Fracture Complexity: Analysis Methodology and Signature Pressure Behavior of Hydraulic Fracture Propagation from Horizontal Wellbores

Abstract

Abstract During hydraulic fracture propagation three regions may be identified from the pressure response referred to as: 1) near-well, that extends tens of inches, 2) mid-field, that extends tens of feet and 3) far-field, that extends hundreds of feet from the wellbore. Each region can experience simple, tortuous, and complex fracture behavior creating unique pressure signatures. It has been observed that complex fractures with extended fracture storage can result from hydraulic fracture stimulation in highly deviated and horizontal wells. Complexity is created as a result of induced hydraulic fractures turning and twisting as they exit the wellbore in the near-well region, propagating into the mid-field region, and then re-orienting in the direction of principal stresses in the far-field. This results in anomalously high apparent net pressures as evident by increased ISIP's and rapidly declining pressures that dissipate minutes after shut-in. This paper presents minifrac and post-job pressure matching case studies that identify and describe mid-field fracture complexity (MFC), or extended wellbore pressure storage. The pressure behavior supports complex fracture propagation, high fracturing pressures and pressure fall-off responses typically observed in horizontal shale wells. High apparent fracturing stress gradients are often seen that are much greater than the over-burden stress-gradients. Although suggestive, these high stress gradients are not indicative of horizontal fractures in the far-field. Uncharacteristically high MFC is also not necessarily related to fracture complexity in the far-field. A methodology is presented that identifies the "actual" ISIP which allows for in-situ stress calibration, true net pressure identification, proper minifrac interpretation and an improved fracture treatment design. Rapidly declining pressure during a shut-in as a result of MFC resembles pressure dependent fluid loss and often is misinterpreted as such. However, the pressure response is a result of extended fracture storage and energy dissipation in the mid-field region, which can result in multiple closure signatures. Multiple closure events are indicative of complex fracture network behavior as a result of stress anisotropy and creation of multiple non-planar fractures in the mid-field region. Additionally, hydraulically induced secondary fractures perpendicular to the maximum horizontal stress can provide insight into stress anisotropy. Identifying and incorporating MFC into pressure interpretation analyses will enhance fracture treatment design and post-job pressure matching providing a systematic methodology for designing and analyzing horizontal shale fracture treatments. Information regarding MFC is critical in interpreting fracture treatment pressure responses and optimizing fracture treatment designs in horizontal wells, including well spacing and fracture interference.