Estimating drainable gas reserves by the method of material balance

The algorithm of estimating the reserves of gas deposits by the material balance method is described. The advantages of this method are the use of rather precise field technological information for calculations and the possibility of regular checks for compliance with the actual nature of field development. However, there are problems of practical application of this method. One of them is the determination of average weighted pressure for the whole gas-saturated volume, especially for low permeable reservoirs with deep depression craters around wells. The process of correct determination of weighted average pressure by means of isobar map and map of effective gassaturated thicknesses of gas reservoirs is proposed and described. Another problem is the long time interval between measurements of reservoir pressure during the field development. We offer the method that allows increasing frequency of isobar mapping by interpolation of reservoir pressure values by calculating this value through the productivity of wells in order to perform operational analysis of the field development. The input data for the calculation are the dynamics of gas flow rate and wellhead pressure dynamics. This approach allows us to estimate formation pressure around each well outside the de-resistivity funnel at any time. The article presents the results of testing the proposed method under field conditions at one of the Yamal fields. The results of the analysis show the difference in the volume of drained reserves with the geological model for a low permeable layer. The difference in the reserves volume estimation by the material balance method using the isobar map was 18 % of the initial geological reserves. The analysis of the reasons of divergence of the drilled reserves was carried out and the fact of differently sorted excavation along the section and the presence of uninvolved reserves was revealed.