Neutron and density logging responses to gas reservoir for well-balanced and under-balanced logging: Gas reservoirs of sandstone in a western China field

Abstract

Studying the response differences between neutron and density logging of gas reservoir for wellbalanced and under-balanced logging will be of significance in evaluation of gas reservoir under the under-balanced condition and application of logging data. With Monte Carlo simulation technique, the paper obtains the relationship between neutron and density logging measurement and borehole diameter, porosity or gas saturation for well-balanced and under-balanced logging. The conclusions show that the response trend of under-balanced logging to gas reservoirs agrees with that of well-balanced logging with small borehole, and under-balanced logging data can be used usually as well-balanced logging data. When borehole diameter is large, under-balanced logging data should be corrected for the influences of borehole.under-balanced logging, density logging, neutron logging, gas reservoir, response Gas reservoirs of sandstone, which have low porosity and permeability, are contaminated easily by drilling fluid, and their slim filtering channels such as pore and throat are jammed. Thus, some measures should be taken to prevent gas reservoirs from being contaminated. Now, there are two protecting methods, one is adding protecting reagent into gas reservoirs, the other is under-balanced drilling. Results indicate that under-balanced drilling can effectively protect gas reservoirs with low porosity and permeability [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] .Under-balanced logging is carried out when formation pressure is higher than that of mud column. Under-balanced drilling and logging can protect the gas reservoirs with low porosity and permeability. Under-balanced logging data, which is affected certainly by logging physical property, borehole condition, working condition and so on, should be compared with that of well-balanced logging before it is used to evaluate reservoirs.With Monte Carlo simulation program MCNP [19] , the paper compares density and neutron logging responses of under-balanced condition with those of well-balanced condition for gas reservoirs with low porosity and permeability in a field located in western China.