Real Time Production Optimization of Offshore Oil and Gas Production Systems: A Technology Survey

Abstract

Abstract The information flow used for optimization of an offshore oil production plant is described. The elements in this description include data acquisition, data storage, processing facility model updating, well model updating, reservoir model updating, production planning, reservoir planning, and strategic planning. Methods for well allocation, gas lift and gas/water injection optimization and updating of the models are reviewed in relationship with the information flow described. Challenges of real time optimization are discussed. Introduction In the daily operation of an oil and gas production system, or plant, a lot of decisions have to be taken that affects the volumes produced and the cost of production. These decisions are taken at different levels in the organization, but eventually they will reach the physical plant. For such plants this is related to the choke/valve openings, compressor, and pump settings at every instance of time. These are the control elements. In the efforts towards better performance of the plant, the question to be answered is therefore how to decide how to operate the control elements. In the process of finding good control settings, information about the plant is used. This information may be the physical properties such as pipe diameters and lengths, or it may be measurements from the plants. The environment in which the production is performed is under constant change, and this will affect the quality of the control settings being used. If the cooling capacity of the plant is an operational bottleneck at a given moment, this may not longer be the case if the sea water temperature drops. Incidents at the plant may also affect the quality of the control settings; partial shut down of the plant due to maintenance will most likely affect the bottlenecks. Real Time Optimization (RTO) is a method for complete or partial automation of the process of finding good (optimal) control settings. By continuously collecting data from the plant, the data are analyzed and optimal control settings are found. These settings are then either implemented directly in the plant or they get presented to an operator. If settings get implemented directly, the RTO is said to be in a closed loop. The main aim of RTO is to improve utilization of the capacity of a production plant to get higher throughput. The idea is to operate the plant, at every instant of time, as near optimum as possible.[1] To achieve this, a model of the plant is optimized giving optimal control settings. The model is continuously being updated by plant measurements to better fit the actual input-output behavior of the processing facilities, wells/network, and reservoir. A general RTO system used in for example downstream petrochemical plants consists of the following four components [2] as shown in Figure 1: RTO was defined by Saputelli et al. [3] as "a process of measure-calculate-control cycles at a frequency, which maintains the system's optimal operating conditions within the time-constant constraints of the system". Even if the definition was written with oil and gas production systems in the mind, it is general in the sense that it is not restrictive to some specific type of plant or method, and it can be related to Figure 1. Recently, SPE started a technical interest group that focuses on RTO on oil and gas production systems. The driver behind this development is, as in any industry, the demand for more profitable plants. This survey will help to organize previous work related to RTO. The focus will be on offshore oil and gas production systems; however relevant references from other industries are also included.