Characterizing Natural Fractures and Their Interactions with Hydraulically Induced Fractures
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Natural fractures are preexisting micro-cracks and fissures that can have a critical impact on hydraulic fracture treatments in shales. Most shale formations contain natural fractures, but the characteristics of these natural fractures can vary significantly. For example, the natural fractures in the Barnett Shale are mostly narrow, long, and sealed with calcite cement. The natural fractures in the Wolfcamp Shale are much more heterogeneous as a whole, but tend to be clustered in similar groupings based on the lithology of certain areas of the formation. The creation and development of natural fractures prior to any hydraulic fracturing treatments is primarily a function of mineralogy, total organic carbon, and in-situ stresses. During hydraulic fracturing treatments, certain characteristics, such as the relative angle between the natural and hydraulic fractures, the length of the natural fractures, the differential stress of the formation rock, and certain completion design variables, will determine how the natural and induced fractures interact and create a fracture network. The creation of a natural fracture network can have a positive effect on the ultimate hydrocarbon recovery in some cases. Natural fractures provide accumulation space and travel pathways for hydrocarbons, which is critical in low porosity and low permeability shales. However, natural fractures can result in higher rates of fluid leakoff, which will result in less efficient hydraulic fracture treatments overall. Also, natural fractures can provide an undesirable connection to water accumulations, which can negatively impact the economics of a well because of the disposal costs associated with water production. This thesis seeks to characterize natural fractures and detail the author's contributions to a hydraulic fracture simulation program that takes natural fractures into account.