Study on the Plugging Limit and Combination of CO2 Displacement Flow Control System Based on Nuclear Magnetic Resonance (NMR)

https://doi.org/10.3390/pr10071342

“CO₂ displacement is an important technology to reduce emissions and improve crude oil recovery, as well as prevent CO₂ escape. Effective storage is key to the successful implementation of this technology, especially for medium and high permeability reservoirs. The current flow control systems that are applied to seal gas escape are mainly gas/water alternation, CO₂ foam, and CO₂ foam gel, but there is no clear understanding of the plugging limits of various flow control systems and the mechanism of their combined use of residual oil. Therefore, in this paper, a series of core replacement experiments are conducted for different flow control systems and their combinations. The quantitative characterization of the core pore size distribution before and after the replacement is carried out using the NMR technique to try and determine the plugging limits of different plugging systems, and to investigate the residual oil utilization patterns of self-designed flow control system combinations and common flow control system combinations under two reservoir conditions with and without large pores. The results show that the plugging limits of water/gas alternation, CO₂ foam, and CO₂ foam gel systems are 0.86–21.35 μm, 0.07–28.23 μm, and 7–100 μm, respectively, as inferred from the T₂ (lateral relaxation time) distribution and pore size distribution. When different combinations of flow control systems are used for repelling, for reservoirs without large pore channels, the combination of flow control systems using higher strength CO₂ foam first can effectively improve the degree of crude oil mobilization in small pore throats, compared to using gas/water alternation directly. For reservoirs containing large pore channels, using high-strength CO₂ foam gel first to seal the large pore channels increases the degree of utilization of the large pore channels; using water/gas alternation first causes damage to the middle pore channels; High-strength CO₂ foam gel seals the large pore channels when the plugging strength is not enough; and using water/gas alternation can effectively improve the degree of utilization of small and medium pore channels. The results of this paper can provide theoretical guidance for the multi-stage flow control of CO₂ displacement in the field.”

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2.1. Experimental Flow

The experiment mainly consists of an NMR instrument, thermostat, foam generator, non-magnetic core holder, ISCO constant speed and constant pressure pump, gas cylinder, intermediate vessel, vacuum pump, pressure gauge, and measuring cylinder. The experimental flow is shown in Figure 1.

Figure 1. Flow of high temperature and high-pressure online NMR core repulsion experiment.

2.2. Experimental Steps

(1) Core drying. The samples are dried for 24 h at a temperature of 105 °C and then weighed for dry weight. The dry sample is placed into the NMR equipment and tested for dryness. (2) Core saturation with water. The core is vacuumed using a vacuum pump so that the water is pumped into the core and saturation is completed. (3) Core is saturated with oil to establish bound water. After saturating with water, the saturated oil is repelled from one end of the core, and the saturated oil ends, and the water remaining in the core becomes bound water, which is left for some time (aging) to test the T₂ spectrum and nuclear magnetic images in the saturated oil state. (4) Alternating water/gas system plugging. Manganese chloride water and CO₂ are injected alternately to simulate CO₂-water alternate plugging, and the alternation is continued until the T₂ spectrum signal does not change. The T₂ spectrum and NMR images are tested during the experiment. (5) CO₂ foam plugging. Next, 0.6 PV foam (heavy water configuration) and CO₂ are injected into the foam generator at the front of the core, according to the gas–liquid ratio of 1:2. The foam is formed and then injected into the core to simulate CO₂ foam system plugging. It is then repelled until the T₂ spectrum signal does not change. (6) CO₂ foam gel plugging. The 0.6 PV gel and foam mixture with CO₂ are injected into the foam generator at the front of the core, according to the gas–liquid ratio of 1:2, in order to form a CO₂ foam gel system. This is then injected into the core to simulate the plugging of the CO₂ foam gel system and repelled until the T₂ spectrum signal does not change. The T₂ spectrum and NMR images are tested. (7) The changes in oil-repelling efficiency and crude oil utilization in different pore throats are calculated according to the changes in the T₂ spectrum. (8) The T₂ spectra that are measured by the NMR experiments are transformed into pore radius distribution, according to the transformation relationship between relaxation time distribution and pore radius distribution [14,15].

3. Experimental Results and Analysis

3.1. Plugging Boundary

3.1.1. Water/Gas Alternate System Plugging Boundary

Core No. 1 is a medium permeability core with a permeability of 127 × 10⁻³ μm². The NMR T₂ spectra and pore throat radius distribution before and after water/gas alternation in the core are shown in Figure 2 and Figure 3.

Figure 2. NMR T₂ spectrum of core 1 before and after water/gas alternation.

Figure 3. Distribution of pore throat radius before and after water/gas alternation in core No. 1.

From Figure 2, it can be seen that after the water/gas alternating system is carried out in core No. 1, the oil drive efficiency can reach 50.84%, and the high amplitude value area on the T₂ spectrum decreases obviously, indicating that the degree of large pore throat activation is higher during the water/gas alternating process.

From Figure 3, it follows that the remaining oil after the water/gas alternation is mainly distributed in 0.045–0.65 μm and 0.86–3.02 μm pore throats, and a small amount is distributed in 9.24–16.15 μm pore throats. The pore throat activation range is 0.86–4.60 μm and 4.60–21.35 μm; the recovery rate is 0%, 40.9%, 88.2% from the left peak to the right peak, in order; and the water/gas alternating system plugging limit is 0.86–21.35 μm.

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