Laboratory Studies of Capillary Interaction in Fracture/Matrix Systems

Abstract

Summary Current study of fractured petroleum reservoirs often is based on the assumption of capillary discontinuity between matrix blocks. Both theoretical analysis and examination of the field performance of some fractured reservoirs, however, indicate a degree of capillary interaction between matrix blocks. Experiments performed to gain an understanding of the capillary continuity across a stack of matrix blocks are described. Three matrix blocks with a total length of 3 ft [90 cm] were stacked and placed in a transparent core holder. Before the experiments with stacked placed in a transparent core holder. Before the experiments with stacked blocks were conducted, rock properties, capillary pressure, relative permeabilities, and the aperture/overburden relationship were measured. permeabilities, and the aperture/overburden relationship were measured. Drainage capillary pressure data showed a threshold height of the same size as each of the individual matrix blocks. Experimental results showed a strong capillary interaction (capillary continuity) between the neighboring blocks. Recovery from the top two matrix blocks far exceeds that from the same blocks when the assumption of capillary discontinuity is used. We believe that incorporation of the capillary-continuity concept in dual-porosity models will result in more realistic simulation of fractured reservoirs. Introduction Capillary, gravity, viscous, and diffusive forces generally are believed to be the major forces affecting the recovery and performance of fractured and unfractured petroleum reservoirs. The influence of these forces, however, is different in fractured and unfractured reservoirs. The share of these forces also depends on the type of process in a given reservoir. Capillarity and gravity usually are the process in a given reservoir. Capillarity and gravity usually are the major forces in fractured reservoirs, while viscous forces could dominate in unfractured reservoirs. The role of gas/gas and gas/liquid diffusion could also be more pronounced in a fractured petroleum reservoir with small matrix blocks than a similar process petroleum reservoir with small matrix blocks than a similar process would be in an unfractured reservoir. The effect of capillary forces in a multiphase flow process in a fractured porous medium is accounted for by both matrix and fracture capillary pressures. Defining the matrix blocks of a fractured petroleum reservoir as discontinuous blocks is appropriate only if the fracture capillary pressure is assumed to be zero. No reason exists, however, to believe that the assumption of zero fracture capillary pressure is appropriate. On the contrary, theoretical analysis and examination of the field performance of some fractured reservoirs indicate a degree of capillary continuity between the matrix blocks. Currently, the numerical simulation of fractured petroleum reservoirs often is based on the assumption of capillary discontinuity across the matrix blocks. For a field-scale example, Quandalle and Sabathier assumed zero fracture capillary pressure, but in a test example consisting of eight 30-ft [9.1-m] cubic matrix blocks, they assumed a nonzero water/oil capillary pressure for the fractures. For fractured reservoirs with small matrix-block heights (say, less than 5 ft [less than 1.5 m]), this assumption could result in an erroneous per-formance prediction of the simulated reservoir. Fracture capillary per-formance prediction of the simulated reservoir. Fracture capillary pressure will influence both the gas/oil gravity-drainage and pressure will influence both the gas/oil gravity-drainage and water/oil capillary-imbibition processes. However, the effect of fracture capillary pressure is usually more pronounced in a gravity-drainage process than in a capillary-imbibition process. Capillary continuity is perhaps the most important parameter affecting the performance and oil recovery of some gravity-drainage fractured performance and oil recovery of some gravity-drainage fractured reservoirs. Fig. 1 shows the ultimate recoveries of a tall matrix block and a stack of three small matrix blocks where each small block height is equal to a third of the tall block. If the capillary pressure in the fractures between the small matrix blocks is assumed pressure in the fractures between the small matrix blocks is assumed to be zero, most of the oil will be kept inside the matrix. The existence of vertical continuity between the matrix blocks in the stack, however, may cause the ultimate oil recovery to increase substantially to the extent that it matches the recovery from the tall matrix block (see Fig. 1).