Summary
Several options (prevention and mitigation) for controlling wax deposition
inside production wells operating in an Arctic environment are evaluated.
Preventive measures include improved heat retention using vacuum-insulated
tubing (VIT), active heating using electrical heating elements inside the well
tubing, jet pumps using water as a power fluid, and downhole injection of
paraffin inhibitors. In the field cases studied, none of these measures is 100%
effective in preventing wax deposition. Mitigation methods include mechanical
scraping using wireline tools and hot oiling. Again, the mitigation methods are
not completely effective, and even a method with 99% effectiveness can damage
the well seriously after several jobs. Cost of the wax jobs and the production
downtime during the jobs are added losses. The current study showed that an
optimal combination of appropriate prevention and mitigation measures is needed
for adequate wax control and maximization of economic returns. Extensive field
data (production rates and flowing wellhead temperatures) were collected to
develop and tune the well thermal model. Laboratory flow-loop data were
obtained for wax-deposition scaleup and predictions. The wax-deposition model
was tuned to match the field data, including a production-rate decline
attributable to wax deposition. An economic model was developed to evaluate the
benefits of VIT with appropriate polyurethane coupling insulation in achieving
higher production rates and lower production downtime during the wax jobs
versus the capital expense of the VIT segment. The results showed that it is
highly beneficial to run the VIT, at least in the permafrost layer. Additional
length of VIT below the permafrost layer has limited benefits.
Introduction
Vacuum-insulated double-walled tubular products have been used for a number
of years to provide downhole-temperature management. Production of a paraffinic
crude oil in an Arctic environment is a major flow-assurance challenge.
Significant productivity losses may occur because of flow restrictions caused
by wax deposition inside the production tubing. This paper presents a
comprehensive study of this phenomenon, including key field data used for the
evaluation of VIT.
Crude oil is a complex mixture of hydrocarbons that contains different
functional types such as paraffins, aromatics, naphthenes, resins, and
asphaltenes. Among these types of hydrocarbons, high-molecular-weight paraffins
(i.e., waxes) and asphaltenes are responsible for the various problems
encountered during transportation and processing of these complex fluids.
Paraffins, a broad fraction of crude oil, are straight-chain normal alkanes
with carbon numbers ranging from 5 to 100 or even higher. One of the main
features of high-molecular-weight paraffins is their low solubility in most of
the paraffin-, aromatic-, naphthene-, and other oil-based solvents at room
temperature.
At reservoir temperatures (>50 to 70°C), the solubility of these
paraffinic compounds is sufficiently high to keep these molecules fully
dissolved in the mixture. Wax molecules start precipitating out of the liquid
phase below a certain temperature known as the wax-appearance temperature
(WAT). Below the WAT of a crude oil, waxes can start plating out on cold
surfaces of tubulars, flowlines, surface equipment, or pipelines. When the
thickness of deposited wax increases inside crude-oil production tubing (as
anticipated in the Arctic environment), crude-oil production declines rapidly
because of the flow restriction. In the worst cases, a complete wax plug forms
in the production tubing, and production must be stopped to remove the
plugging.
In the Arctic wells discussed in this paper, a combination of slickline
scraping and hot-oil treatments is commonly used to remove wax from production
tubing. Wax deposition can be controlled by thermal insulation, injection of
wax inhibitor, or both. An artificial-lift technique that uses jet pumps to
hydrolift crude oil using water as a power fluid has also been used to prevent
wax deposition inside production tubing. Water helps to decrease the
wax-deposition rate in two ways: by increasing the thermal mass, resulting in a
higher wellhead temperature (the heat capacity of water is twice that of oil),
and by making the production tubing water-wet. Hydraulic lift was considered
for artificial lift and wax control, but was rejected because of higher capital
and operating costs.
Several attempts have been made to develop internal-surface-coating
materials that are less adhesive with paraffin molecules than conventional pipe
surfaces. Laboratory experiments indicate that the internal-surface coatings
are effective to some extent in reducing the initial rate of wax deposition;
however, once wax deposition occurs, the pipe wall is coated with an incipient
wax deposit, and the interaction between the coated surface and paraffin
molecules becomes irrelevant. Coatings also may be rejected because of concerns
about possible damage to the coatings from regular wireline pressure and
temperature surveys.
VITs have been used over the years to provide downhole temperature
management. VITs have been used successfully for thermal isolation to prevent
the transfer of heat from well fluids, thus preventing the problems associated
with flow-assurance issues such as paraffin deposition and hydrate plugging in
deepwater Gulf of Mexico fields, and annular-pressure buildup (Azzola et al.
2004a), especially in the case of trapped or sealed annular spaces that cannot
be vented. VIT has been demonstrated to be a technical success (Purdy and
Cheyne 1991) as a passive thermal solution to the problem of paraffin
deposition in well tubing in the Norman Wells field.
For waxy-crude-oil production in an Arctic environment, VIT application is
considered to be a possible solution for wax control. The first step in
evaluating a VIT application is to develop a thermohydraulic model that can
accurately predict the heat transfer and thermal profile in the existing wells
in the area. The modeling work was done with OLGA, a commercial transient
multiphase-flow-simulator, which is capable of computing temperatures of the
flowing streams in the production tubing by incorporating heat transfer into
the surrounding formation through different casing and annulus materials. The
second step is to develop a wax-deposition model that can be coupled with the
thermohydraulic model to predict the wax-deposition rate inside the production
tubing.
© 2007. Society of Petroleum Engineers
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History
- Original manuscript received:
29 May 2006
- Revised manuscript received:
18 December 2006
- Manuscript approved:
13 March 2007
- Version of record:
20 June 2007
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