Asphaltenes Pipeline Cleanout: A Horizontal Challenge for Coiled Tubing

Abstract

Crude produced in the North of Monagas Estate, East Venezuela, has a high asphaltene content, which comes out of solution in both the wellbore tubulars and pipeline. This can eventually lead to complete plugging of the pipeline. This increases the cost of maintaining production because of the need to periodically remove these organic deposits. In a specific case, 9 km of 8–5/8-in. outside diameter (OD) production pipeline was successfully cleaned out using 2-in. OD coiled-tubing (CT) to regain pipeline production. As there is limited literature or documentation on the use of CT for this specific application, the operator and the service company established a joint team to do the feasibility study and engineering. Some of the key points were; the design of a frame to lay down the injector head, define the entry points along the pipeline, the selection of the bottomhole assembly (BHA), and fluids to use. Other issues were the measurement of stresses (push/pull) on the CT so that the CT could be run as far as possible in the pipeline without damaging either the CT or the pipeline. The pipeline was successfully cleaned, the CT being run seven times from five different entry points in the pipeline. This resulted in savings of USD 1 million for the operator and significantly reduced the time to recover normal production in the pipeline. Background The characteristic asphaltene content in the crude produced from the northeastern Venezuelan oilfields,1 requires periodic CT cleanouts with solvents and mechanical means to remove the obstructions that plug the wells and reduce production. These asphaltenes precipitate from solution in the wellbore tubular, caused by the pressure and temperature differentials, and progressively reduce the flow area in the tubing. Surface facilities are not exempt from this phenomenon. As the crude flows through the pipeline, the asphaltenes settle at the bottom due to the lower temperatures and pressures in the system.To prevent total plugging, the operators pump solvent mixtures and pigs through the pipelines, however without regular treatments the pipelines become restricted and eventually the production rate drops. In the case studied, this phenomenon led to an increase in the differential pressure in the line and ended in complete production stoppage on the surface system. This forced the operator to look for alternative means to reestablish and maintain production. Fig. 1 illustrates the progressive increase in the pipeline's differential pressure caused by the asphaltenes accumulation. Conventional pipeline maintenance practices could not be performed. Pumping aromatic solvents was not an option given the environmental constraints. Temporary Production Assisted by a Mobile Testing Unit To maintain the well's production, a program designed to produce the well using a mobile testing unit (MTU) was implemented. A choke manifold, a flare, two separators, and twelve 500-bbl capacity storage tanks were connected to the well and reconnected to the production line. Because of the severity of the pipeline plugging, it was necessary to transport the produced oil using trucks, increasing the risk associated with human error and environmental incidents. Fig. 2 illustrates the MTU layout. An attempt was performed to unplug an entire pipeline section, with the assumption that the plugged section would be below the river crossed by the pipeline, as this was the coldest and the lowest point in the pipeline. Two pipeline entry points (HT1 and HT2) were constructed to perform a first test across the river section by pumping water at a maximum pressure of 2,900 psi. Different pipeline sections were tested following the same procedure. When the pressure built up and there was no resultant flow at the other end, a plugged section was identified. Two main plugged sections were identified. The results showed that these plugged sections were located between the pipeline entry points HT3 and HT2 and between HT1 and HT4. Fig. 3 illustrates the pipeline layout on the field and the pipeline entry point locations.

Fractionation of Asphaltenes in Understanding Their Role in Petroleum Emulsion Stability and Fouling

“…SARA analysis has successfully guided the processing and refining paths of crude oils based on the quantity of asphaltenes in crude oils. Asphaltenes precipitate when the temperature/pressure changes, or the oil loses its light components, is mixed with a paraffinic solvent, or is blended with a paraffinic crude oil. , Such precipitation may lead to the deposition of asphaltenes in production wells and pipelines (Figure a), resulting in the flow restriction or even bringing oil production to a halt. ,− In addition, asphaltenes play a significant role in the stabilization of water-in-oil (W/O) emulsions − (Figure b), which lead to severe corrosion problems in production and transportation as a result of the dissolved salts and entrained fine solids carried by emulsified water droplets . Moreover, crude oil with a high content of asphaltenes tends to form coke when heated, which is highly detrimental to heat exchangers and catalyst beds in upgrading and refining. ,,, Because of its simplicity, SARA analysis is a reasonable first step in characterizing oils, providing warning for potential asphaltene-related losses suffered by the oil industry.…”

“…(a) Pipeline plugged with asphaltenes. Reprinted with permission from ref . Copyright 2005 Society of Petroleum Engineers.…”

Interactions of Asphaltene Subfractions in Organic Media of Varying Aromaticity

“…Despite their applications in coatings and paving, asphaltenes are generally considered a nuisance. − Their ability to self-associate and partition at oil–solid and oil–water interfaces is the root cause of several major issues encountered during oil production and processing. The adsorption of asphaltenes onto solid surfaces can (i) change the wettability of reservoir rocks and plug wellbores , and (ii) block pipelines and foul equipment, thereby reducing oil recovery and potentially halting oil production. On the other hand, the adsorption of asphaltenes to the oil–water interfaces significantly contributes to the stabilization of undesirable water-in-oil (W/O) emulsions, which pose severe corrosion problems to pipelines and refining facilities due to salts and fine solids associated with emulsified water droplets. − These asphaltene-induced problems are closely related to the inevitable self-aggregation behavior of asphaltenes as a result of the strong adsorption of asphaltene aggregates at oil–water interfaces or onto mineral and metallic surfaces. ,,, …”

“…2−4 Their ability to self-associate and partition at oil− solid and oil−water interfaces is the root cause of several major issues encountered during oil production and processing. The adsorption of asphaltenes onto solid surfaces can (i) change the wettability of reservoir rocks and plug wellbores 5,6 and (ii) block pipelines and foul equipment, 7 thereby reducing oil recovery and potentially halting oil production. On the other hand, the adsorption of asphaltenes to the oil−water interfaces significantly contributes to the stabilization of undesirable water-in-oil (W/O) emulsions, which pose severe corrosion problems to pipelines and refining facilities due to salts and fine solids associated with emulsified water droplets.…”

Asphaltene Aggregation and Assembly Behaviors in Organic Solvents with Water and Inhibitor

“…Naturally occurring asphaltenes in crude oil are known for causing considerable problems in the petroleum industry . Primary damages caused by asphaltenes include plugging wellbore, fouling transportation pipelines, producing a stabilized oil–water emulsion, and catalyst deactivation . Most of these problems can be attributed to the aggregation and precipitation behaviors of asphaltenes, which have been widely observed in experimental studies .…”