The Impact of Horizontal Bedding Plane Fractures on Reservoir Fluid Production in Shale Oil Plays with High Pore Pressure

Abstract

Massive hydraulic fracturing stimulation generates complex induced fracture systems in shale reservoirs. The complexity increases in mudrock plays characterized by overpressure and associated small differential stresses. Such conditions favor interactions between induced vertical hydraulic fractures and mechanically weak bedding planes. When these planes delaminate easily, they may hydraulically stimulate large horizontal fracture components. In such situations, weak bedding planes are critical during hydraulic fracture propagation, impacting the fracture geometry and associated hydrocarbon production. We examine the effect of stimulated mechanically weak horizontal bedding planes on reservoir fluid production in multilayered mudrock plays distinguished by high pore pressure. We propose two idealized but viable geometries (‘fracture scenarios’) reported to occur in some overpressured shale plays. Our reference scenario comprises only vertical and planar hydraulically induced fractures. In the second geometry, we add stimulated horizontal bedding plane fractures that intersect the vertical hydraulic fractures. Next, we integrate the predetermined fracture geometries into a commercial reservoir simulator (CMG-IMEX) and assess the wellbore flow performance. Finally, we perform sensitivity analyses on horizontal fracture closure-mechanism, position and number of horizontal fractures, and reduced vertical fracture permeability. The results reveal that large horizontal fractures compromise hydrocarbon production. We conclude that horizontal fracture compressibility and vertical hydraulic fracture permeability are critical parameters during reservoir simulation. Compared with the reference scenario, the unpropped (i.e., highly compressible) and large stimulated horizontal fractures may reduce the initial oil production by 13% and the cumulative oil production at 15 years by 10%, assuming the highly conductive vertical hydraulic fractures. In contrast, horizontal fracture propagation results in shorter and narrower vertical hydraulic fractures. Thus, the lowered vertical hydraulic fracture conductivity predicts that initial oil production may decline by up to 77%. Finally, we show that vertical and planar hydraulic fractures geometry is not always an accurate assumption. This assumption may lead to an overestimation of hydrocarbon production during shale reservoir simulation studies. Our unique reservoir simulations show a numerical justification for the massive stimulation jobs and the unexpectedly low hydrocarbon production obtained in several mudrock plays worldwide. Consequently, we demonstrate that massive fracturing treatments may not always be a successful development method in mudrock plays.