Improvement of the Calculation of Naphtha Shrinkage and Evaporation Effects in the Allocation Process of a Heavy Oil Blend
Authors: Paz Carlos Jose , Peña Rojas Carlos Ferney , Prada Mejias Jorge Andres , Laiton Corredor Leonardo Andres , Sena De Lima Jesley , Claib Meinhardt Amin Adolfo , Lozano Ruiz Henry , Vaquez Garcia Claudia , Tello Bahamon Cristhian , Perez De La Cruz Francisco , Ramirez Olarte Hector Eric , Soberanes Hernandez Roberto Eduardo 
Published: April 2022

To overcome the challenge of transporting heavy, 8.5 API, oil through a long pipeline, the highly viscous fluid is mixed with naphtha (74 API). This increases the API density of the mixture and improves fluid flow to the endpoint. The method involves a complex calculation, and it requires reallocating the production for all networks that handle naphtha. Because the calculation is prone to failure when performed manually, an automated solution was sought.

An interactive calculation was designed in which all volumetric and laboratory data are considered to estimate the shrinkage and evaporation index for the heavy oil produced by different reservoirs. The process starts by obtaining a shrinkage factor based on the crude quality, designed specifically for the heavy oil. This triggers a recursive calculation process to provide the mass, viscosity, and API of the heavy oil blend at the collection station. Those calculations are added to the allocation process by an iterative prorating task to obtain several intermediate calculated variables, added to the network that is being allocated.

The most accurate and effective way to perform those estimations is to calculate the shrinkage of the naphtha into heavy oil, calculate naphtha evaporation during the transport, and, finally, calculate the final blend viscosity and API based on naphtha volume contraction using several iterations. The iterations are stopped when the difference between measured viscosity and calculated viscosity is below the 0.01 threshold. This workflow directly affects production allocation because it adjusts the blend of the heavy oil. Besides having an accurate volumetric distribution, the operational team can realize how much naphtha volume has been volatilized and how much has been sent into the taxable oil. Subsequently, the team can identify how much oil has been produced by each well. In addition, the intermediate calculation steps offer a reference of how much volume of naphtha is needed to ease the heavy oil mobility inside the pipeline and the expected quality when this blend is delivered at the final point.

The benefits of systematizing the workflow of naphtha shrinkage and evaporation can be measured by production cost, time, and quality of the daily activities. Previously, the time consumed in performing all activities was greater than 1 hour and required a dedicated operation engineer in addition to the time to set up the calculation spreadsheets. With automated calculations, the process takes less than 10 minutes and does not require human intervention. In addition, the automated calculations provide data security and accurate results by minimizing data handling errors. This solution provides high data traceability and demonstrates the PDMS capability to include external variables in the allocation process by additional programming, with high confidence in the results.