Role of connate water salinity in gas dispersion during enhanced gas recovery by carbon dioxide injection and sequestration

Abba, MK

Abstract

A better understanding of the factors that influence mixing between CO2 and CH4 in natural
gas reservoirs can provide an avenue to minimise the gas dispersion during Enhanced Gas
Recovery (EGR). This highlights EGR’s field scale adoption as a potential method for
simultaneously reducing CO2 emissions through sequestration and enhancing natural gas
recovery and, thus, showcases it economic viability. An important aspect of the reservoir is
connate water. So, what is the role of its connate water salinity on mixing during EGR?
In this investigation, three (3) different sandstone core samples (Grey Berea, Buff Berea, and
Bandera Grey) with different petrophysical properties were used in this research. Phase I of
this study entailed the cleaning and the characterisation of the core samples using
experimental core analyses to determine the petrophysical properties. A novel practical
approach to grain diameter determination of the core samples using image analysis was
developed. The measurement showed that Buff Berea had the largest average grain size of
165.70 μm amongst the core samples used, followed by Grey Berea with 94.66 μm, and lastly
Bandera Grey with 57.15 μm. This facilitated the determination the Peclet number during the
displacement which helped develop a robust injection strategy for displacement of the CH4
with minimum contamination by providing an optimum injection rate ranges for this
application.
Phase II involved core flooding process to simulate the displacement of CH4 by CO2 that was
carried out at 1300 psig and 50oC with varying injection rates of 0.2, 0.3, 0.4, and 0.5 ml/min.
This was performed on dry core samples at different injection orientations –horizontal and
vertical - to ascertain the effects of these variations on the displacement efficiency. The
optimum injection rate was determined based on the dispersion coefficient and the CH4
recovery efficiency obtained from testing individual core samples. Grey Berea at 0.3 ml/min
in the vertical orientation gave the best results based on the criteria adopted and provided the
benchmark for subsequent sensitivity analyses.
The Phase III of the study focused on the impact of connate water salinity of the mixing and
dispersion of CO2 into CH4 during the displacement at the simulated reservoir conditions
during EGR with different brine salinities (0, 5, 10 wt% NaCl) using the optimum conditions
determined in Phase II for consistent results. The results from the core flooding process
indicated that the dispersion coefficient decreases with increasing salinity, hence the higher
the density of the immobile phase (connate water) the lower the dispersion of CO2 into CH4.
This is the first investigation into the relationship between the connate water salinity and the
dispersion coefficient in EGR. Consequently, feasibility of the solubility trapping as a
secondary mechanism for CO2 storage during EGR was experimentally investigated through
core flooding process. Solubility trapping was found to increase the CO2 storage capacity of
natural gas reservoir by about 60% during EGR and the higher the connate water salinity the
higher the sequestration potential of CO2 but lower the CH4 recovery was realised.
With this new information, the effect of connate water salinity on EGR is substantial and its
inclusion in simulations studies will be helpful for field scale applications of EGR technique.

Citation

Abba, M. (in press). Role of connate water salinity in gas dispersion during enhanced gas recovery by carbon dioxide injection and sequestration. (Thesis). University of Salford

• Thesis Type: Thesis
• Acceptance Date: Jun 7, 2019
• Deposit Date: Jun 24, 2019
• Publicly Available Date: Jul 24, 2019