Summary
Application of cyclic solvent injection into heavy and viscous crude oil followed by in-situ combustion of heavy residues is explored from a laboratory perspective. The solvent reduces oil viscosity in-situ and extracts the lighter crude-oil fractions. Combustion cleans the near-well region and stimulates thermally the oil production. Both solvent injection and in-situ combustion are technically effective. The combination of the two methods, however, has never been tried to our knowledge.
Hamaca (Venezuela) and West Sak (Alaska) crude oils were employed. First, ramped temperature oxidation studies were conducted to measure the kinetic properties of the oil prior to and following solvent injection. Pentane, decane, and kerosene were the solvents of interest. Second, solvent was injected in a cyclic fashion into a 1-m-long combustion tube. Then, the tube was combusted. Hamaca oil presented good burning properties, especially following pentane injection. The pentane extracted lighter components of the crude and deposited preferentially effective fuel for combustion. On the other hand, West Sak oil did not exhibit stable combustion properties without solvent injection, following solvent injection, and even when metallic additives were added to enhance the combustion. We were unable to propagate a burning front within the combustion tube. Nevertheless, the experimental results do show that this combined solvent combustion method is applicable to the broad range of oil reservoirs with properties similar to Hamaca.
Introduction
This article investigates the effect of solvent injection on the subsequent performance of in-situ combustion. The work is based on experimental results obtained by a combination of these two successful in-situ upgrading processes for viscous oils. It is envisioned that application in the field occurs first by a cycle of solvent injection, a short soak period, and subsequent oil production using the same well (Castanier and Kovscek 2005). By mixing with oil, the solvent decreases the oil viscosity and upgrades the crude by extracting in-situ the lighter ends of the crude oil. The heavy ends, that are markedly less interesting, are left behind. Injection of solvent and oil production occurs for a number of cycles until the economic limit is reached or until the deposition of crude oil heavy ends damages production. The solvent injection phase is followed by in-situ combustion that burns the heavy ends left from the solvent injection. By switching from air to nitrogen injection, the combustion is extinguished. Again, oil is produced by the same well used for injection in a cyclic fashion. Combustion enhances the production by decreasing thermally the oil viscosity and adding energy to the reservoir through the formation of combustion gases. The combustion also upgrades the oil through thermal cracking (Castanier and Brigham 2003).
For our experiments, two oils of particular interest were used. The first experiments employed crude oil from Hamaca (Venezuela), where the field location requires important costs of transporting crude to upgrading facilities. The second set of experiments was conducted with viscous West Sak oil (Alaska), where steam injection currently appears to be unsuitable because of heat losses to permafrost.
While the presence of oil in the Orinoco heavy-oil belt, in Central Venezuela, was discovered in the 1930s, the first rigorous evaluation of the resources was made in the 1980s, and the region was divided into four areas: Machete, Zuata, Hamaca, and Cerro Negro. It contains between 1.2 and 1.8 trillion recoverable barrels (Kuhlman 2000) of heavy and extra-heavy oil. The 9-11° API density crude is processed at the Jose refinery complex on the northern coast of Venezuela. The cost of transporting heavy oils to the northern coast provides an incentive to investigate in-situ upgrading. In 2003, the total production from these projects was about 500,000 B/D of synthetic crude oil. This figure was expected to increase to 600,000 B/D by 2005 (Acharya et al. 2004).
West Sak is a viscous oil reservoir located within the Kuparuk River Unit on the North Slope of Alaska. It is part of a larger viscous oil belt that includes Prudhoe Bay. The estimated total oil in place ranges from 7 to 9 billion barrels, with an oil gravity ranging from 10 to 22°API. The reservoir depth ranges from 2,500 to 4,500 feet, with gross thickness of 500 feet and an average net thickness of 90 feet. The temperature is between 45 and 100°F, and there is a 2,000-ft (600-m) -thick Permafrost layer. In March 2005, 16,000 BOPD were produced and 40,000 BOPD are planned for 2007 (Targac et al. 2005). Within the scope of this study, West Sak is of particular interest because there are technical difficulties with steam injection that include (Gondouin and Fox 1991):Surface-generated steam passing through a thick permafrost layer; the well would sink if the permafrost melted.The reservoirs consist of thin, medium-permeability layers.The formation may contain swelling clays that reduce the rock permeability when exposed to steam condensate.
Solvent injection and in-situ combustion are effective in a variety of fields. Both techniques upgrade the oil directly in the reservoir, thereby making heavy resources easier to exploit. The combination of these two processes is applicable at large scale to recover viscous oil, or in-situ combustion could be applied on an ad hoc basis to clean the wellbore region, increase the permeability, and thus act as a stimulation process.
