Managing fluid behavior, solids deposition to ensure optimum flow

Flow assurance and multiphase transport

Flow assurance is the ability to produce petroleum fluids economically from the reservoir to a production facility, over the life of a field in any environment. It is the key issue to economic development of energy reserves in deepwater and ultra-deepwater. Most of these deepwater assets have highly productive wells and huge petroleum reserves for recovery.

Any interruption of the production and processing of oil and gas streams from these assets has unfavorable economic consequences. The importance of flow assurance has prompted extensive work on improving the present technologies and developing new ones. The efforts have resulted in many technological advances.

The main flow assurance issues in deepwater operations are disturbed fluid behavior, such as foaming and emulsification, and deposition of solids, such as hydrates, waxes, asphaltenes, and scales. Any upset in fluid behavior or excessive solid deposition can cause operating problems and, in some severe cases, shut down the production.

To minimize these costly occurrences, issues of flow assurance must be addressed as early as possible, so that all management strategies can be incorporated in the project design phase and established before the first production. An extensive flow assurance review can also be conducted on the existing projects for potential process optimization or for incorporation of new technologies to enhance flow assurance.

Flow assurance can be effectively managed following the steps of prediction, prevention, monitoring, intervention, and improvement:

  • All flow assurance concerns expected through out the life of production must be identified. Preliminary laboratory studies as well as computer simulations based on the models created from past experience can assess the severity of associated problems.
  • Problems, once identified and quantified, must be controlled with implementation of an effective flow assurance program. The program should include preventive measures, monitoring plans and intervention strategies to address all types of problems that might occur.
  • A good flow assurance program should also have a continuous improvement component to maintain the program's effectiveness.

Flow assurance

The word "flow assurance" was first used by Deepstar, the producer-service company organization developed to deal with deepwater Gulf of Mexico technology challenges, who define it as "the ability to produce fluids economically from the reservoir to a production facility, over the life of a field in any environment". In other words, flow assurance means economically quantifying and managing the risks to production under both normal operating and transient conditions throughout the production lifetime.

Transient operations deal with startup, shut-in, and re-start of production. A successful flow assurance program must effectively control the solids deposition and the hydraulic performance of petroleum fluids through better understanding of fluid behavior and the use of innovative technologies (Holder et al - 1993).

The economic implications of flow assurance are tremendous. The cost of remediation to resume deepwater production can be millions of dollars. Considering the fact that most deepwater projects are justified by their huge energy reserves, the loss of revenue as a result of unexpected shutdown can easily exceed the high remediation cost.

Key issues

There are two key issues that must be considered to achieve flow assurance.

(1) Ensure that the passage of flow, such as through pipeline and tubing, is clear without any blockage. This involves mainly preventing and controlling the deposition of hydrates, waxes, asphaltenes, mineral scales, and solids (sands). The deposits can progressively block the fluid flow and fill up process vessels and tanks. Excessive deposits can interfere with the operation of valves and instrumentation.

(2) Control the transportation conditions and fluid behavior to achieve the most cost-effective way of operation. For example, excessive foaming and emulsification can increase the energy and chemical demand while reducing the production throughput and the sales value of the produced fluids. These, therefore, result in an increased cost of production and decreased revenues.

Fluid behavior

Managing fluid behavior and controlling solid deposition should be a cohesive task as they are closely related to each other. Fluid behavior in oilfield operations is very complex. Understanding fluid behavior requires the integration of fluid properties (such as phase behavior, rheological characteristic, physical properties, and water chemistry) and transportation phenomena (such as momentum/heat/mass transfer).

Fluid behavior directly impacts the operability of the fields and will vary over the production lifetime. Solid deposition is closely tied to the production chemistry. Therefore controlling production chemistry should be an integral part from project design to field operation. Many of these issues are not new to production operations because the petroleum fluids found offshore are not any different from those discovered inland. These issues are operational nuisances to the onshore operation, but become operational challenges to the offshore deepwater production.

