흐름 전위에 기초한 자연 전위 탐사법의 원리 및 활용

A Technical Review on Principles and Practices of Self-potential Method Based on Streaming Potential

Abstract

흐름 전위는 지하 공극 내 유체의 흐름에 의해 발생하는 자연 전위로 이러한 흐름 전위를 이용한 자연 전위 탐사는 유체 흐름 뿐만 아니라 지층 유체에 대한 정보도 파악할 수 있는 탐사법이다. 지하 매질과 유체 사이에 존재하는 전기 이중층과 같이 입자가 대전하고 있을 때 여러 원인에 의해 유체가 유동하여 발생하는 계면 동전기 현상 중 하나인 흐름 전위는, 발생 기작이 복잡하고 측정 전위 값이 비교적 작아 잡음에 취약하다는 어려움도 있지만 이를 이용한 자연전위 탐사는 인공적인 송신원이 없어 탐사가 용이할 뿐만 아니라 반복성도 좋기 때문에 지층 유체 모니터링 탐사에 적용이 확대되고 있다. 이 논문에서는 지금까지 흐름 전위의 발생 기작에 대한 연구와 지배 방정식들을 정리하여 설명한 뒤, 매질의 물리적 특성 변화에 따른 흐름 전위 반응 특성 및 다양한 국내외 유체 흐름 자연 전위 탐사의 사례 분석을 수행하였다. 이 기술보고에서는 흐름 전위에 대한 이해도를 높임과 동시에 적용 가능한 다양한 분야를 소개함으로써, 국내에서의 흐름 전위를 이용한 자연전위 탐사의 현장 적용 방안을 제시하고자 한다.

Streaming potential (SP) arises from fluid flow through effectively connected pores. From this potential, formation water information as well as fluid flow properties can be estimated. As micro particles being located in boundary between subsurface porous media and fluid are charged to form electrical double layer, fluid flow caused by several reasons generates SP, one of electrokinetic phenomena. Occurrence mechanism of SP is complex and signal strength is relatively weak compared to noise. However, application of self potential survey using SP to monitoring of formation fluid is expanding because of its' convenience of exploration without artificial source and repetitiveness of signal. This paper accounts for the occurrence mechanism of SP studied before, including governing equations and analyzes previous various case studies of SP according to the change of physical properties of materials. It helps to increase understanding about SP and also lays the foundations of the application of SP to fields.

Fig. 1. Half-space heterogeneous model with water pumping well. (a) is plan view of model and (b) is cross-section of the model. Background of the model is silty sand and that has physical properties of K(1×10^-6$\frac{m}{s}$), L^sat(1×10^-5$\frac{A}{㎡}$) and σ^sat(5×10³$\frac{s}{m}$) and more permeable zone(sand aquifer) that has different physical properties is located on the center in the plan view. The pumping well is located at the center of the sand aquifer and it’s pumping rate is 500 m3/day (Sheffer and Oldenburg, 2007).

Fig. 2. Vertical cross-sections of streaming current source density about homogeneous and heterogeneous models. (a) is streaming current source density about water pumping when the physical properties are same within whole model (K = 1×10^-6$\frac{m}{s}$), L^sat = 1×10^-5$\frac{A}{㎡}$, σ^sat = 5×10³$\frac{s}{m}$). (b) is set in the model that has different hydraulic conductivity, K (1×10^-4$\frac{m}{s}$). (c) is set in the model that has heterogeneous hydraulic conductivity, K (1×10^-4$\frac{m}{s}$) and crosscoupling conductivity, L (3×10⁵$\frac{A}{㎡}$ ) (Sheffer and Oldenburg, 2007).

Fig. 3. Surface plan map of SP about homogeneous and heterogeneous pumping models. (a) homogeneous half-space model, (b) heterogeneous K half-space model, (c) heterogeneous K and L halfspace model and (d) heterogeneous K, L and σ half-space model (Sheffer and Oldenburg, 2007).

Fig. 4. Surface profiles of SP at y = 500 m. (a) homogeneous halfspace model, (b) heterogeneous K half-space model, (c) heterogeneous K and L half-space model and (d) heterogeneous K, L and σ half-space model (Sheffer and Oldenburg, 2007).

Fig. 5. SP anomaly and resistivity profiling to compare results at fracture zones in a hard rock. Resistivity survey was conducted to use Wenner configuration with 150 m current electrode separation (i.e. a = 50 m) and SP response was measured along the same profile line with a 10 m potential electrode separation. Positive SP anomaly is interpreted to show groundwater flow in the fractured formation (Pozdnyakova et al., 2001; Sharma and Baranwal, 2005).

Fig. 6. SP distributions with 5 different time steps of water (white) moving model toward the production well, which is located at 0 m on the horizontal axis. (a) shows 2 snapshots of 231 days and 463 days of the saturation distribution within the reservoir layer. (b) is SP about 1D horizontal section per different times. The peak of the SP curve per each time is located at the water front of same time (Saunders et al., 2008).

Fig. 7. Vertical sections of 3 models containing shale (black) with different percent; (a) 17%, (b) 35% and (c) 50% randomly. From the different contents of the model, conductivity structure of the reservoir is changed (Saunders et al., 2008).

Fig. 8. Maximum SP measured at the well versus time for models with varying contents of shale. SP increases in early time about the model of higher shale content while the water front is far away from the well. And increased conductivity by shale content makes potential reduced when the water front is close to the well (Saunders et al., 2008).

Fig. A-1. Examples about three types of electrical double layer:(a), (b) and (c). (a) is Helmholtz double layer that surface ofcolloid is charged with negative ions and positive ions areabsorbed to negative ions symmetrically consisting layer. (b) isGouy-Chapman double layer. In this layer model, ions havingsame charge with charge on the surface of colloid also consist ofdiffuse layer. (c) is stern double layer. This system consists of twolayers of stern layer and Gouy layer. Stern layer is fixed and Gouylayer is diffuse layer that can be moved. Three types of electricaldouble layer are all electrical equilibrium state as long as therearen't any external current sources or any other forces.

Fig. A-2. 4 types of electrokinetic phenomena are all connectedeach other. These phenomena can be divided into two groups; oneis under being in the given electric field and the other is if electricfield. Electrophoresis and electro-osmosis have complementaryrelation under the electric field.