Abstract
An algorithm has been evaluated to offer a fast and simple design of a primary crude dehydration system consisting of a low capacity wash tank train, installed as Early Production Facilities (EPF) to achieve a crude oil specification not greater than 0.5% Basic Sediment and Water (BS&W) with average water cut at inlet of 30%.
The estimation method allows to perform routine calculations to determine the crude oil residence time available for water separation inside the equipment, water droplet diameter and water cut within the oil pad at different heights using a short-circuit factor (F) always greater than 1. As a result, short-circuit factor of 1.4 is verified to be a good value to ensure separation of phases in low-capacity tanks. In addition, tank dimensions are given based on API-12F specification for a nominal capacity of 500 bbl. for the 12 ft diameter and 25 ft height. Moreover, the designer must take into account different tank internal configurations to improve its efficiency.
Different dehydration configuration can be arranged using 500 bbl. low-capacity wash tanks installed in series. One (01) train consists of two (02) wash tanks, and as the required processing fluid capacity is higher, similar trains in parallel must be added to increase the dehydration system throughput.
Retention time and settling theory were used and applied to the system described above and determining that the oil quality is related to the maximum size of water drops carried-out over O/W interface and being smaller as the oil reaches the outlet connection which is close to the top of the tank. These sizing techniques allowed the BS&W and water droplet diameter being estimated as a function of the oil-column height, residence time and temperature.
Different effective heights through the wash tank oil pad are checked at various operational temperatures which allow the dispersed water droplets to settle out from the oil with different velocities and, hence, being able to evaluate several water cuts (%BS&W). EPF conformed by two trains of 4 low-capacity tanks each; working in parallel, provided a processing capacity of 12000 BFPD to meet a desired 0.5 %BS&W specification for given oil and water physical properties. Initial investment, operational costs and implementation time were favourably reduced during the early production stage until the permanent facilities were installed, thereby avoiding sophisticated technologies that could result in expensive and late industrial project deployments.