Flow assurance is extremely important in deepwater production because of the hostile deepwater environment (low ambient temperature and limited accessibility) and the very high cost of failures. The challenge of transporting petroleum fluids from highly productive wells to the very limited number of floating or stationary production units has led to new hardware designs (such as subsea completions, long tiebacks, deep composite risers, flexible flowlines, and long umbilicals) and new production processes (such as multiphase transportation). The design of new developments traditionally has been focused on the structural and mechanical aspects of the systems. The recent booming of deepwater projects caused a paradigm shift to the much greater emphasis on flow assurance.

Field optimization

The conventional approaches to flow assurance use preventive programs and remedial operations. These are accomplished by heat management, chemical treatment, mechanical intervention, or a combination of techniques. Complex subsea systems are used to facilitate these techniques and the cost is considerably high.

Recently, there has been a paradigm shift towards field optimization when projects are still in the design phase. The design engineers are teamed with operation personnel along with service providers to address various aspects of the production. Therefore, the most cost-effective design for flow assurance can be integrated into the project. Retrofitting is costly and often times the optimum fix may not be feasible due to the constraints from the previous design. Considering flow assurance issues up front, in the project design phase, allows the optimum balance of operating expenditure (OPEX) and capital expenditure (CAPEX).

Because flow assurance is so important, it is not surprising that the energy industry is overly conservative when it comes to managing the flow. That conservative attitude results in an excessive operating cost that can be saved by substituting the conservatism with the confidence gained from technological advances.

For example, the risks to continuous production can be addressed by reliable projection with modeling. The effectiveness and the cost of preventive measures and remedial operations can be quantified by the use of many innovative technologies.

Although corrosion is one important issue in flow assurance, it has been well covered and documented in a large number of technical articles. Suffice it to say that corrosion of production equipment and deposition of corrosion products should not be overlooked when addressing flow assurance. Reservoir management, such as zone shutoff and water shutoff, is another important flow assurance issue that will not be addressed in this paper.

Flow assurance program

Flow assurance programs must effectively address the preceding issues through better understanding, effective management, and the use of improved and innovative technologies. This involves the ability to:

  • Predict the flow assurance problems up front in the project design phase
  • Design the effective management programs to achieve flow assurance
  • Monitor the effectiveness of flow assurance programs
  • Intervene and fix the problems should they occur.

The following are important control strategies for flow assurance:

  • Flow assurance must be considered and integrated into the project design. It is essential that the design team be fully represented from all working groups, including the production team to cover flow assurance issues.
  • Understanding the fluid behavior is a prerequisite for developing proper management strategies for flow assurance. Fluids should be properly characterized in the laboratory and simulation tools may be used for predicted properties.
  • Computer modeling techniques can be used to establish a safe operating window and to quantify the associated flow assurance risks.
  • Proper balance of OPEX and CAPEX must be considered in the design of cost-effective flow assurance programs.
  • All flow assurance programs, especially the chemical treatments, should be properly evaluated in a simulated system prior to deployment. Good correlation of laboratory test results with field performance data is a key element to increase the certainty of success and to minimize unnecessary field trials.
  • Proactive prevention measures are always better than reactive intervention fixes.
  • If the system is prone to solid deposition, a regular intervention program, such as periodic pigging in troubled pipelines, should be included with other preventive programs.
  • All flow assurance programs should be properly monitored to ensure they remain effective.
  • A review can be conducted on the existing production systems for potential process optimization to enhance flow assurance.
  • Innovative technologies were keys to the past successes in flow assurance and should be seriously considered and incorporated whenever applicable.

Author

Bob Fu received a BS in Chemical Engineering at National Tsing Hua Univ in Taiwan, and a PhD in Chemistry from Texas A&M University. He has 10 years experience in oilfield production chemical technologies with Nalco and Nalco/ Exxon. He currently works on the Flow Assurance team and the Corrosion team. Fu is a member of SPE and NACE International.

Editor's Note: The is a synopsis of a paper presented at the Deeptec 2000 conference, hosted by IIR limited, on 1/26-28/2000 in Aberdeen, UK.

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