07121-DW1603B

Goal
This research will introduce a new technology for characterizing hydrate plugs in subsea equipment and criteria for selecting the most effective dissociation technique. Knowledge of typical hydrate plug character, including permeability and porosity, will be key to the evaluation of dissociation technique feasibility.

Performers
University of Tulsa, Tulsa, OK 74104-3189
BP America, Inc., Houston, TX 77079

Background
In deepwater wells, thermodynamic conditions are favorable for the formation of hydrates, which tend to agglomerate and eventually plug pipelines. One of the offshore industry’s major concerns is how to cost effectively eliminate hydrate plugs from pipelines after they form. While a number of case histories have been recorded related to the formation and recovery of hydrate plugs, very few have been quantified to prevent future plugging. It is very important to be able to predict the location and timing of plug formation to prevent emergency situations from arising.

Different remediation strategies, such as melting, depressurization, and the application of inhibitors, may be implemented, little is known about the properties of the plugs themselves, in particular, their porosity and effective permeability to gas or liquids. Therefore, little quantitative information is known about the efficiency of dissociation methods as a function of these properties and environmental conditions. The aim of this project is to bridge the knowledge gap between plug characterization and dissociation, leading to the selection of the most effective plug dissociation method for different plug scenarios.

Different dissociation strategies, such as depressurization, wall heating, and thermodynamic inhibitors will be evaluated, and a comparison of their efficiency will be provided based on characterization parameters. Databases for dissociation will be compared with existing models for wall heating. An engineer’s estimation tool will be developed to help facilitate the design of appropriate dissociation methods for different operational conditions.

The scope of work for this study includes modification of the University of Tulsa’s high pressure Flow Assurance Loop (FAL) to generate solid hydrate plugs and evaluation of the efficiency of hydrate dissociation strategies. To do this, after hydrate plugs are formed under different scenarios, plug characteristics, such as porosity and permeability, plug length, and pressure drop across the plug will be determined. The FAL consists of a 3” pipe flow loop mounted on an 80-ft long tilt table. The flow path is 160 feet long and fluids can be set in motion by a multiphase pump or by the rocking motion of the flow loop deck.

Solid hydrate plugs will be formed in the high pressure flow loop and the length and density of each plug will be obtained by using a scanning gamma densitometer to obtain porosity values. A fluid handling system will be utilized for displacing the liquids out of the system by injecting gas and pressure drop data will be acquired. Permeability values will be calculated from the pressure drop data and plug length measurements. Finally, a variety of dissociation strategies will be applied to the plug: depressurization, wall heating, and inhibitor injection (MEG or Methanol). The dissociation times will be assessed and compared.

Potential Impacts
This research will provide a tool that a production engineer faced with a hydrate plugging problem can use for selecting the most appropriate dissociation strategy. This will be a step change improvement, as models for determining the best dissociation strategies and expected dissociation time are not available for plugs formed in oil producing environments. Application of this tool will lead to safer and more profitable operation of deepwater production facilities and subsea equipment.

Accomplishments
The design of the facility was completed and the facility was constructed.

Eighteen plugging experiments were conducted with a model oil of 19 cp viscosity. Preliminary permeability calculations indicated that plugs may have permeabilities ranging from zero to 100 D (equivalent to a gravel bed). At 50% liquid loading the plugging repeatability was not very good. Better repeatability was achieved at liquid loadings of 75%, which now constitutes the base case. At 50% liquid loading and less than 50% water cut the hydrates circulate as a slurry under stratified flow conditions and no bridging of the pipe takes place. A plug impermeable to gas was generated with fresh water at 75% liquid loading and 50% water cut. The plug sustained differential pressures up to 160 psi without letting gas flow through. Tests with salt water appeared more permeable and less severe. This is as expected, due to the unconverted water remaining in the hydrates and the lower driving force for hydrate formation.

Plug characterization tests were continued where a new procedure allowing draining of the plugs from free liquids and improved permeability measurements with the 19 cp lube oil. Plugs formed when the slug of hydrate slurry stopped flowing due to wall friction. Plug permeability was on the order of 10s to 100s of Darcys if calculated using Darcy’s law, even though non-permeable plugs were formed, mainly with fresh water. Preliminary results indicated that the permeability of the plug decreases as gas is circulated through it.

Eighteen experiments were conducted to investigate plug formation and dissociation with the facility in the low spot configuration, simulating the leaky valve scenario. The effect of gas leak rate and salinity (fresh, 3.5% and 7.0% wt.) were tested and the data are currently being processed. The observed mechanism for plug formation did not change for the parameters tested and plugs are fairly reproducible with respect to porosity. The plug density varies from 0.28 to 0.36 g/cc. A permeability of around 10 D was observed for 100 psi differential pressure across the plug. As water saturated gas continued to flow through the plug the permeability continued to decrease. Should flow continue long enough, the plug would become nearly impermeable. It was also observed that the plug permeability was very sensitive to spikes in flow rate or pressure; several times the plugs collapsed with 100 psi differential pressure across them.

The simulated results for the heating and depressurization tests were compared to the experimental results. Plugs were found to melt radially and the predicted trends were in good agreement with the experimental data. The experimental results show that the plugs did not dissociate uniformly using two sided depressurization. As a result, they cannot be compared using the CSMPlug simulator. The dissociation model was modified to account for pressure profile input and modeling of the depressurization experiments was conducted. Modeling with variable temperature and pressure inputs yields a better match between simulation results and experimental data than modeling without variable inputs.

Current Status
Four major tasks were envisioned for the proposed project: The final report is available below under "Additional Information".

Facility Modifications. Modifications to the existing flow loop facilities were needed to complete the proposed experimental program. This task is complete.

Formation of plugs and measurements of plug characteristics. The initial effort was dedicated to studying the feasibility of creating compact and repeatable hydrate plugs under two different scenarios: (1) water and natural gas and (2) light mineral oil and natural gas. A sensitivity analysis was performed to study the feasibility of obtaining accurate permeability calculations from pressure drop data and accurate porosity calculations from density measurements utilizing the existing scanning gamma densitometer. Reproducible plugs were made and characterized. Plugs formed while pumping were impermeable. Plugs formed in a low spot configuration were also reproducible and the permeability was found to continually decrease as water saturated gas flowed. If plow continued long enough the plug became nearly impermeable to gas. These experiments are complete.

Evaluation of dissociation methods. Depressurization, methanol injection or MEG injection, and wall heating were carried out and their impact on plug dissociation measured. Plugs were formed and dissociated via heating, depressurization and MEG. These experiments are complete and the results are still under evaluation.

Data analysis and processing. This element of the research effort will include development of the engineer’s tool for dissociation strategies, completion of the final technical report, and fulfillment of the student researcher’s thesis writing and defense. Modeling with variable temperature and pressure inputs yields a better match between simulation results and experimental data than modeling without variable inputs. We were not able to capture inhibitor dissociations using equilibrium data input, so a first generation Excel spreadsheet inhibitor model was developed. This effort is ongoing.

Project Start: September 22, 2008
Project End: September 21, 2010

DOE Contribution: $120,000
Performer Contribution: $61,719

Contact Information:
RPSEA – Jim Chitwood (jchitwood@rpsea.org or 713-372-2820)
NETL - Jay Jikich (Sinisha.Jikich@netl.doe.gov or 304-285-4320)
Performer Company – Dr. Michael Volk, Jr. (michael-volk@utulsa.edu or 918-631-5127)

Additional Information:

Final Project Report [PDF-5.20MB] - September, 2010