The Impact of Reservoir Confining Stress on Nmr T2 and Pore Frequency Distribution in Some Carbonate Samples

Recent developments allow high-pressure mercury intrusion experiments to be performed under reservoir-representative confining stress. NMR relaxation measurements on core samples are increasingly being used in addition to mercury intrusion data since the two techniques yield complementary information about the pore network. Integration of the different measurement techniques result in a more detailed pore network topology at representative conditions. New apparatus is described in this paper which enables NMR T2 distributions to be determined for core samples under reservoir confining stress. Data for a number of samples originating from two producing carbonate reservoirs are presented. Both high pressure mercury intrusion and NMR relaxation tests were performed at series of increasing confining stresses to observe the transformation in both pore frequency distribution and NMR T2 distribution. Additional core characterisation measurements were performed on the same samples using multi-speed centrifuge at similar confining stresses. The selected reservoir core samples were earlier characterised according to reservoir rock types (RRT) based on conventional poroperm data, ambient condition mercury intrusion data, thin section analysis and X-ray CT scanning. The systematic changes observed in the pore size and pore throat distributions as a function of increasing confinement is discussed in the context of the different pore space phenomena to which the two types of measurement relate. Comparison between the two methods gives insight into issues such as pore connectivity and accessibility, and the impact on these and other properties as a function of increasing applied stress is also discussed. The results show the importance of representative reservoir core measurements in reducing uncertainty in developed static models and consequent simulation validation for field development models. INTRODUCTION High pressure mercury intrusion testing possesses the advantage over gas or oil–brine drainage capillary pressure experiments of great speed of data acquisition at the cost of