Coning Phenomena in Naturally Fractured Reservoirs

Abstract

SPE Members Abstract Development of water coning in naturally fractured reservoirs and the issues of critical rates and cone breakthrough times are examined in terms of appropriate reservoir properties. A correlation is proposed to account for fracture acceleration effects in the computation of critical rates and the breakthrough times for uniformly distributed fractures. For a fractured systems with non-uniform fracture distribution, cone development is asymmetrical and the concept of partial completion to prevent coning becomes inapplicable. As such, for heterogeneously fractured reservoirs, estimation of critical rate and breakthrough time requires modeling with an understanding of fracture pattern around the producing wells. Introduction Optimization of completion interval and production scheduling requires consideration of factors affecting cone development in bottom water drive reservoirs. Water coning is a phenomena caused by an imbalance between the gravitational and viscous forces around the completion interval. From a force balance relationship, an estimation can be made of the maximum rate above which a cone may develop for a particular wellbore and reservoir configuration. Additionally, correlations have been developed for estimation of water breakthrough time for wells producing at a constant rate and above the critical rates. Furthermore, the water oil ratio behavior after cone breakthrough has been discussed in the literature. Correlations published for homogeneous reservoirs incorporate the effect of vertical permeability in terms of an anisotropy ratio; kv/kh. Presence of shale breakers and the effect of compaction cause reductions in the vertical permeability. There are, however, cases where the kv/kh can be larger than unity. A prime example is the situation with naturally fractured reservoirs. High vertical permeabilities in fractures are bound to accelerate the coning process resulting in lowering of the critical rates and more rapid breakthrough times. Additionally, the preferential path for fluid flow through the fractures and the uneven fracture conductivities commonly observed in naturally fractured reservoirs is expected to affect wells regardless of their structural position. Furthermore, the role of the tight matrix in controlling the interporosity flow needs to be studied. For naturally fractured reservoirs that can be represented by a dual porosity model of the type described by Warren and Root, two interporosity parameters, and are convenient means for representation of the storativity and transmissivity of the system. Intuitively, the critical rates for such systems should be influenced by the storativity of the fracture network. The saturation profiles developed in matrix and fractures are expected to be non-similar in extent and breakthrough times. The issue of coning behavior in naturally fractured reservoirs has received limited attention. The purpose of this paper is to examine the nature of cone development in naturally fractured reservoirs, to scrutinize the application of conventional correlations for such systems and to propose alternative correlations and guidelines for completion strategies and optimal rate determinations. PREDICTION METHODS Early work on analysis of critical rates to prevent coning, such as the method published by Muskat and Wyckoff focused on a force balance between the gravitational and viscous effects. P. 551^