Introduction

As fluid from a reservoir is produced and brought to the surface, the fluid remaining in the reservoir experiences changes in the reservoir conditions. The produced fluid also experiences changes as it is brought to the surface. The reservoir fluid typically sees only a decrease in pressure, while the produced fluid will experience decreases in pressure and in temperature. As the pressure decreases, it is common to observe gas that had been dissolved in the oil or water be liberated. Reservoir engineers use terms, such as the solution gas-oil ratio (R_so), to account for this. There are many variations on this term. R is generally used to denote any ratio, while the subscripts denote which ratio is being used. R_soi, for example, is the initial gas-oil ratio, and R_sw is the solution gas-water ratio.

As the fluid is produced from the reservoir, the pressure on the rock from the overburden or the rock above it remains constant but the pressure of the fluid surrounding it is decreasing. This leads the rock to expand or the pores in the rock to be compressed. This change in pore volume due to pressure is called the pore volume compressibility (c_f). The compressibility of the gas is also of interest. The gas compressibility (c_g) involves a compressibility factor (z). The compressibility factor is simply a ratio of how the gas would behave ideally compared to how it behaves in actuality. The compressibility of oil (c_o) and water (c_w) can also be determined, but their magnitude is far less than that of the gas. The determination of each of these properties, as well as those defined is critical in predicting the performance of a reservoir. The contains a discussion of the pertinent rock and fluid properties with which a reservoir engineer will work.