Observations of Heavy Oil Primary Production Mechanisms from Long Core Depletion Experiments

ABSTRACT

Foamy oil solution gas drive mechanisms are complex and our knowledge and understanding is limited despite extensive studies in the literature. In order to advance our understanding of heavy oil solution gas drive mechanisms, long core depletion experiments were designed. These experiments were performed on sand-filled or glass bead-filled tubes that are x-ray transparent and have pressure transducers along their length. The novelty of the experiments is the length that they extend (over 18 m) and the duration of the experimental runs. The results of the longer experiments should be able to provide data that bridge the gap between the field scale and the shorter laboratory experiments that have been performed in the past. Thus, production, pressure transient and saturation data are presented in this ‘extended’ scale. In addition, CT scanner images are expected to provide information about the evolution of gas.

Introduction

Sand production increases the permeability of unconsolidated sand reservoirs through the establishment of wormholes. These higher permeability regions, in combination with heavy oil solution gas drive, recover heavy oil through primary production (Cold Heavy Oil Production with Sand or CHOPS). A better understanding of the fluid-rock interaction can be established by studying the effect of permeability and geometry of the experiments.

Bubble growth in a porous medium is initially controlled by the geometry of the pores, the pore walls and capillary forces (1). Dumore (2) compared two different permeability sandpacks and saw that the gas remaining dispersed for longer in the high-permeability sandpack. Wall and Khurana (3) observed that lower permeability cores resulted in higher gas saturation within the core. Therefore, they suggested that the free gas saturation depends on capillarity. Sarma and Maini (4) found that, although higher production was obtained with a higher permeability core, the general trend for pressure and production as a function of time were the same as the lower permeability core. Firoozabadi et al. (5) observed lower supersaturations and lower critical gas saturation were obtained from lower permeability depletion experiments. Tang et al. (6) saw that poorly packed areas had higher gas saturation as a result of lower capillary forces in higher porosity areas.
In higher permeability porous media, trapping due to capillary forces is lower as a result of larger pore sizes. Therefore, the flow of the fluid is less hindered, giving the newly nucleated gas less time to grow within the pores before it begins to move with the oil. This causes the gas to remain dispersed within the oil for a longer time before the gas coalesces, compared to lower permeability sandpacks.

High depletion rates are necessary when field observations of heavy oil solution gas drive are reproduced in the laboratory. As a result, there have been numerous investigations in the literature that study the effect of the depletion rate (7-10). High depletion rates are considered representative of near wellbore behaviour while low depletion rates represent field conditions. By increasing the length of the sandpack to a much larger scale, it should be possible to capture the pressure, saturation and production behaviour both near and further from the wellbore in a single experiment.

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