Study of Liquid–Liquid and Liquid–Liquid–Vapor Equilibria for Crude Oil Mixtures with Carbon Dioxide and Methane Using Short-Wave Infrared Imaging: Experimental and Thermodynamic Modeling

Abstract

In this work, phase behavior was analyzed for recombined mixtures of a Brazilian pre-salt crude oil with carbon dioxide and methane. This oil was sampled at stock tank conditions during a well-test, and mixtures with gas were prepared with a gas molar composition ranging from 20.0 to 86.0 mol %. Their phase behavior was investigated from 293.15 to 378.15 K and up to 100.0 MPa. Direct phase transition detections were achieved by using a full visibility PVT cell, coupled with a short-wave infrared camera. At this wavelength, crude oil has a lower absorbance, and it has been noted that macroscopic phase transitions can be easily detected by visual inspection. For the pseudo-binary system crude oil + carbon dioxide systems, liquid−liquid phase transitions were observed when the gas content is higher than 70.0 mol %. In addition, at the higher carbon dioxide composition, an asphaltic phase was formed at high pressures together with liquid−liquid phases. Similarly, crude oil and methane systems presented a liquid−liquid immiscibility region at methane composition above 60.0 mol % in all temperature domain studied. The Peng−Robinson equation of state was used for modeling purposes, and liquid−liquid and liquid−liquid−vapor transitions were qualitatively described. Additionally, it was observed that liquid−liquid equilibria behavior was highly dependent on the crude oil heavy fraction immiscibility because of the increasing system asymmetry by increasing mixture gas content.

Measurement of Fluid Phase Equilibria for High Gas Ratio Mixtures of Carbon Dioxide, Methane, and Brazilian Presalt Crude Oil

“…This same molar gas composition was tested in our past works, using pure methane mixed with the same crude oil 14 or CO 2 with a comparable presalt oil. 10 Similarly, no asphaltene precipitation or phase separation was observed. It is important to mention that CO 2 is considered to have less influence on asphaltene precipitation than methane, which is related to their higher solubility parameter.…”

“…As a possible analysis, we have used NIR imaging for total visualization of a similar presalt fluid, revealing a more extended multiphasic region with similar dispersion characteristics. 10 More precisely, LLE has been found in presalt reservoir fluids at high temperatures and pressures, presenting low density differences that make their separation difficult. 10,11 Additionally, the LLE boundaries determination can be limited .…”

“…10 More precisely, LLE has been found in presalt reservoir fluids at high temperatures and pressures, presenting low density differences that make their separation difficult. 10,11 Additionally, the LLE boundaries determination can be limited . Particles count and SDS signal for RC3 (75.8 mol % gas, 37.6 mol % CO 2 content in gas), RC4 (78.0 mol % gas, 26.6 mol % CO 2 content in gas), RC5 (78.9 mol % gas, 22.0 mol % CO 2 content in gas), and RC6 (79.8 mol % gas, 17.9 mol % CO 2 content in gas) systems during the HPM test.…”

“…34 Recently, we have investigated complex phase behaviors obtained for Brazilian presalt crude oil mixtures with methane or CO 2 . 10 These systems present an uncommon high temperature LLVE and LL asphalt equilibria when recombined at a high gas ratio. Similar behavior is observed when this crude oil is mixed with the separate gases.…”

“…For these complex reservoirs, diverse multiphasic conditions can be encountered during crude oil production. For instance, liquid–vapor equilibria (LVE), liquid–liquid equilibria (LLE), liquid–liquid–vapor equilibria (LLVE), and solid–liquid equilibria (LSE) have been reported and studied. − To properly assess these phase transitions, recent research has been done regarding analytical techniques for complex fluid evaluation, , along with experimental procedures for complex phase transition detection. − Because of the complex fluid phase behavior of these oils, experimental evaluation requires different approaches than the typical PVT measurements.…”