Gas–water two-phase productivity model for fractured horizontal wells in volcanic gas reservoirs with natural fractures

Abstract

ABSTRACT Volcanic gas reservoirs, as new exploration and development fields, are rich in resources and have broad exploration and development prospects. However, compared with the conventional sandstone gas reservoirs, volcanic gas reservoirs have more complex storage and permeability mode, with multiple media including pores, caves and fractures, which determines the strong heterogeneity, complicated seepage mechanisms, and difficult development of volcanic gas reservoirs. To further improve the prediction accuracy of fractured horizontal well productivity in volcanic gas reservoirs, Xushen low-permeability volcanic gas reservoirs are presented as examples herein using the equation of motion based on the principle of equal flow. Considering the stress-sensitive effect, starting pressure gradient, high-speed turbulence effect, and dual porous medium structure, as well as combining the nature of gas and real gas state equations, a gas–water two-phase productivity model is established for a matrix-natural fracture-artificial fracture-wellbore volcanic gas reservoir fractured horizontal well. In addition, factors affecting production are studied. Results indicate that stress-sensitivity coefficient of different media has different effects on the production. The natural-fracture stress-sensitive coefficient has the greatest influence on production. With the increase of the natural-fracture stress-sensitive coefficient, the gas production decreases and the reduction amplitude changes little, which is almost uniform. When natural-fracture stress-sensitive coefficient is 0.08, gas well production decreases by 14.24%. The second is the artificial main fracture stress-sensitivity coefficient. With the increase of the artificial main fracture stress-sensitivity coefficient, the decrease of gas production becomes larger. When artificial main fracture stress-sensitivity coefficient is 0.1, gas well production decreases by 6.6%. The matrix stress-sensitivity coefficient on production is the least, almost zero. The high-speed nonlinear coefficient significantly affected the production. With the increase of the high-speed nonlinear coefficient, the gas well production decreases, and the decrease rate is faster and faster. When the high-speed nonlinear coefficient increases to 10000 m−1, gas well production decreases by 45.65%. However, the starting pressure gradient had little effect on production. Finally, the model is verified by the actual production data of Xushen1-H2 well. The average errors of the gas production, water production, and pressure calculations are 9.65%, 9.63%, and 9.8%, respectively. This study contributes positively to the productivity prediction of fractured horizontal wells in low-permeability volcanic gas reservoirs.