Inhalt
Diffusion
Diffusion experiments are conducted by applying a chemical potential gradient across the water-saturated sample in the absence of a pressure gradient. The non-steady (transient) state method used provides information both on the molecular mobility (effective diffusion coefficient) and the bulk concentration of the diffusing species (CO2) in the rock (Schlömer & Krooss, 1997; Zhang & Krooss, 2001).
Figure 2 shows the flow scheme of the experimental set-up for the measurement of gas diffusion through water-saturated rocks. It comprises a high-pressure system (flow cell, pumps, valve Vmanual) and a sampling section (valve V2 - V1). The flow cell is pressurised by means of a liquid chromatography (HPLC) pump with a maximum pressure of 50 MPa (500 bar) and a maximum flow rate of 10 ml/min. Through the sample loop on the 6-port switching valve (Vmanual) a certain volume of the source gas (CO2), can be introduced into the source chamber of the flow cell. Special precautions were taken to prevent backward diffusion of the gas through the communicating tubing system into the sampling chamber, that is a 1/8'' distance loop in order to increase the diffusion distance along this bypass and an adjustable relief valve for flushing the system. An automatically operated sampling valve (V5) is connected to the sampling chamber of either flow cell 1 or 2 (V2), followed by a capillary flow restrictor (25 mm, i. d.) to prevent a pressure drop in the flow cell during fluid sampling. The fluid is collected in a purge-vessel and CO2 is removed from the water-phase by a He-current (V4) and directly transported to the CO2 trap. CO2 will be released from the trap by electrical heating (up to 300°C) and transferred to the gas chromatograph (V0).
