Abstract

A survey of the stress dependent compressibility of unconsolidated sand reservoirs will be presented, including examples from the deepwater Gulf of Mexico, Brazil, West Africa, and Malaysia. Framework mineralogy, texture (grain size and sorting), stress history, and incipient overgrowth cements all influence the stress dependence of the compressibility curve as a function of depletion stress. Stress history is a first order control on compressibility. In general as maximum effective stress increases, the magnitude of the compressibility curve decreases and the peak compressibility occurs at higher depletion stress. Framework mineralogy also exerts a first order influence on the magnitude and shape of sand’s compressibility curve. As the volume of ductile grains increases the peak of the compressibility curve occurs at progressively lower depletion stresses, and the magnitude of the peak compressibility increases. Over a range of mineralogies, as mean grain size increases so does the pore volume compressibility, although grain size exerts a second order influence on compressibility when compared to stress history or framework mineralogy. Incipient overgrowth cements (quartz and feldspar) present in very small volumes (on the order of 1.5%) enhance the rigidity of the framework at low depletion stress.