A New Technology to Meet Tight Gas Formation

We develop a new method to accurately determine gas saturation in tight gas reservoir using a three detector neutron logging tool with pulsed neutron source. This tool obtains the thermal neutron counting ratio R13 between near and far detectors and the ratio R23 between the middle and far detectors. With this method, the difference D between R13 and R23 is related to gas saturation and porosity. Monte Carlo simulation is used to evaluate the performance of the tool. The simulation results are analyzed with the new method to estimate gas saturation. The result shows that D can be used to determine gas saturation and using D to determine gas saturation has higher sensitivity than other ratio of thermal neutron counts or other difference of ratios. Another finding is that D logging response is affected by factors such as lithology, shale content, formation water salinity, borehole fluid, borehole size which should be corrected in the log data interpretation. We have applied the new method to process log data from two wells, yielding reliable interpretation result. The field data application result validates our new method and demonstrates its potential value and prospect in the gas saturation evaluation for tight gas reservoirs.