Calculation Of Horizontal Well Productivity Equation And Analysis Of Influencing Factors In Tight Sandstone Gas Reservoirs

The compact sandstone gas reservoir has the characteristics of low yield and poor reservoir property,so how to increase production is an urgent problem.Segregated fracturing horizontal well is currently considered as an effective way to solve this problem.However,this technology is not mature in terms of production forecast so it needs further improvement.In this situation,This paper studied the productivity forecast.By comparing the tight sandstone gas reservoir with the conventional sandstone gas reservoir,especially in the feature of geology,seepage and development,it can be founded that the former should take account of several effects which are caused by threshold pressure gradient,slippage effect,main fracture conductivity and secondary fractures in the process of predicting production capacity.Based on the Visual equivalent well diameter model and principle of pressure drop superposition,the paper established three capacity equations namedⅠ,Ⅱ,Ⅲ :(1)equation Ⅰ considers secondary fractures and non-darcy flow in fractures;(2)equation Ⅱconsiders start-up pressure gradient,secondary fractures and non-darcy flow in fractures;(3)equation Ⅲ considers slippage effect,start-up pressure gradient,secondary fractures and non-darcy flow in fractures.This paper used MATLAB to compile the solution program,and the numerical solution of the equation was obtained by iterative method.Then the calculated value was compared with the actual test gas data.The results indicate that the calculation results of the equation considering the above factors are more accurate than those of equation without considering factors.Through analysis,this paper got the following understandings:(1)after staged fracturing of the horizontal well,neglecting secondary fractures and slippage effects will underestimate productivity,neglecting the start-up pressure gradient and limited conductivity of fractures will overestimate productivity.(2)In segregated fracturing horizontal wells,the distribution of crack production is symmetrical,and the contribution from the crack at the heel end and the crack at the toe end to the production is larger.The production contribution curve is "U-shaped" distribution.(3)non-darcy flow in fractures will reduce production,and its external crack production capacity will be reduced more than internal cracks.(4)starting pressure gradient will reduce production,and its internal crack production capacity will be reducedmore than external cracks.(5)slippage effect increases the production,and the increase of the crack production capacity on the outside is greater than the internal crack.(6)With the increase of the number of cracks,the output of horizontal wells increases,but the increase trend gradually decreases.Take a horizontal well with a length of 1000 m as an example.When the number of cracks increases to more than 10,the output of the well is no longer increasing.Based on the understandings above,the appropriate number of horizontal fractures should be between seven and ten.