Inhalt
Gas breakthrough
The dominant transport mechanism in hydrocarbon migration and dismigration is separate phase fluid flow (Darcy flow), either through matrix porosity or fracture networks. In the presence of two or more immiscible fluid phases in a porous rock, two-phase or multiphase flow will occur which is associated with capillary effects. Fluid transport is then controlled by the interfacial tension of the fluids involved, the wettability of the solid surface (wetting angle) with respect to the fluids, and the structure of the pore system. According to the Washburn equation (Washburn, 1921) intrusion of a non-wetting fluid into a cylindrical capillary of radius r only occurs if the capillary pressure Pc (i.e. the pressure difference between the two immiscible fluids) within a pore is exceeded.
Experimental procedure
Gas breakthrough experiments in this study are performed by imposing instantaneously a high gas-pressure gradient (i.e. exceeding the expected gas breakthrough pressure) across the rock samples and monitoring the resulting gas flux by means of the pressure changes (non-steady state, capillary controlled experiments). Complete water-saturation of the pore system is required as a defined initial condition for the experiments. Therefore all samples are first subjected to single-phase flow tests. After constant single-phase flow was registered for an extended period of time, complete water-saturation of the conducting pore system is assumed.
In a typical breakthrough experiment the pressure in the downstream chamber increases (P2) after a certain lag time and approaches the up-stream pressure (P1). A complete pressure equilibration between the two chambers is usually not reached, but a residual pressure difference Pc,residual ("minumum capillary displacement pressure" = Pc,threshold/breakthrough; Figure 4) remains which is a characteristic parameter for the rock sample analysed.
Apart from the displacement pressure required to force a gas phase into the initially water-saturated pore system of a mudstone, the quality of a seal is characterized by the effective permeability to the gas-phase after breakthrough. The effective gas permeability (keff) of the sample varies with time as a function of the excess gas pressure and thus the gas phase saturation. It is calculated from the pressure change in the downstream compartment using Darcy's law for compressible media:
Figure 4 shows a scheme of the experimental parameters recorded during the experiments and their interpretation in terms of capillary processes: (a) pressure history of a gas breakthrough experiment; (b) capillary pressure of the gas phase as a function of water saturation;. (c) relative permeability curve for the gas phase as a function of water saturation during drainage (I) and imbibition (II).
