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21

Systems Analysis

In this textbook the petroleum production system, shown schematically in Fig. 1-6, is decomposed into individual parts, each representing a pressure difference in series. The system and the corresponding pressure drops describe the path of a petroleum fluid beginning in the reservoir, through the completion zone (which can be damaged, equipped with gravel packs and screens or perforations), up the tubing, through the wellhead, and finally along the horizontal lines into separators or other surface facilities. The individual components of the system are interdependent. At any point along the system the rate that the upstream pressure drop may deliver must coincide with the rate that the downstream pressure drop may allow. This concept is demonstrated amply in Chapter 8, where appropriate flow performance relationships (IPR-describing what the reservoir can deliver) are combined with the well vertical lift performance for a given wellhead pressure (as shown inchapter 7). The combination of the two is the expected production rate at exactly the expected flowing bottomhole pressure. Thus, this rate, resulting from the pressure difference between the reservoir and bottomhole pressures, is equal to the flow rate that would result in a pressure difference in the tubing equal to the determined flowing bottomhole pressure minus the given wellhead pressure. Well deliverability is one use of system analysis. All individual pressure differences depend on well-understood variables but whose values in certain cases may be unknown. By deducing the corresponding pressure difference, the value of an unknown or group of variables may be determined, and remedial action may be taken. The idea of analyzing individual components of a production system becomes possible because pressure measurements-either direct, such as at the surface, across a choke, or downhole, or indirect (e.g., determining average reservoir pressure from a buildup test)-can be readily available.