Research on Multiple Coal Seams Relative Permeability Calculation Method Based on Production Data Inversion

Abstract

To obtain the actual gas-water relative permeability of the coalbed methane (CBM) reservoir and further deepen the cognition of the gas-water production law of multiple coal seams, a relative permeability calculation method based on production data inversion is constructed. Based on production data, historical fitting is carried out through the multiple coal seams whole process coupling flow model, and the basic physical parameters of each layer are inversed. Based on the obtained physical parameters, the productivity prediction of the whole production cycle is carried out. By calculating the average water saturation and gas-water relative permeability in each iteration time step, the average gas-water relative permeability curve of the reservoir in the target period is finally obtained. The results show that the calculation method proposed in this paper can realize the acquisition of the relative permeability curve in the given period. Compared with the input relative permeability curve, there is a reverse point on the output relative permeability curve that can represent the continuous production of desorption gas. Gas production is affected significantly by different types of initial input relative permeability curves, and is mainly influenced by the input relative permeability curve at the initial production stage. Under ±30% deviation, the average difference in cumulative gas production is 16.92% (3 years). During the production of CBM wells, the average water saturation was maintained at a high level. At the end of the production of multiple coal seams commingled production well, the average water saturation change is less than 15%. Restricted by high water saturation, the average relative permeability of the gas is always maintained at a low level, less than 0.1 at the end of production of actual production wells. The fundamental technical difficulty in realizing the initial high production and subsequent sustained and stable production of CBM wells lies in how to reduce the reservoir water saturation effectively and improve the relative permeability of the gas, so as to promote the desorption of adsorbed gas and the sustained CBM production.