Asphaltene Precipitation and Deposition during Nitrogen Gas Cyclic Miscible and Immiscible Injection in Eagle Ford Shale and Its Impact on Oil Recovery

Abstract

Cyclic gas injection methods have been shown to improve oil recovery in conventional reservoirs. Even though similar technologies have been used in unconventional reservoirs with some success stories in shale resources, cyclic gas injection enhanced oil recovery (EOR) is still a little-understood subject in boosting oil recovery from unconventional reservoirs. During gas injection, asphaltenes start to deposit and precipitate, which causes pore plugging and reduces oil recovery. Studies of asphaltene deposition challenges during cyclic nitrogen (N 2 ) gas injection and oil production in unconventional reservoirs are yet relatively limited. Therefore, a comprehensive experimental study was conducted using 12 Eagle Ford shale cores (dynamic mode), and filter paper membranes (static mode) were used to evaluate whether miscible and immiscible huff-n-puff (cyclic) N 2 injection increases oil recovery and aggravates asphaltene precipitation. To ensure that miscibility can be examined in cyclic experiments, N 2 minimum miscibility pressure (MMP) was determined using a slim tube technique. The factors studied included the injection pressure, number of cycles, production time, and injection cyclic mode, all conducted at 70 °C. The findings showed that a high asphaltene weight percent was calculated during static experiments (i.e., using filter membranes), and this increase was severe on smaller pore size structures. Dynamic tests (i.e., using shale cores) showed that miscibility increased oil recovery, but a stronger intermediate-wet system was observed when measuring the wettability of cores after N 2 cyclic tests. When starting with shorter soaking times, more oil recovery could be achieved. Oil recovery reduction and asphaltene depositions were observed at later cycles. Microscopy and scanning electron microscopy (SEM) imaging of the Eagle Ford cores showed asphaltene clusters inside the cores after cyclic tests. A mercury porosimeter emphasized the degree of pore plugging after cyclic tests, and the findings revealed a smaller pore size distribution after N 2 tests due to the asphaltene deposition process when compared to cores that had not been pressured. This extensive study focuses on the effects of asphaltene deposition on oil recovery under cyclic N 2 -miscible and immiscible conditions in shale resources.

Experimental Investigation of Asphaltene Deposition and Its Impact on Oil Recovery in Eagle Ford Shale during Miscible and Immiscible CO2 Huff-n-Puff Gas Injection

“…The novelty of the work lies in presenting a comprehensive experimental evaluation of asphaltene instability in tight shale reservoirs during miscible and immiscible conditions using shale cores and filter paper membranes. This study further expands the work of Elturki and Imqam, − who evaluated the effect of continuous and huff-n-puff immiscible/miscible N 2 injections on the deposition of asphaltenes. The ultimate goal of this research is to highlight the process of asphaltene damage during the miscible and immiscible CO 2 huff-n-puff process, especially in ultra-small-permeability reservoirs (mainly unconventional reservoirs).…”

“…The performance of OR under CO 2 and N 2 gas injections, as well as the effect of asphaltene deposition, is comprehensively compared in this section. The results of OR under N 2 gas injection are from our previous work . For the comparison, two immiscible pressures (i.e., 1000 and 1300 psi) and two miscible pressures (i.e., 1750 and 2000 psi) for the two gases were selected with a production time of 15 min and 6 h soaking time, as summarized in Table .…”

“…The results of OR under N 2 gas injection are from our previous work. 98 For the comparison, two immiscible pressures (i.e., 1000 and 1300 psi) and two miscible pressures (i.e., 1750 and 2000 psi) for the two gases were selected with a production time of 15 min and 6 h soaking time, as summarized in Table 8. Figure 23 shows the performance of OR during immiscible and miscible CO 2 and N 2 injections.…”

Review on Oil Displacement Technologies of Enhanced Oil Recovery: State-of-the-Art and Outlook

“…N 2 comes from a wide range of sources and brings no pollution or corrosive effects in the application process. Therefore, N 2 has more merits in production and application than other energy sources, which pushes it to become the core content of existing gas injection for oil displacement . N 2 is insoluble in water and less soluble in oil, so continuous injection of nitrogen can increase the number of contact between gas and crude oil, forming immiscible flooding .…”

“…Therefore, N 2 has more merits in production and application than other energy sources, which pushes it to become the core content of existing gas injection for oil displacement. 243 N 2 is insoluble in water and less soluble in oil, so continuous injection of nitrogen can increase the number of contact between gas and crude oil, forming immiscible flooding. 244 The immiscible nature between N 2 and crude oil is used to form an artificial gas cap so that the remaining oil at the top of the reservoir and the pores, which cannot be affected by water, can be displaced by gravity displacement.…”