A FAST AND ACCURATE METHOD TO MEASURE THRESHOLD CAPILLARY PRESSURE OF CAPROCKS UNDER REPRESENTATIVE CONDITIONS

Assessing representative threshold capillary properties of fine grained rocks is very important in the context of the reservoir production, the basin modeling and the geological storage (natural gas or CO2) This paper presents an innovative approach to measuring representative values of threshold capillary pressure under in-situ conditions based on the analysis of a dynamic displacement of the non-wetting fluid in the sample. When the injection is conducted at a constant pressure drop (higher than the threshold capillary pressure), the pressure profile within the core can be divided into three parts : the upstream invaded region, the front zone with a pressure jump associated with the threshold capillary pressure and the virgin region still saturated with the brine. Just before the non-wetting phase contacts the inlet face of the core, the brine production rate corresponds to Darcy's law using the overall pressure drop and the absolute permeability value. When the non-wetting fluid starts flowing within the core, a reduction of the production rate is observed due to the threshold pressure jump at the displacement front. Because the upstream invaded region extension is limited, its pressure drop can be neglected. The pressure drop in the virgin region can then be calculated from Darcy's law using the reduced production rate and the absolute permeability value. Finally, the threshold capillary pressure is deduced by subtracting this pressure drop value from the overall pressure drop value. The proposed approach has been compared to the other available techniques to measure threshold capillary pressure : 1) step-by-step increase of the non-wetting phase pressure (standard approach) and 2) transient evolution of inlet and outlet pressure (residual capillary pressure approach). The results demonstrate that the new method is as accurate as the standard approach and is as fast as the residual capillary pressure approach. When local pressure taps are available, the method can also detect threshold Pc variations along the sample.