Miscible Gas Injection in Fractured Reservoirs

Abstract

Abstract This paper discusses miscible gas injection in fractured reservoirs, based on compositional reservoir modeling. Developed miscibility in conventional one-dimensional systems is dependent on fluid properties alone, and can be estimated by well known experimental and numerical procedures. This is not the case for heterogeneous and multi-dimensional systems such as fractured reservoirs. Based on a multi-cell algorithm a method is proposed to determine conditions of developed miscibility in fractured reservoirs. The proposed method makes it possible to evaluate gas injection processes, and developed miscibility in particular, in specific regions of the reservoir. A systematic step-by-step extension from a single one-dimensional matrix block to more complex matrix-fracture systems shows that the minimum miscibility pressure/enrichment (MMP/MME) level in a fractured reservoir is significantly higher than for a conventional one-dimensional single-porosity system. This is mainly due to multi-dimensional flow and molecular diffusion. On the other hand, even at pressures significantly below the fracture system MMP, substantial enhancements in the recovery rate and ultimate oil recovery can be expected by non-equilibrium gas injection. It is shown that reduction of the gas-oil interfacial tension and internal Darcy flow induced by interfacial tension gradients play a key role. This capillary driven process may lead to accelerated production and very high ultimate recoveries. Introduction Conventional recovery strategies in fractured reservoirs normally involve depletion and/or immiscible injection schemes (water or lean gas injection). An alternative to these conventional recovery methods is enriched gas injection, potentially leading to a miscible situation and high ultimate recoveries. Miscibility in conventional single porosity reservoirs has been studied extensively over the past years. Experimental and numerical procedures have been developed that give a definitive measure of the miscible development for a one-dimensional (1-D) flow system1–3. The 1-D miscible flow process has been shown to be dictated by phase behavior alone, and not at all by rock or other fluid properties. For fractured reservoirs, however, the physics behind developed miscibility is unknown. It is even uncertain whether miscibility can be obtained in a fractured system. We have not found references on this subject in the literature. Hence, the objective of this work was mainly to gain a better physical understanding of the potential for miscible displacement in fractured reservoirs and to develop numerical procedures for determination of miscibility in dual-porosity systems. With the introduction of fractures, the displacement process no longer depends on fluid properties alone. The fracture-matrix geometry, size and interaction, and other physical phenomena also play an important role. Instead of a well-defined displacement of oil by gas, the injection gas tends to flow in the highly permeable fractures, surrounding the oil in the matrix blocks. The gas composition inside the matrix blocks is usually different from the composition in the fractures. Provided the matrix is higher than the capillary entry height, gas enters at the top and may the upper sides of the matrix block. This means that the gas composition near the displacement front will be different than for purely vertical flow. For significant differences in fluid compositions between the matrix and fracture media, diffusion may play an important role in fractured reservoirs. All this suggests that the total composition, (and hence the miscible process), near the displacement front will be different than from a one-dimensional slimtube displacement. Away from the injectors in a fractured reservoir, the fracture gas composition may be influenced by the upstream matrix-fracture fluid exchange. Intuitively, the gas entering the matrix blocks some distance away from the injectors will be richer than the original injection gas. That is, one might expect more favorable conditions of developed miscibility and improved recoveries from the matrix blocks in the reservoir regions away from the injectors.