The Effect of heterogeneity on the Performance of EOR Processes with Horizontal Wells
Authors: Gharbi R.B. , Peters E.J. , Elkamel A. , Afzal N. 
Published: December 2007

The effect of heterogeneity on the performance of enhanced oil recovery (EOR) processes with horizontal wells was investigated using a three-dimensional, finite difference, chemical flood simulator (UTCHEM). We have used the reservoir and fluid properties of a reservoir typical of an oil field from the Middle East region. Several three-dimensional heterogeneous permeable media, each with different permeability variation and spatial correlations, were generated geostatistically using the matrix decomposition method. These permeability fields cover a wide range of permeability variation and correlation lengths. Initially, simulations of miscible and immiscible displacements with one horizontal injector and one horizontal producer were performed in all these heterogeneous reservoirs. The displacement performance of the two horizontal wells in each of the heterogeneous reservoirs were then compared with that obtained with two vertical wells (one injector and one producer). For all the permeable media, the effect of the amount of cross flow on the displacement behavior was also investigated.

The study shows that the performance of enhanced oil recovery processes with horizontal wells is strongly affected by the permeability variation and the spatial correlation of the reservoir heterogeneity. The combination of high permeability variation and high correlation length significantly reduces the performance of horizontal wells.

Introduction

Heterogeneity and the interaction of several forces, namely viscous forces driven by adverse mobility ratios, capillary forces from interfacial forces between immiscible fluids, gravity forces driven by fluid density gradients and dispersive forces caused by concentration gradients between miscible fluids can conspire to make many enhanced oil recovery (EOR) processes unstable and somewhat unpredictable. The unfavorable mobility ratio of most EOR processes may cause unstable displacement and poor oil recovery. Even when the porous medium is homogeneous, unstable displacements can occur and result in reduced sweep efficiency and early breakthrough of the injected fluid. Reservoir rocks are seldom homogeneous and variation in permeability occurs on a variety of length scale. With the advent of computed tomography (CT) scanning and nuclear magnetic resonance (NMR) imaging, many rocks that were once thought to be homogeneous, have been shown to contain permeability variation. Even sand packs may have packing problems that can create channels, especially along the boundaries. An unfavorable mobility ratio coupled with the heterogeneity of the porous medium results in bypassing of oil. Furthermore, at low displacement rates, gravity forces which segregate the less dense fluid from the more dense fluid, can dominate the other forces. This may lead to gravity override or underride of the oil by the injected fluid, and thus poor sweep efficiency.

During the last decade, the petroleum industry all over the world has experienced a rapid increase in the number of horizontal wells being drilled. The primary reason for this upsurge in the number of horizontal wells is due to the fact that the recent advances in drilling technology has reduced the drilling and completion costs of horizontal wells. A second reason for the increase in the number of horizontal wells is that a horizontal well is drilled parallel to the bedding plane thus opening up more of the formation to well bore. This large surface area of a horizontal well with the reservoir may be able to deliver two to ten times the production of conventional vertical well.

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