CHANGES OF POROSITY VALUE IN ABNORMALLY HIGH PRESSURE GAS RESERVOIRS AND ITS EFFECTS ON THE PRECISION OF RESERVE CALCULATION——A CASE STUDY ON JIA-2 GAS RESERVOIR,MOXI GASFIELD

Since Porosity is a basic parameter for the calculation of gas reservoir reserves,determining the porosity under the reservoir condition is the only way to enhance the precision of the calculation.Based on the measurement of the porosity changes with effective stress and the irreversibility of porosity changes with stress,this paper proposes a conceptual model of porosity change history with the confining pressure.Through the study,two methods to calculate the in-situ porosity value are set up.According to the experimental data of core samples from Jia-2 gas reservoir of Moxi Gasfield,the ratio of conventional confining porosity φ_f to core analysis porosity φ_0 is 0.846(average).The error of confining porosity φ_f with in-situ porosity φ_0 is 5.7%.The error of conventional porosity φ_0 with in-situ porosity can reach 11.5%.It is suggested that it is necessary to modify the porosity and connect it with the logging porosity to make interpretation charts for the reservoir reserve calculation.