Abstract

Reactivation of natural fracture systems within an unconventional reservoir can provide significant deliverability benefits during production. Our industry is replete with extensive studies of the interaction between structural fabric and present-day completion activities. Horizontal image logs, microseismic surveys, core study, complex geomechanical modeling, and extensive completion parameter testing can all be part of in-depth study. However, these studies can be cost and time prohibitive to smaller operators. This paper proposes a methodology and workflow that can bridge the gap between low capital costs and sophisticated, geomechanical knowledge of the reservoir.

Key pieces of information include detailed structural interpretation from well control or seismic surfaces, nearby stress estimations from cross-dipole sonic logging, and basic completion information, such as ISIP and breakdown pressure. Interpretation of structural surfaces provides insight about the large scale structural fabric that exists at the reservoir level and deeper horizons. Stress estimations are important to predict the in-situ stress tensor and generalized rock strength. ISIP is generally related to closure pressure, or minimum horizontal stress. Increases in ISIP could be attributed to increases in closure stress. Fluctuation in closure stress can be attributed to stress state change or change in mechanical stratigraphy, which is why it is important to define mechanical stratigraphy through sonic log analysis, as stated before. Breakdown pressure is generally referred as a proxy for fracture initiation pressure. Breakdown pressure should not be directly related to rupture stress, as it is a rate-dependent, size-dependent, and fluid-dependent value. However, relative changes in breakdown pressure, while completion parameters are held somewhat constant, can provide insight on the stress required to initiate a fracture.

This study will utilize 4 Diamondback horizontal wells within the Lower Spraberry. We have targeted the organic-rich facies at the base of the Lower Spraberry. Geomechanically, this interval is relatively consistent, save for a few, thin, high-stress, low porosity turbidites. Structural interpretation was derived from dense well control and seismic analysis. Significant faulting is present throughout the area in the Pennsylvanian, but does not entirely translate to the Lower Spraberry. Some assumptions of meso-scale and micro-scale structural fabric can be made by investigating the macro-scale, such as relative fracture intensity and orientation. Combining these observations and interpretations with the completion information provided in post-job reports lend corroborating evidence to support the claim that some stages within our laterals exhibit a higher degree of natural fracture reactivation through higher fracture densities.