Technology Today Series articles are general, descriptive representations that summarize the state of the art in an area of technology by describing recent developments for readers who are not specialists in the topics discussed. Written by individuals recognized as experts in the area, these articles provide key references to more definitive work and present specific details only to illustrate the technology. Purpose: to inform the general readership of recent advances in various areas of petroleum engineering.
It seems that all the large, easily accessible reservoirs have been developed. The prospects we encounter now usually are smaller and, often, in more-remote areas. New facilities must be placed in more-challenging locations- on offshore structures, in remote areas, subsea, or even downhole in the well. Lowering facility capital costs can make marginal fields economical any where. Because of ever-increasing geographical challenges and economic pressures, development of innovative and lower-cost facilities has advanced rapidly in recent years.
In early efforts to reduce facility costs, attention focused first on reducing the cost and size of key pieces of equipment, in the belief that smaller, lower-cost equipment was the key to reducing overall facility costs. Liquid/gas and oil/water separators were a substantial portion of the equipment cost, size, and weight; therefore, these were targeted first. Most separator manufacturers rushed to develop more compact separators. The use of this new equipment did reduce costs, but did not result in the anticipated magnitude of savings. In search of higher stakes, and in response to more challenging environments such as deep water, attention then turned to how these new devices could be used as components of innovative production systems and used to debottleneck facilities that were already limited. The economic effect was greater, but still left room for improvement. More recently, it has been recognized that facility costs are driven strongly by the field conceptual development plan, which ultimately has the most influence over the finalfacility costs. By taking advantage of existing infrastructure when available, capital and operating costs can be lowered dramatically.
Traditionally, separation of oil, gas, and water has been achieved by gravity segregation in large, heavy vessels on the surface. Recent efforts to reduce cost, size, and weight have resulted in separators that are a fraction of the size and even in some that may be placed downhole in the well or on the sea floor. This change was accomplished by adding some form of cyclonic flow, which centrifuges the fluids and enhances gravity separation beyond 1 g. Forliquid/gas separation, the spiral flow is induced passively by either internal vanes or by a tangential entry angle. Many separator manufacturers have developed these more compact designs. An overview of new separation equipment is given in Ref. 1.
Hydrocyclones and centrifuges have been used increasingly for oil/water separation. Hydrocyclones have no internal vanes or moving parts. The tangential inlet and converging nozzle promote cyclonic rotation. Because of their small diameter, they have been adapted for downhole separation as well. However, the water cut required for successful hydrocyclone separation is relatively high. A rule of thumb is that the stream must be at least 50 to 75%water. Also, limits exist for processing heavy oil in that the oil density is close to that of water and oil viscosity is greater than that of water. Centrifuges for oil/water separation on the surface and rotating gas separators for downhole electric submersible pumps both have rotating vanes that spin the fluids. With this equipment, the tradeoff between smaller size and increased maintenance costs associated with rotating equipment must be considered. However, they have a wider operating range than some of the alternative compact separators.
Because of the large size, weight, and inspection requirements for vessels, as well as subsea and downhole space limitations, separators constructed of pipe instead of large-diameter vessels have been developed. Some advantages are that they can easily tie into existing piping, are typically much lower cost, can be replaced or inspected easily, are more suitable for downhole geometry, and are better for applications having high internal or external pressures.
When the purpose of separation is only for metering, and when accuracies within 5 to 10% are acceptable, multiphase meters, which require no separation, can be used. These meters are much smaller and lighter than test separators and can perform more well tests in a given time period because they require lessstabilization time compared with a test separator.
In addition to separation technology, recent advances have been made in developing smaller gas compressors, gas dehydration units, and remote power-generation turbines that use wellhead gas.
New developments in remote locations have driven changes in the way that facilities are configured. Long distances from wells to a central processing facility/platform have spurred the development of subsea test separators and multiphase meters to avoid the high cost of providing a separate flowline for well testing. Downhole or local separation can be used to reduce the energy, flowlines, and surface processing equipment that is used to handle water or stranded gas, for which there is no monetary value.
Increasingly, new discoveries are produced through existing facilities. Often, this method means that some form of debottlenecking must be used to accommodate that extra production. Alternatively, debottlenecking opportunities may exist because water or gas production has increased over time because of depletion, waterflood breakthrough, or coning. The first step in debottlenecking projects should be system modeling. This step will show the bottlenecks clearly as well as the effect of the proposed facility changes. Ideally, this model should combine reservoir, gathering system, and facility-limit models because all of these are codependent. This modeling promotes a common vision of complex systems, identifies other system responses that may alter the expected benefits, and quantifies the benefits of the proposed project.