Crude Oil Process Enhancement and Water Conservation Through Industrial Revolution Initiatives
Authors: Alhajri Nasser A. , White Ramsey J. , Lopez Andreu Miguel 
Published: March 2019

One of the primary functions of Saudi Aramco Gas-Oil Separation Plants (also known as GOSPs) is to separate emulsified water from the crude. The water is typically highly concentrated with salt, so crude desalting is required to meet the standard quality specifications. GOSPs are typically designed with standard Proportional-Integral-Derivative (PID) controllers to control demulsifier and wash water flow for injection into wet crude. Demulsifier and wash water injection rates are normally left to operator judgement. The challenge with manual adjustment of flowrate is the high risk of overdosing or underdosing as there are several variables that impact the required demulsifier and wash water rates. Overdosing will result in wastage of demulsifier and wash water and higher operating expenditures. Underdosing may lead to operational upsets and potentially off-spec crude production.

To overcome this challenge, innovative schemes (Smart Demulsifier Control & Wash Water Ratio Control) have been developed in-house. Smart Demulsifier Control optimizes the separation efficiency (or percentage of total produced water separated) of an upstream High Pressure Production Trap (HPPT or 3-Phase Separator) based on a dynamic target by adjusting the demulsifier injection rate and concentration in the wet crude. Simultaneously, wash water ratio control ensures that an adequate wash water rate is injected to satisfy salt-in-crude specifications. These control schemes eliminate the need for operators to determine the required dosage rate, thereby avoiding both overdosing and underdosing of demulsifier and wash water.

The Smart Demulsifier Control (SDC) scheme controls demulsifier injection using two control layers. The first layer controls the Concentration of the Demulsifier in the Wet Crude so that demulsifier flow is automatically adjusted based on the Production Rate to achieve the set point concentration determined by the second layer of control. The second layer adjusts the demulsifier concentration to control the Separation Efficiency of the HPPT, or the amount of water separated in the HPPT vs. the Dehydrator, to achieve the Target Separation Efficiency Set Point determined by a site specific process model. In case of a dehydrator upset, another PID controller with more aggressive tuning will override the HPPT Separation PID Controller to set the required demulsifier concentration to mitigate the upset.

Wash water ratio control scheme controls the flow of wash water to ensure that the salt-in-crude specification is met. A site specific target ratio is determined through a salt mass balance.

These innovative controls have reduced desalting train upsets by 78% as the process related upsets are practically eliminated. This is achieved while optimizing the demulsifier dosage where 20-40% of demulsifier dosage reduction was realized, especially during the winter season. Moreover, savings of 20% wash water have been achieved throughout the utilization of these self-calculated and smart controls that were developed in-house with minimal costs.