Study Compares Fluid-Tracking-Modeling Approaches for Mature Onshore Field
Authors: Carpenter Chris 
Published: October 2021

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 202747, “Fluid-Tracking Modeling for Condensate/Oil Production and Gas Use Allocation: An Abu Dhabi Onshore Example,” by Yun Wang and Gary Jerauld, SPE, BP, and Yatindra Bhushan, SPE, ADNOC, et al., prepared for the 2020 Abu Dhabi International Petroleum Exhibition and Conference, Abu Dhabi, held virtually 9–12 November. The paper has not been peer reviewed.
A reservoir in a giant field onshore Abu Dhabi has been producing for 6 decades. The reservoir was already saturated at the time of production commencement, with a large oil rim and a gas cap. This paper presents a comprehensive comparison of two modeling-based approaches of fluid tracking for condensate allocation and gas usage: a tracer modeling option in a commercial reservoir simulator and a full-component fluid-tracking approach.

Introduction
Examples of benchmarking fluid-tracking options against an independent fluid-tracking approach are rare in the literature. The goal of this paper, therefore, is to document a comprehensive and detailed comparison between the fluid-tracking option (TRACK) in NEXUS (a commercial reservoir simulator used by companies of the coauthors of this paper) and a full-component fluid-tracking (FCFT) approach. This work is motivated by the commercial arrangement of a concession covering an onshore field in Abu Dhabi. Because of different equity entitlements among the shareholders for the oil rim and gas cap per the concession commercial terms, a need exists to allocate the condensate vs. oil-rim oil production and the injected lean-gas usage.

FCFT
The idea of FCFT is relatively straightforward. Before discussing the approach, it is important to note that the focus of this paper is to compare the modeling of different tracking approaches. It is assumed that a fit-for-purpose fully compositional reservoir simulation model already exists. For fluid-tracking modeling with a full-field model (FFM), this means that the compositional reservoir model has already been history matched properly at both field and individual well levels and that no additional reconciliation is required before the hydrocarbon liquid and vapor streams are split into tracked substreams. In this paper, FCFT is completed on the field level for the comparison with the TRACK option in NEXUS. One could easily extend the field level tracking to either regional well-group levels or individual well levels.
In the case of the onshore reservoir, lean-gas injection has been active during much of the producing history. In future development schemes under consideration, lean-gas injection, carbon dioxide (CO2) injection, and gas lift are all possible scenarios, raising the question of how to treat injected gas components within the FCFT framework. Different approaches exist to handle the injected gas components. One may treat the injected gas components as either the gas-cap components, the oil-rim components, or entirely new components. Lean gas injected in the onshore field example is actually a dry gas, according to the 11-component equation-of-state (EOS) prediction. Thus, the lean gas can be reasonably modeled with only one new component as long as that component replicates the volumetric behavior of the injected lean gas.