Abstract:

Water/gas percolation in low-permeability gas reservoirs is rather complicate because of the reservoirs'low porosity and low permeability.As a result,the traditional percolation law is inapplicable to this kind of reservoirs.Laboratory research and field application testify the nonlinearity of percolation and multi-variability of fluidal in the low permeability gas reservoirs.The percolation of fluid will not only need to overcome start-up pressure gradient,but will also be enslaved to stress sensitivity.Horizontal well technique is an effective method in low permeability gas reservoirs development.However,prediction of productivity of horizontal well in low permeability gas reservoirs is mostly carried out within the framework of traditional percolation theory and therefore fails to take start-up pressure gradient and stress sensitivity into consideration.In an attempt to fix the flaw,the conformal transformation is introduced in the establishment of a productivity model for horizontal wells in low-permeability reservoirs and the effects of start-up pressure gradient and stress sensitivity is studied based on the real data from a low-permeability gas reservoir.Results indicate that productivity of horizontal wells in low-permeability gas reservoirs decreases with start-up pressure gradient and stress sensitivity at a linear and an exponential relationship respectively.The stress sensitivity has a stronger impact on the productivity compared with start-up pressure gradient.When start-up pressure gradient reaches 0.00025 MPa/m,the productivity will be reduced to 77%;while when rock elastic distortion coefficient exceeds 0.15 MPa^-1,the horizontal gas well will have to be shut down.It suggests that start-up pressure gradient and stress-sensitivity effect should be considered to correctly predict the productivity of horizontal gas well in low permeability reservoirs.

Key words: low-permeability gas reservoir, start-up pressure gradient, stress sensitivity, productivity of horizontal well