Experimental Hydraulic Fracture Propagation in a Multi-Fractured Medium

Abstract

Abstract In scaled laboratory experiments, we investigated the interaction of apropagating hydraulic fracture with natural discontinuities. We observed the hydraulic fracture geometry as a function of horizontal stress difference, stress regime, flow rate and discontinuity pattern. We observed the smallest amount of interaction for high flowrates. When the horizontal stress differencewas increased, the fracture was smoother. However, when we represented the tectonic stress ratio's, with a lower vertical stress compared with the maximum horizontal stress, we observed a higher fracture tortuosity. Introduction Mineback experiments1,2,3 in formations that were hydraulically fractured have provided the first views into complex hydraulic fracturegeometries in fractured reservoirs. They showed the formation of multiplefractures and large-scale interaction between a hydraulic fracture and natural fractures. These features will negatively influence the hydraulic fracture dimensions and affect the near-wellbore geometry. Encounters between apropagating hydraulic fracture and discontinuities may lead toarrest of fracture propagation,fluid flow into discontinuities,creation of multiple fractures andfracture offsets. The first three will result in areduced fracture length. Fracture offsets and multiples will result in areduced (local) fracture width. These width reductions may cause proppant bridging and consequent pre-matureblocking of proppant transport (so-called screen-out). Problems with early screen-out have been encountered in actual fracturing treatments in a fractured reservoir, the Minami-Nagaoka gas field in Japan4. In particular, abnormally high net pressures have been reported and are attributed to simultaneously propagating multiple fractures5,6,7. The combined analyses of fracturing and production performances also indicate complex fracture geometries. Most attention has been given to complex hydraulic fractures near the wellbore. Near-wellbore tortuosity is probably the dominant reason forpremature screen-out8. The influence of natural fractures (in thispaper termed discontinuities) on premature screen-outs has not received much attention. Observations, both from field8 and laboratory9data suggest that a high flow rate and very viscous fluid can induce a clean fracture in the preferred fracture plane. Presently, the recommended procedure for mitigating near-wellbore tortuosity is to initiate the fracture with viscous fluid (Cross-linked gel). The opposite conclusion could be drawn from numerical simulations of fluidflow into a fractured reservoir. In these simulations, the fluid flows in tojoints that are oriented in the direction of the preferred fracture plane for low flow rates. At high flow rates, the fluid flows in a radial pattern away from the wellbore10. The explanation is that the high flow rate induces a high net pressure and when the net pressure is much higher than the horizontal stress difference, the fluid can flow into fractures with any orientation. However, a limitation of these simulations was that the blocks in between the natural fractures could not break. Perhaps, in reality the formation of new fractures would yield a dominant hydraulic fracture at high flow rate. In this paper, we will describe the experimental set-up and the preparation of the fractured blocks. Then we discuss the test results for variation of the injection rate and stresses. Although a part of the experimental settings is prepared for the Minami-Nagaoka gas field, the conclusions drawn in this study are rather general.