Spontaneous imbibition is vital for oil recovery in porous media, especially when considering fluid flow behavior in fractured reservoirs. Small-scale laboratory tests on reservoir rock samples show that spontaneous imbibition becomes crucial in estimating field behavior according to oil recovery by imbibition performance. This paper presents experimental work on oil recovery by spontaneous water imbibition on kerosene/brine/Berea systems. The main objectives of this study were to investigate the effect of shape factor (SF), characteristic length (CL), and boundary conditions (BC) on the rate of spontaneous imbibition, and improve oil production simulation by spontaneous imbibition from naturally fractured reservoirs using derived shape factors. General mathematical definitions for shape factor and characteristic length are typically derived using irregular geometric shapes, but this investigation utilized regular geometric shapes of core samples (i.e., cylindrical, square prism, and triangular prism) for simplicity. Experiments were conducted on different sizes of core samples with cylindrical shapes yielding the highest imbibition rates and triangular prisms the lowest imbibition rates. Characteristic length had an inverse effect on the rate of spontaneous imbibition within the same shape family of core samples. Boundary condition (BC) experiments were performed to investigate the effect of the boundary on the rate of spontaneous imbibition. The four different boundary conditions applied to the samples were (1) all faces open (AFO), (2) two ends closed (TEC), (3) two ends open (TEO), and (4) one end open (OEO) systems. BC experiments showed that an increase in total surface area open to imbibition promoted the rate of spontaneous imbibition. In this study, kerosene/brine/Berea systems showed that imbibition recovery rates increase with an increase in shape factor while decreasing in characteristic length specific to the same shape of core samples. (c) 2006 Elsevier B.V. All rights reserved.