An Experimental Study of Carbon Dioxide Dissolution into a Light Crude Oil

Carbon dioxide (CO2) dissolution into a heavy oil has been extensively investigated as an effective solvent-based enhanced heavy oil recovery method for several decades. However, fewer attempts have been made to study CO2 dissolution into a light crude oil mainly because of possible occurrence of density-driven natural convection in the CO2- saturated light crude oil phase. The primary objective of this thesis is to experimentally study the phase behaviour of a Bakken light crude oil‒CO2 system and the complex mass-transfer process of CO2 dissolution into the light crude oil. First, a series of PVT tests were conducted to measure CO2 solubilities in the light crude oil, oil-swelling factors, and CO2-saturated light crude oil densities at different equilibrium pressures and the actual reservoir temperature of Tres = 56.0°C. Second, the onset pressure of the initial quick light-hydrocarbons (HCs) extraction was determined by applying the axisymmetric drop shape analysis (ADSA) technique. Third, five comprehensive CO2 diffusion tests in the light crude oil were performed at five different initial test pressures (Pi = 4.1, 5.2, 6.3, 7.7, and 9.0 MPa) and Tres = 56.0°C by applying the pressure decay method (PDM). Two CO2 diffusion tests were repeated and conducted at Pi = 6.2 and 7.9 MPa to determine the experimental repeatabilities. Fourth, three gas samples in the diffusion cell were collected at three different CO2 diffusion times (t = 8 min, 1 and 24 h; t = 20 min, 1 and 24 h) in two CO2 diffusion tests (Pi = 5.2 and 7.7 MPa), respectively. Then the so-called graphical method was applied to analyze the measured pressure versus time data and determine the CO2 diffusivities in the light crude oil under different initial test pressures. The equilibrium pressures required in the graphical method were predicted from the measured CO2 solubilities and oil-swelling factors, as well as the predicted HCs extraction by using the Peng‒Robinson equation of state (P‒R EOS). Finally, the pressure history matching (PHM) method was also employed to determine the CO2 effective diffusivity in a short period of each diffusion test, in comparison with the graphical method. The experimental results show that CO2 solubility in the crude oil was increased from 1.301 to 8.101 kmole/m3 in the pressure range of 2.46‒10.20 MPa. A quicker increase in CO2 solubility was found at a higher test pressure. The swelling factor of the light crude oil was increased from 1.05 to 1.62 in the pressure range of 2.01‒9.29 MPa. The measured density of CO2-saturated light crude oil was increased with the equilibrium pressure or CO2 concentration in the range of 2.01‒9.29 MPa or 1.150‒7.079 kmole/m3. The onset pressure of the initial quick light-HCs extraction was determined to be 5.1 MPa. It was found from the five diffusion tests that there were three distinct periods of CO2 dissolution into the light crude oil: the natural convection-dominated period (Period I), the transition period, and the molecular diffusion-dominated period (Period II). The determined CO2 effective diffusivities (Deff) in Period I and CO2 molecular diffusivities (D) in Period II from the graphical method were in the ranges of 0.28‒1.75×106 m2/s and 0.87‒1.94×109 m2/s at the initial test pressures of Pi = 4.1‒9.0 MPa, respectively. The CO2 effective diffusivity (Deff) was much reduced once CO2 reached the supercritical state because of possible formation of the CO2-enriched second liquid phase. However, the CO2 molecular diffusivity (D) was almost independent of the initial test pressure. A constant Z-factor used in the graphical method may lead to a relatively large error in the determination of Deff or D.

Name thesis:degree_name

Master of Applied Science (MASc)

Level thesis:degree_level

Master's

Discipline thesis:degree_discipline

Engineering - Petroleum Systems

Grantor dc:publisher

Faculty of Graduate Studies and Research, University of Regina

Year dc:date.issued

2016

Author dc:creator

• Du, Fengshuang

Advisor dc:contributor.advisor

• Gu, Yongan (Peter)

Committee members dc:contributor.committeemember

• Zeng, Fanhua
• Jin, Yee-Chung

Language dc:language.iso

en

DOI

https://doi.org/10.82465/4491

OAI identifier oai:identifier

oai:uregina.scholaris.ca:10294/7621