PRODUCTIVITY CALCULATION OF HORIZONTAL WELLS IN LOW-PERMEABILITY GAS RESERVOIRS CONSIDERING STARTING PRESSURE GRADIENT

The development of low-permeability gas reservoir plays an important role in petroleum industry.Owing to its peculiar stimulation advantage,horizontal wells have been widely used in the development of low-permeability gas reservoirs.Due to the comparatively high cost input of horizontal wells,it is necessary to forecast their productivity accurately so as to provide a scientific basis for designing the drilling and production engineering projects of horizontal wells.The productivity prediction of a horizontal well in low-permeability gas reservoirs mostly neglected starting pressure gradient effect,which leads to the great difference between the predicted and the actual productivity.Based on the Joshi percolation theory of horizontal wells and taking the low-permeability gas reservoirs with enclosed top and bottle boundaries as an example,the formula for productivity calculation of horizontal wells in low-permeability gas reservoirs considering starting pressure gradient was deduced and the dimensionless parameter s was introduced to represent the degree of production loss caused by starting pressure gradient.The field data have verified the productivity formula's accuracy and analyzed the influence trend of starting pressure gradient to gas wells' deliverability.Results showed that with the increasing of starting pressure gradient,the loss of gas amount would increase linearly,which provides a theoretical support for the development of horizontal wells in low-permeability gas reservoirs.