Investigation of Permeability Impairment Due to Asphaltene Precipitation During Gas Injection EOR in a Major GoM Field
Authors: Fassihi M. R. , Turek E. , Honarpour M. M. , Fyfe R. 
Published: August 2020

As part of studying miscible GI in Deep Water GoM, four key areas were identified that could pose risk or uncertainty to value delivery. These were: reservoir containment, injectivity, reservoir sweep and reservoir optimization. Top seal integrity, column capacity, fault seal risk and asphaltene formation risk were identified as key drivers underpinning containment risk. As pointed out by Abouie et al. (2018), deposited asphaltene on the rock causes wettability alteration. This, in turn, significantly changes the displacement dynamics in miscible gas floods by lowering the gas mobility, increasing the mixing length, and reducing the displacement and sweep efficiency. The industry has not conducted many experiments to quantify the impact of asphaltenes on reservoir and well performance under gas injection (GI) conditions. This paper discusses a novel laboratory testing for evaluating the asphaltene impact in a major oil field in GoM. The goals of the study were to: a) Define the Asphaltene Precipitation Envelope (APE) using blends of reservoir fluid and injection gas, and b) Measure permeability reduction due to asphaltene precipitation in a core under gas injection.

To properly analyze GI impact, a full suite of fluid characterization were conducted including restoring oil samples, compositional analysis, constant composition expansion and differential vaporization. The miscibility conditions were defined through slim tube displacement tests. Then, gas solubility-swelling tests were conducted. This was followed by Asphaltene Onset Pressure (AOP) testing. The permeability impairment experiments were carried out in a long core. A unique procedure was developed to estimate the impact of asphaltene deposition on core permeability. Using a fully saturated core with the live oil and injection gas mixture, several depletion tests were repeated starting from a pressure above the AOP (9000 psi) down to a pressure just above the bubble point followed by reloading the core with the same mixture at 9000 psi. These tests were meant to mimic continuous flow of oil along the path of injected gas and thereby observe the accumulation of asphaltene on the rock surface.

The test results indicated that during this cyclic asphaltene deposition process, the core permeability to the live mixture decreased in the first few cycles but appeared to stabilize after cycle 5. The deposited asphaltenes were analyzed further through Environmental Scanning Electron Miscroscopy (ESEM), and their deposition was confirmed by mass balance before and after the tests. Finally, a relationship was established between permeability reduction and asphaltene precipitation. The results from the asphaltene deposition experiment show that for the sample, fluids, and conditions used, permeability is impaired as asphaltene flocculates and begins to coat the grain surfaces. This impairment reaches a plateau and subsequent permeability measurements indicate that stability is being established at approximately 40 percent of the initial permeability. Distribution of asphaltene along the core was measured at the end by segmenting the core and conducting solvent extraction.

A novel systematic approach was developed for characterizing the permeability impairment due to asphaltene deposition during a gas injection EOR process. Numerical modeling of these test results and using this model to forecast the magnitude of the permeability impairment to be expected in a reservoir setting during miscible gas injection is our recommendation.