Abstract

Cold heavy oil production (CHOP) is an aggressive recovery process that has been used in the development of unconsolidated heavy oil reservoirs. It is believed that wormhole structures are generated during this process and the structures provide the main conduits for fluid flow; therefore, an understanding of the effects of wormhole structures is critical for primary and post cold production.

A comprehensive semi-analytical model coupling the reservoir/wormhole flow has been developed to study the effects of wormholes. This model is based on the source and Green's functions method, which has been widely practiced in horizontal and multilateral well analyses to solve the unsteady flow problems. This model is also quite flexible to incorporate any wormhole hydraulic model, which is necessary to consider the foamy oil and sand flow in terms of the pressure drop along wormholes; moreover, the model can be used under a variety of reservoir boundary conditions, such as closed boundary, constant pressure boundary, etc. In this paper, the effects of wormhole patterns, wormhole scales, wormhole diameters, wormhole branching, and pressure drop along the wormhole on the cold production have been analyzed extensively. The model not only provides a powerful tool to understand the effects of wormholes, but also offers a way to analyze the possible wormhole structures and to optimize/predict the reservoir fluid production in the heavy oil cold production process.

Introduction

Cold production is a non-thermal process in which sand is aggressively produced to obtain a higher oil production rate. It is believed that wormhole structures are generated during this process and the wormhole structures provide the main conduits for fluid flow. The presence of wormholes has been confirmed according to the observations made in oil fields and investigations in laboratory experiments. Numerous tracer surveys were conducted in the Clearwater reservoir, Linderberg1and S. E. Pauls Valley, Oklahoma2and rapid communications were observed between wells confirming the existence of linear conduits which contain little to no matrix material. The results of a seismic survey in Burnt Lake3had ruled out the formation of a big cavity surrounding the wellbore. Husky Oil Operation Ltd.4 conducted a short-term test and fracture-like behavior was observed at the injection pressures much lower than the generally accepted fracture strengths of the undisturbed formation. Pressure tests conducted by Mobile Oil Canada in the Celtic field5indicated that inflow was dominated by linear flow and the permeability increase was on the order of three to five times of the undisturbed matrix permeability. An extremely high porosity channel was also observed in the laboratory experiment conducted by Tremblay.6

The use of Source and Green's functions method to solve the unsteady, pseudo-steady or steady flow problems is widely practiced in vertical, slant, horizontal, and multilateral wells. For example, based on Source and Green's functions, Yildiz and Ozkan et al.7, 8 proposed a three-dimensional analytical model to analyze the transient flow into perforated vertical, horizontal, and slant wells. In their models, it is possible to consider non-uniform distribution of perforations and each perforation may have different penetration length and skin factor.

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