| Description |
Recorded pressure is often the most reliable and inexpensive data available to evaluate fracture characteristics, and to optimize the hydraulic fracturing program. Pressure diagnostics are evaluated to provide additional value to the operator. Three scales are addressed in this work to investigate fracture characteristics: 1. In-laboratory, large-block hydraulic fracturing experiments under in-situ stress conditions were conducted to investigate the role of interfaces in hydraulic fracture propagation. To replicate subsurface conditions in laboratory experiments, a technique was developed to produce reasonable experimental times, and stable fracture propagation. The hydraulic fracturing test results identified situations for crossing and not crossing interfaces, and for crossing with offset or "step over" behavior. The role of fluid velocity in the fracture propagation is also discussed. The experimental measurements confirm that fracture propagation can occur in steps and high fluid velocity in the fracture is accompanied by a higher pressure drop, which has consequences in field operations. 2. Moving up in length scale from laboratory block tests but covering a smaller relative time scale, pressure falloff response from a Diagnostic Fracture Injection Test (DFIT) was evaluated. A technique is developed iv to determine the spacing between natural fractures from falloff response in a DFIT. Numerical experiments with 3DECTM were carried out to simulate DFIT response in a naturally fractured reservoir. A simplified equation that uses dual porosity reservoir parameters is developed to determine the spacing between natural fractures from a history-matched pressure falloff response. 3. The third large-scale indicator of fracture performance is derived by analyzing wellbore storage following long-term injection and falloff. The field case data from the RRG-9 ST1 injection well in a geothermal reservoir at a Department of Energy (DOE) site in Raft River, Idaho demonstrates the methods to characterize an induced fracture network from the wellbore storage coefficient estimated from the pressure falloff response. These interpretations are constrained by long-term seismic monitoring which tracked the fracture propagation with time over a period of years. Multiple length and time scales have been used to infer relevant properties of natural fractures. Observational techniques are suggested for diagnosing the presence, character, and potential impact of the fractures. |
