Abstract

In this project, steam was tested to stimulate damaged limestone cores. Several dynamic experiments were conducted to determine the effect of asphaltene on the permeability of limestone cores. Plugging effects due to asphaltene deposition were evaluated through comparison with reference permeability measured prior to and after oil flow. Damaged cores were then subjected to steam soak process. The permeability of the cores was measured after each treatment. Core impairment resulting from in-situ asphaltene deposition, was found to cause a 56 to 89% loss of initial oil permeability depending on rock permeability and injection rate. Core stimulation resulting from steam soak treatment, was found to cause 24 to 230% improvement on the damaged core permeability depending on rock permeability. On average, 95% improvement in damged core permeability was obtained.

Introduction

Many field cases and laboratory studies have reported precipitation and deposition of asphaltenes during the recovery of oil. Asphaltenes deposition generally appeared in the field first in surface facilities, especially in separators, during the oil final depressurization step.1,2

Another critical point along the production chain is within tubing in which precipitate form at depths corresponding to the bubble point pressure of produced oil3,4. In some field cases, the reported asphaltene deposition in reservoirs has been so severe that reduced well productivity and injectivities.5,6 Asphaltenes are generally found in all crude oils.4 They are defined as a solubility class of the heavy components in the crude oil that are insoluble in a non-heavy (n-alkenes) solvents such as pentane. They are complex polar macro-cyclic molecules that contain carbon, hydrogen, oxygen and sulfur7,8 Asphaltene are aromatic and occur partly dissolved and partly dispersed colloidal suspension stabilized by non-polar resins and maltenes fraction of the crude. The ratio of polar to non-polar components and the ratio of high to low molecular weight components control the solubility of asphaltenes, resins and maltenes within the crude.9 Crudes with high resin and maltene content are more stable. Any actions of chemical, mechanical or electrical nature will result in alteration these ratios and consequently depeptize the asphaltene micelle and cause their destabilization/flocculation. For example, crudes that have a low aromatic content have a higher degree of asphaltene instability and the addition of aromatics will increase the crude stability for asphaltene, thereby reducing the likelihood of asphaltene precipitation. Precipitation and deposition can also be caused by variation in pressure and temperature.10 Temperature effects are important since the higher the temperature the greater the solubility of the resins in the n-alkanes and therefore the less soluble the asphaltenes in the crude. All crudes will experience some compositional changes during production from reservoir to surface pressure. The molar volumes of the fractions of the crude will change due to depressurization of the crude. The light ends, which are predominantly straight chain n-alkanes will expand relatively more than the heavier components of the crude. The increase in molar volumes of the light ends will continue up to the fluids bubble point pressure. Below the bubble point the light ends will leave the fluid as gas. This will result in a decrease in the molar volume of the light ends causing a decrease in the n-alkanes concentration. Since the solvency of asphaltenes in crude oil is dependent on the on the ratio of aromatics to n-alkanes all crudes will experience a minimum solvency just above the bubble point. Wether this will be sufficiently low to result in asphaltene precipitation will depend on the factors above i.e. the ratio of resins to asphaltenes, the crude solvency and the temperature4.

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