Behavior of Flow Through Low-Permeability Reservoirs

Abstract

Abstract While pores and throats of low-permeability reservoir rock are very tiny, they have significant influence on flow behavior in those reservoirs. There may be large differences in throat sizes and their distribution in low-permeability formations, even though they have similar storage space and flowing channels. Equivalent radii of flowing throats for low-permeability reservoirs with similar permeabilities may be very different. Stronger liquid-solid interaction within finer pore throats having a large interface area and abundant hydrophilic clay tends to prevent water from flowing through. This paper presents a systematic study of flow behavior in low-permeability reservoirs usinga constant-rate mercury injection test,a nuclear magnetic resonance (NMR) core analysis, anda seepage experiment. NMR results show that mobile fluid saturation in low-permeability formations is very different from that in normal reservoirs. In general, the less clay content and larger throat radii of the low-permeability rock, the larger the mobile fluid saturation. We conducted a series of seepage tests, with the results showing that at low velocity, the flow rate in low-permeability rock is not linearly correlated with pressure gradient, but rather is a function of pressure gradient. Based on experimental results, we propose a modified Darcy's law for describing the observed nonlinear flow phenomenon, which is implemented into a black-oil model simulator. Simulation results incorporating the nonlinear flow-behavior show that effective permeability near production and injection wells is higher, whereas the permeability further away from the wells is lower. Well performance predicted by the linear model overestimates production rates, while that predicted by the new, nonlinear model better matches the field production data. Introduction Pores and throats in low-permeability reservoir rock are very tiny, but their interface area is quite large, and interaction between pore surface and fluid is strong. Therefore, Darcy's law may not be in general valid for low-velocity flow. The relationship between flow velocity and pressure gradient in such porous media is a buckled curve. Only when the pressure gradient or flow velocity reaches a certain value does the relationship curve become a straight line. However, the straight line cannot intersect the origin when extrapolated. The intersection of extension for the straight line and the x-axis is called the "pseudo-threshold pressure gradient"[1–4]. Such a pseudo-threshold pressure gradient concept has been commonly used to describe nonlinear flow behavior in a low permeability reservoirs: