Effect of pressure on simultaneous elastic and electric measurements of sandstones, The

Abstract

Sedimentary rocks are composed of complexly shaped grains deposited in random orientation. This arrangement allows void space to be present between grains, also called pores. Pore spaces are as complex as the grains themselves; when connected, the pore spaces create a network of micro-structures. This network, or porosity, and its geometry, or aspect ratio, are essential for geophysical data interpretation and reservoir quality determination. In this thesis, the effect of pressure on sedimentary rocks and the hysteresis caused by pressure cycles are studied. In the search to understand the fundamentals of the joint physics between elastic and transport properties, simultaneous elastic and electrical measurements and porosity and permeability measurement were performed, both under confining pressure allowing the study of elastic and transport properties under simulated reservoir conditions. Further, Nuclear Magnetic Resonance (NMR) results are used between pressure cycles to evaluate hysteresis. Ultimately the purpose of the proposed studies is to generate a reliable data set for elastic and electrical joint studies, better understand how the rock microstructure is modified by pressure, and how hysteresis can affect pressure dependent tests. The data presented by this study were produced from five sandstone outcrops samples and a sandstone analog constructed from fused glass beads. The results from simultaneous elastic and electrical measurements under confining pressures were compared to literature and models to ensure data reliability. Multidirectional elastic measurements were used to verify variability since all samples used were homogeneous and isotropic. Finally, the NMR results were used to understand how the pore-size distribution of each sample was being modified during pressure cycles. After evaluating the results I was able to conclude that the data acquired during this work are reliable with low variability. Further, the results obtained in this thesis show: 1. Significant hysteresis after different pressure cycles make simultaneous measurements vital. Subsequent measurements after a pressure cycle can be affected by changes in the sample microstructure. 2. The imaginary conductivity shows a shift of peak frequency towards lower frequencies. 3. With pressure, the decrease in surface area is larger than the increase in tortuosity. 4. Finally, the effects of pressure on the measured data were evaluated, allowing the conclusion that elastic data changes in a exponential path with pressure, while porosity, permeability and conductivity behave more linearly between 3 and 20.7 MPa.