One of the problems encountered in the production of crude petroleum is the accumulation and deposition of asphaltenes in the wellbore and production lines. Asphaltenes may deposit in the wellbore and the production lines in the form of a solid deposit or a sludge. Solid deposits of asphaltenes may be a result of growth of asphaltene aggregates on formation surfaces, while sludges may form as large aggregates suspended in crude petroleum that settle out of the suspension.
The asphaltic materials which compose the deposits may originally be in solution in the crude oil as it exists in the reservoir. Often times in the production or transporting of crude petroleums, the equilibrium of the solution is altered or destroyed and these asphaltic materials, being the least soluble constituents, separate and accumulate in the wellbore and in production equipment at locations where the flow velocity is less than that required to maintain the asphaltene aggregates in suspension. Formation of asphaltenes deposits may also be exacerbated by interruptions in formation fluid flow through wellbores and production lines, such as the interruptions caused by routine maintenance and/or preparations for extreme weather such as hurricanes. The accumulation of these asphaltic materials progressively decreases the rate of movement of the petroleum and must therefore be periodically removed.
Various solvents have been utilized to solubilize asphaltenes that have deposited in oil-bearing formations. U.S. Pat. No. 5,425,422 discloses injecting deasphalted oil into an oil-bearing formation to solvate asphaltene deposits near a wellbore in the formation and thereby improving production of oil from the formation. The injected oil may be produced from the formation and deasphalted prior to being injected into the formation. The use of aromatic solvents such as o-xylene and toluene to dissolve asphaltene-based deposits in a formation near a wellbore is also known.
Disulfide solvents have also been used to dissolve asphaltene-based deposits in a formation for near-wellbore formation remediation. U.S. Pat. No. 4,379,490 discloses the use of an amine activated disulfide oil for treating and removing unwanted asphaltene deposits from the pore spaces of oil-bearing formations. U.S. Pat. No. 4,379,490 further discloses that carbon disulfide is one of the most effective asphaltene solvents known, and that it has been utilized for the removal of asphaltene-based deposits from oil-bearing formations.
Such solvents, however, have certain disadvantages attached to them. Injection of aromatics such as toluene and o-xylene may be subject to regulatory limitation, and is economically inefficient since such aromatics are even more highly processed and valuable than deasphalted oil. Disulfide solvents may be subject to hydrolysis within the formation, and, in the case of carbon disulfide, may result in souring the formation. Carbon disulfide is also highly toxic.
It is desirable to develop an improved method of remediating asphaltene deposits in wellbores and production lines.