Fractured Reservoirs Modelling and Interpretation

Abstract

Abstract. Fractures are common features of many well-known reservoirs. Naturally Fracture Reservoirs (NFR) consist of fractures in igneous, matamorphic, and swedimentary rocks (matrix). Faults in many naturally fractured carbonate reservoirs often have high-permeability zones and are connected to numerous fractures with varying conductivities. In many NFRs, faults and fractures frequently have discrete distributions rather than connected fracture networks. Because fractures are often created by faulting, faults and fractures should be modeled together. Accurately modeling naturally fractured reservoir pressure transient behavior is important in hydrogeology, the earth sciences, and petroleum engineering, including ground-water contamination to shale gas and oil reservoirs. For more than 50 years, conventional dual-porosity type models, which do not include any fractures, have been used for modelling fluid flow in naturally fractured reservoirs and aquifers. They have been continuously modified to add unphysical matrix block properties such as the matrix skin factor. In general, fractured reservoirs are heterogeneous at different length scales. It is clear that even with millions of grid blocks, numerical models may not be capable of accurately stimulating the pressure transient behavior of continuously and discretely NFRs containing variable conductivity fractures. The conventional dual-porosity type models are obviously an oversimplification; their serious limitations and consequent implications for interpreting well test data from NFRs are discussed in detail. These models do not include wellbore-intersecting fractures, even though they dominate the pressure behavior of NFRs for a considerable length of testing time. Fracture conductivities of one to infinity dominate transient behavior of both continuously and discretely fractured reservoirs, but again dual-porosity models do not contain a single fracture. Our fractured reservoir model is capable of treating thousands of fractures that are perdiocially or arbitrarily distributed with finite- and/or infinite-conductivities, different lengths, densities, and orientations. Appropriate inner boundary conditions are used to account for wellbore-intersecting fractures and direct wellbore contributions to production. Wellbore storage and skin efffects in bounded and unbounded systems are included in the model. Three types of damaged skin factors that may exist in wellbore-intersecting fracture(s) are specified. With this highly accurate model, the pressure transient behavior of conventional dual-porosity type models are investigated, and their limitations and range of applicabilities are identified. The behavior of the triple-porosity models are also investigated. It is very unlikely that triple-porosity behaviorf is due to the local variability of matrix properties at the microscopic level. Rather, it is due to the spatial variability of conductivity, length, density, and orientation of the fracture distributions. Finally, we have presented an interpretation of a field buildup test example from an NFR using both conventional dual-porosity models and our fractured reservoir model.

Pressure Transient Behavior of Horizontal Wells Intersecting Multiple Hydraulic Fractures in Naturally Fractured Reservoirs

“…As reported by Kuchuk et al (2014), if the permeabilities of the fracture segments are a few orders of magnitude larger than those of the matrix blocks, but not k m k f , then a macroscopic representative elementary volume can be constructed (i.e., dual-porosity models can therefore be used for continuously fractured reservoirs). If k m k f , then a macroscopic representative elementary volume (REV) cannot be constructed.…”

“…1 of the Kuchuk and Biryukov (2015) papers], particularly in carbonate formations (Al-Thawad et al 2001. It was reported by Kuchuk and Biryukov (2015) and Kuchuk et al (2014) that wellbore-intersecting fractures dominate the pressure transient behavior of both continuously and discretely naturally fractured reservoirs.…”

“…Thus, analytical or numerical techniques should be used to model the pressure transient behavior of fractured reservoirs without fracture-segment homogenization or upscaling (i.e., fractures have to be modeled explicitly). In general, dual-porosity models should not be used for discretely fractured reservoirs (Kuchuk et al 2014). Kuchuk et al (2014) also showed that the resistance interface condition used by Barenblatt et al (1960) and Warren and Root (1963) to specify fluid transport between the equivalent matrix and fractured media, dominates the dual-porosity model pressure transient behavior, irrespective of whether it is a vertical or horizontal well.…”

“…In general, dual-porosity models should not be used for discretely fractured reservoirs (Kuchuk et al 2014). Kuchuk et al (2014) also showed that the resistance interface condition used by Barenblatt et al (1960) and Warren and Root (1963) to specify fluid transport between the equivalent matrix and fractured media, dominates the dual-porosity model pressure transient behavior, irrespective of whether it is a vertical or horizontal well. The pseudosteady-state flow in the equivalent matrix medium that we observe in the Warren and Root (1963) model is a consequence of the resistance interface condition.…”