How the Presence of Oil and Water Affects Chalk Mechanics at Isotropic Stresses

Abstract

This study focuses on chalk mechanics for oil and water saturated cores. Tests were carried out on four Kansas chalk cores initially saturated by 1.1 M NaCl-brine. Two cores were wettability altered by a mixture of Heidrun oil - heptane (60/40 volume ratio) and then aged for three weeks at 90°C. The wettability index was estimated from test series of equally treated cores. All four cores were hydrostatically loaded to 1.5 times above yield at 130°C and left to creep. During the first 15 days of creep the fluids in the pores were stagnant. It was followed by flooding of 0.219 M MgCl2-brine at two flow rates (0.01 ml/min and 0.04 ml/min) for different durations. It is observed that during loading with stagnant fluids inside the pores, the stiffness and strength are affected by wettability. The axial creep strain curves for different wettability cores overlap during the stagnant fluid creep period and during flooding of MgCl2-brine at both flow rates. We see that the creep curves, given that the creep stress is 1.5 times the yield stress, are insensitive to the initial wettability and oil/water saturation. Hence yield stress is the determining factor for these experiments.