Optimization of Vertical Miscible Flood Performance through Cyclic Pressure Pulsing

ABSTRACT

The concept of improving the performance of vertical hydrocarbon miscible floods is addressed. Improvement is attempted through cyclic pressure pulsing. The idea is to force recovery from dendritic structures or other unswept areas through pressure pulsing of the solvent and gas phases. A series of vertical miscible floods was performed on a-core from the Rainbow Keg River area. Methane (solvent) displaced n-pentane (oil) under a variety of rate and pressure scenarios. All runs were performed at rates significantly higher than the critical rate for the specific system. Through the experiments it was established that at the core level, the rate of displacement has a considerably larger effect on oil recovery than the pressure pulse. However, even if the solvent has broken through, reduction of frontal advance rates can provide significant incremental oil recovery.

Introduction

Vertical hydrocarbon miscible floods are frequently used in the Rainbow Lake reefs of northern Alberta. Oil is displaced downward by a less viscous solvent. Viscous fingering, which is in part triggered by local heterogeneities is blamed for reduced sweep efficiencies and oil recoveries which are less than what was expected in the design stage. In most cases, the solvent is chased by another gas. When both solvent and chase gas break through, the gas to oil ratio (GOR) increases dramatically with a comparable decrease in oil production. The standard approach in controlling excessive gas or solvent production is workovers in which the current perforations are cement squeezed and new perforations are made lower in the reef. This procedure is costly and is not always successful. Husky proposed another procedure in optimizing the displacement strategy which can improve the vertically directed miscible floods, by applying a cyclic flow interruption consisting of periodical shut-in (1). Field observations lead to the formulation of the concept (2,3). Conceptually, periodic shut-in and pressure pulses cause reduction and even reversal in the advancement rate of the solvent/oil flood front. These changes would allow not only the cones and fingers of solvent to heal back, but also the healing process would increase the length of the mixing zone. Longer mixing zones reduce the tendency for fingers to form. Also, these changes would change the pathways of dispersion and diffusion, potentially accessing bypassed oil. The problem with the cyclic flow interruptions is that production is discontinued. The question was raised whether a similar effect could be observed through a continuous process. It was proposed that cyclic pressure pulsing under restricted flow could provide increased mass transfer between solvent and oil and also allow for the solvent to access previously inaccessible oil through lateral spread. It was decided to test the proposed ideas in the experimental system previously used to test the applicability of the shut-in concept (2.3).

FIGURE 1: Schematic of the experimental apparatus. (Available in full paper)

Experimental

The demonstration experiments were conducted in core from Rainbow Keg River “B ” pool. The properties of the different pieces used in this study are shown in Table 1. Methane was used as the solvent and n-pentane as the oil.

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