공극 구조 내 교차 주입이 비혼성 유체의 포획 및 거동에 미치는 영향

Effect of Cyclic Injection on Migration and Trapping of Immiscible Fluids in Porous Media

Abstract

이산화탄소 지중저장 수행 중 저류층 내부에서 나타나는 비혼성 유체의 대체 과정은 다공성 매체의 공극 표면에 대한 각 유체의 습윤 특성에 따라서 배수(drainage)와 흡수(imbibition)로 구분되는데, 각 과정 동안 나타나는 비혼성 유체 간의 거동 및 포획 양상을 이해하는 것은 주입 효율성 및 저장 안정성을 평가하는데 매우 중요하다. 본 연구에서는 다공성 매체 내 주기적인 배수와 흡수 과정의 수행을 통해 공극 구조 내 비혼성 유체의 거동 양상 및 분포의 변화를 분석하고자 하였다. 이를 위하여 2차원 마이크로모델 내부로 이산화탄소와 공극수의 대체 유체로서 선정된 헥산과 탈이온수를 주기적으로 교차 주입하는 실험을 수행하였다. 관측 결과를 이용하여 각 유체 주입 과정에서 나타나는 두 비혼성 유체의 거동 양상을 비교 분석하고, 잔류 유체의 포화도를 산정하였다. 분석의 결과로서 헥산과 탈이온수의 잔류 포획 유형을 기작에 따라 습윤성(wettability), 모관압(capillarity), 막다른 공극(dead end zone), 포위(entrapment) 그리고 우회(bypassing)로 구분하였다. 또한, 교차 주입이 거듭됨에 따라 공극 구조 내에서 주입 유체의 흐름 경로는 주 흐름 경로(main flow channel)를 중심으로 단순화되었으며, 이로 인하여 주입 유체의 대체 효율은 일정한 값으로 수렴하였다. 실험적 관측과 분석의 결과는 실제 이산화탄소 지중저장 환경에서 습윤성-비습윤성 유체의 주기적인 교차 주입이 야기하는 저장층 내 비혼성 유체의 거동과 분포, 그리고 주입 유체의 대체 효율을 예측하는데 활용될 수 있을 것으로 판단된다.

In geological $CO_2$ sequestration, the behavior of $CO_2$ within a reservoir can be characterized as two-phase flow in a porous media. For two phase flow, these processes include drainage, when a wetting fluid is displaced by a non-wetting fluid and imbibition, when a non-wetting fluid is displaced by a wetting fluid. In $CO_2$ sequestration, an understanding of drainage and imbibition processes and the resulting NW phase residual trapping are of critical importance to evaluate the impacts and efficiencies of these displacement process. This study aimed to observe migration and residual trapping of immiscible fluids in porous media via cyclic injection of drainage-imbibition. For this purpose, cyclic injection experiments by applying n-hexane and deionized water used as proxy fluid of $scCO_2$ and pore water were conducted in the two dimensional micromodel. The images from experiment were used to estimate the saturation and observed distribution of n-hexane and deionized water over the course drainage-imbibition cycles. Experimental results showed that n-hexane and deionized water are trapped by wettability, capillarity, dead end zone, entrapment and bypassing during $1^{st}$ drainage-imbibition cycle. Also, as cyclic injection proceeds, the flow path is simplified around the main flow path in the micromodel, and the saturation of injection fluid converges to remain constant. Experimental observation results can be used to predict the migration and distribution of $CO_2$ and pore water by reservoir environmental conditions and drainage-imbibition cycles.

Fig. 1. Micromodel with glass and pore space (black:glass, sky blue: pore space).

Fig. 2. Schematic diagram for the cyclic injection experimental set-up.

Fig. 3. Image processing procedure (G: glass, W: deionized water, H: n-hexane); (a) a real image before nhexane injection (All of pore was initially saturated with water), (b) a real image after n-hexane injection (A portion of deionized water was trapped by n-hexane), (c) a gray image transformed from the real image (b), (d) the binary image transformed from the real image (a), (e) a binary image of the distribution of deionized water (white) in pore network with n-hexane (black) transformed from the gray image (c), (f) a binary image of the distribution of nhexane (white) in pore network with deionized water (black), (g) a image colored sky blue from the binary image (e), (h) a image colored red and purple from the binary image (f), (j) a finalized multicolor image of distribution for flowing n-hexane (red), residual n-hexane (purple), deionized water (sky blue) and glass (black).

Fig. 4. Sequential images over the course of five drainage-imbibition cycles. (a, c, e, g and i) After the completion of the drainage (n-hexane injection), (b, d, f, h and j) after the completion of imbibition (deionized water injection) (Experimental conditions: 0.1 MPa, 25^oC and 10 μL/min). The different colors represent flowing n-hexane (red), residual n-hexane (purple), flowing deionized water (blue) and residual deionized water (sky blue).

Fig. 5. Enlarged images after the completion of the 1^st Drainage. Residual trapping by (a) wettability, (b) capillarity, (c) dead end zone, (d) entrapment and (e) bypassing. The different colors represent flowing n-hexane (red) and residual deionized water (sky blue).

Fig. 6. Enlarged images after the completion of the 1^st Imbibition. Residual trapping by (a) wettability, (b) capillarity, (c) dead end zone, (d) entrapment and (e) bypassing. The different colors represent residual n-hexane (purple), flowing deionized water (blue) and residual deionized water (sky blue).

Fig. 7. Variation in saturation of n-hexane and deionized water over the course of five Drainage-Imbibition cycles (D: Drainage (n-hexane injection), I: Imbibition (deionized water injection)) (Experimental conditions: 0.1 MPa, 25^oC and 10 μL/min).

Fig. 8. Enlarged images of the yellow square in Fig. 4 (g, h, i and j). The different colors represent residual n-hexane (purple)and residual deionized water (sky blue).

Fig. 9. The number of blobs and saturation of n-hexane over the course of five Drainage-Imbibition cycles. Each data point denotes the average of five replicate experiments (Experimental conditions: 0.1 MPa, 25^oC and 10 μL/min).

Fig. 10. Average blob area of n-hexane after the completion of the Drainage-Imbibition cycles. Each data point denotes the average of five replicate experiments.

Table 1. Fluid properties and interfacial tension of n-hexane and deionized water at ambient pressure