Research On The Productivity And The Method Of Rate Maintenance Of Low Permeability Gas Reservoir

With the increasing demand of natural gas,the production of conventional gas reservoir can not afford all the market demand.Thus,the development of gas reservoir with low permeability is expanded in China.The proportion of demonstrated reserves of low permeability gas reservoir in the gross demonstrated reserves of China's natural gas is higher and higher.The percolation mechanism of low permeability gas reservoir is more complicated comparing with that of conventional gas reservoir,which results in a more difficult exploration and low gas reservoir recovery.Therefore,it's crucial to establish the theory and method of efficient development of low permeability gas reservoirs.This paper focus on the percolation phenomenon of the vertical wells and horizontal wells of low permeability gas reservoir.Firstly,the threshold pressure gradient,the slippage effect,and the stress sensitivity effect in the stratum and the influence of the high-speed non-darcy effect of the near bore zone on gas seepage in the low-permeability gas reservoir were analyzed on the basis of massive researches on related literature,so as to establish a trinomial productivity equation of a vertical well and a horizontal well which covers all of above influence factors.The accuracy and applicability of the productivity equation were verified through instance data,and the influence factors of the gas well productivity were analyzed.Secondly,based on the productivity equation,the vertical well and the horizontal well received reasonable production allocation through three conventional methods,i.e.the empirical approach,the gas production index method and the nodal analysis method as well as the optimal production allocation method under limit of the critical liquid carrying flow rate,the velocity sensitivity effect,the pipe wall erosion velocity and the years of stable production.Furthermore,the production allocation result was analyzed in a comparative way.Finally,the single-well control reserve of each gas well was calculated through the pressure drop method,the flowing material balance method and the productivity accumulation method,and thus the number of stable production years was predicted.Besides,the influences of the threshold pressure gradient,the single-well control reserve,the gas producing rate and the daily gas production rate to the number of stable production years of the gas wells were analyzed.Through the study of this paper,the following conclusions are obtained:(1)The slide effect can promote the flow of gas through the mechanism of seepage flow in low permeability reservoirs.With the increasing of stress sensitivity and the starting pressure gradient,the production is becoming smaller.The seepage area of high speed non-darcy can only exist in the near well and this region is changed with time.The above factors should be taken into account when evaluating the productivity of the gas well.(2)In the case of low permeability gas well production allocation,the thumb rule can give a rough range;The method of nodal analysis and mining gas index is usually larger than the practical productivity.The number we get through the way of optimized production allocation is more reasonable cause we consider the constraint conditions.(3)Through analyzing the factors affecting rate maintenance capability,it is concluded that a positive relationship between stable production time and wellhead,production,starting pressure gradient,speed of the gas production respectively,but a negative relationship between stable production time and single-well controlled reserves.The research of this paper use comprehensive factors to build production predicted model,and it is offer some theoretical guidance and technical support for production prediction.Moreover,the research of optimization allocation and rate maintenance capability also can be the valuable references for the development of low permeability gas reservoirs.