In recent years, more and more multilayer gas reservoirs were discovered in the world. Forecasting the dynamic reserve of multilayer producer is a major challenge for reservoir engineering. Material Balance Equation (p/z plot) is one of the fundamental methods for reserve estimation. It is usually used in single layer gas reservoir.
In this paper, the applicability of multilayer gas reservoir material balance equation for reserve estimation is studied by single well numerical simulation. There are several parameters which influence reserve estimation of multilayer gas reservoir, such as the connectivity between layers of gas reservoir, the difference between formation pressures of the layers, the difference between permeability of the layers.
Results of numerical simulation show that vertical permeability is the main factor that influences reserve estimation. Reserve estimation has a large accuracy when vertical permeability is larger than 1E-4mD. Reserve estimation value tends to be smaller than the true value when the vertical permeability is smaller than 1E-4mD, and the more non-homogeneous layer, the larger the deviation is.
The results of numerical simulation show that difference between the permeability of the layers is another main factor in reserve estimation. For a larger non-homogeneity between layers, calculated dynamic reserve decreases as increase of non-homogeneity between layers, and the dynamic reserve of the layer with low permeability is low.
The dynamic reserve calculation is one of major tasks for reserve estimation in gas field development. The dynamic reserve is the produced OGIP which is calculated with dynamic method when formation pressure reduces to zero. Calculation methods including material balance method [1,2], elastic binomial, extended drawdown test, production decline analysis method and numerical simulation, etc.
Material balance method is a main method for reserve estimation of gas field. One of the basic assumptions of the method is that gas reservoir consists of single and homogeneous gas tank. Degree of reserve recovery shall be higher than 5% as the methods applies to reserve calculation. Several successful analyses have been performed on single layer reservoir . However, classic material balance equation is unable to accurately forecast production performance of multi-layer gas reservoir .
The multilayer gas reservoir can be divided into two types according to whether there is cross flow between layers. One is commingled system which means the layers have been isolated by non-permeable interbed no cross-flow exists, the other is crossflow system where there is connectivity between layers .