Study On Fluid Evaluatuion Of Water-bearing Tight Gas Reservior

The Sulige gas field with reserves more than one trillion cubic meters is the largesttight sandstone gas pool ever discovered in China. It has strong reservoir heterogeneitywith low porosity, low permeability and low reservoir abundance. This study is focusedon the H8and S1formations of Block Su54, which lies in the gas-water transition zoneof western Sulige gas field. Based on geology logging data, analysis data, well loggingdata, and gas testing data, the reservoir characteristics, the relationships amonglithological characteristics, physical properties, gas-bearing properties, and electricproperties, the methods for quantitative formation evaluation and fluid identificationhave been studied.Firstly, sedimentary characteristics, lithological characteristics, physical properties,pore types, pore structures and formation water characteristics have been analyzedsystematically. The sedimentary system of H8and S1formations is braided river. Themain rock types are lithic quartz sandstone and quartz sandstone. The average porosityis7.47%, and the average permeability is0.31mD. The main pore type is secondarypore, such as dissolved pore and intercrystalline pore. The pore throat distribution is notuniform, the selectivity is poor, and the connectivity is low. The type of formation wateris CaCl2, and its salinity is high.Secondly, the relationships among lithological characteristics, physical properties,gas-bearing properties, and electric properties have been analyzed systematically. Withthe increasing of rock grain size and quartz content, the physical properties like porosityand permeability turn better. With the increasing of detritus content, the physicalproperties turn worse. With the increasing of quartz content, natural gamma decrease,thus it can reflect the change of lithological character. When porosity and permeabilityturn better, gas saturation will increase. There is a positive correlation between gassaturation and resistivity.Thirdly, following the core-calibrated-log technique, various models of loginterpretation, such as porosity, permeability, water saturation are established.Compared with neutron logging porosity model, acoustic and density logging porositymodels have higher precision, and they are applicable when the borehole is good.According to the core electricity experiment, the lithology coefficient a is high and the pore structure index m is low in the relationship between formation factor and porosity.Thus, there are two methods for calculating water saturation, one is Archie formula, andthe other one is improved Archie formula.Fourthly, based on the theory that resistivity logging, acoustic logging, densitylogging and neutron logging are affected by natural, four fluid identification methodshave been studied. Those methods include CNL-DEN curve overlapping method,Rt-AC cross plot, M2RX-LLD/M2RX cross plot and AK-DT cross plot. The differencesbetween dual lateral logging and array induction logging in gas zone, water-gas zoneand gas-water zone have been analyzed. Numerical simulation of mud invasionconfirms that mud invasion will cause the resistivity of water-gas zone and gas-waterzone increase. Thus, M2RX-LLD/M2RX cross plot can be used in fluid identification.