Analysis of Front and Tail Stress Shadowing in Horizontal Well Fracturing: Their Consequences With Case History

Abstract

Abstract Creating multiple hydraulic fractures and keeping them open with proppant deforms the formation in their surroundings. These deformations generate normal and shear stresses within the formation, defined as stress shadowing by the oil and gas industry. Front stress shadowing denotes the effect of these stresses on future fractures. Tail stress shadowing defines their effect on previously created fractures. Front shadowing causes a gradual increase in the fracturing pressure of subsequent stages, ISIP, and deviation in fracture growth path. Tail shadowing accelerates fracture closure and also reduces the intensity of front shadowing. Existing analyses of stress shadowing consider only front shadowing and are generally based on numerical techniques with assumed fracture growth models, and varying approximations of the closing/closed fracture width. This paper uses a rigorous analytical approach to compute the effects of stress shadowing at different stages of fracture closure, and its effect on the pressure and orientation of subsequent fractures. As in actual field situations, the analysis allows for each of the previous fractures to be in different partial closure state. It shows that the stress shadow created by each stage of fracturing extends for only a limited distance ahead and behind it. When the cumulative effects of multiple fracture stages and their progressive closures are examined, the result is a gradual increase in fracturing pressure and ISIP during the early stages, followed by relatively constant values thereafter. Although stress shadowing can cause sharp local deflections in the fracture path, even in extreme cases the angle of deflection decreases along the length of the active fracture. Thus stress shadowing is unlikely to cause fracture coalescence in and by itself. Another effect of fracture deflection is to reduce the effect of stress shadowing in the later stages of fracturing. The main consequences of tail shadowing are accelerating closure of previous fractures, and reducing the intensity of front shadowing by transferring more of the induced deformation behind each fracture. Fracture deflection caused by stress shadowing is largest when the two horizontal principal stresses are very close to each other. Another factor that influences stress shadowing is the time lapse between stages. Longer lapses reduce the magnitude and effect of front stress shadowing. Results of this analysis are compared with field data, with reasonable agreement. They show that the effect of stress shadowing is not as large as generally assumed by the fracturing community. Paper includes practical recommendations to help detect the presence and magnitude of front stress shadowing in fracturing data.