Analysis of pressure behaviors and flow regimes of naturally and hydraulically fractured unconventional gas reservoirs using multi-linear flow regimes approach

Abstract

This paper targets the applicability of multi-linear flow regimes approach in gas reservoirs having multiple porous media: matrix, natural fractures, and hydraulic fractures. The approach attempts to understand pressure profiles and flow regimes developed in some of unconventional shale layers that have already been naturally fractured. In this paper, the main focus is given to fluid flow phenomenon from the matrix towards naturally induced fractures and from these fractures to the hydraulic fractures. Studying the impact of matrix, natural fracture and hydraulic fracture parameters on reservoir performance is another objective for this paper.Purposefully, several mathematical models have been developed to investigate transportation phenomenon of reservoir fluid in porous medium. The porous medium is assumed to be finite rectangular reservoir having different configurations. It is divided into two zones; inner porous medium close to the horizontal wellbore where multiple-hydraulic fractures propagate to some distance and outer porous medium where naturally induced fractures exist in the drainage area close to the outer boundary. Reservoir fluids, in the outer zone, are assumed to flow from the matrix to natural fractures and then from natural fractures to the inner zone. In the inner porous medium, fluids flow from matrix to natural fractures in addition to fluids flowing from the outer porous medium. Then all fluids flow by natural fractures to hydraulic fractures and finally from hydraulic fractures to the wellbore.The mathematical models for fluid flow in the two zones of the reservoir have been solved and three main flow regimes have been characterized. For each flow regime, an analytical model is used to describe its pressure profile with time and production performance. Bilinear flow regime, typically observed at early time, refers to simultaneous linear flow inside hydraulic fractures and flow from inner porous medium to the hydraulic fractures while linear flow describes fluids flow from inner zone to hydraulic fractures and late linear or boundary dominated flow regime describes fluids flow from the outer to the inner porous media.The main outcomes of this study are: 1) The ability to characterize reservoirs with triple porous media using multi-linear flow regimes approach. 2) Understanding the impact on pressure distribution of inner porous medium caused by the characteristics of hydraulic fractures such as fracture width and conductivity in addition to hydraulic fracture storativity. 3) Understanding the effect of reservoir configuration, matrix, and natural fractures characteristics on pressure distribution in the outer porous media. 4) Pressure distribution and flow regimes obtained by applying transient flow approach are not similar to those obtained by applying pseudo-steady state flow approach. 5) Characterization of different linear flow regimes, controlled by different parameters, with respect to production time.