrease of bottomhole pressure can enlarge drainage area and prolong the stable production period;(4)Fracturing can effectively improve single well’s productivity,increase gas production rate in the early period and weaken the influence of the permeability ratio;(5)Different initial interlayer pressure intensifies the impact of the permeability ratio;(6)The fluid of the low permeability layer outputs through the high permeability layer when interlayer channeling exists.At this situation,the low permeability layer does not have productivity;(7)The fitting of Arps decline analysis method and material balance method is accurate.Yet the production curve does not behave as Arps decline curve while the reservoir with high permeability ratio has hydraulic fractures or interlayer channeling.4.Material balance method is suitable for the dynamic reserves evaluation in multi-layer unconsolidated sand gas reservoir.However,the calculation results can be affected seriously by a series of factors,such as improper selection of fitting data,inaccurate calculation of bottomhole pressure and wellbore channeling caused by interlayer pressure difference,etc.These all can diminish the calculation of dynamic reserves.In term of the convex and concave phenomenon on the fitting curve,another fitting way is proposed through the numerical simulation.According to the error analysis of the calculated dynamic reserves,an evaluation idea of dynamic reserves in multi-layer gas reservoir is presented.