Defining petrophysical and mechanical properties of target and barrier zones are key components of the hydraulic fracture modeling process; subsequently, the selection of the detail necessary to accurately model fracture/reservoir performance is challenging. This work investigates whether using detailed petrophysical and mechanical properties provides fracture design parameters that better represent actual fracture behavior and subsequent well performance than a single-layered model.
The approach was to model an existing hydraulic fracture treatment and well performance from a well located in the northern Delaware Basin producing from the lower Brushy Canyon Formation. Models varied from a single layer model with simple-averaged, petrophysical properties to a fine resolution 1-ft model with detailed petrophysical values. Detailed core descriptions were constructed to appropriately represent the thin-bedded and micro-laminated sandstones and siltstones.
In addition, point load tests measured values of fracture toughness for specific lithofacies from 600 to 1100 psi-in½. In comparison, the default value for a sandstone system is 1000 psi-in½. Other mechanical properties, e.g., Poisson’s ratio and Young’s modulus were derived from well logs, and were within typical values.
For the fracture modeling phase, the actual treatment volumes, rates and pressures were inputted into the model along with the measured petrophysical and mechanical properties. Model net pressure was matched with the actual values to verify the output. The dimensionless fracture conductivity (FCD) from the various models ranged from 4.8 to 13.6. The range depends on the variation of lithofacies included in the fine resolution models and their associated mechanical/petrophysical properties. Adding micro-laminated and bioturbated siltstones at the expense of clean sandstone in the finer resolution models resulted in higher permeability, fracture toughness and lower stress gradient.
For the production history matching phase, simulation pressures were significantly overestimated compared to actual measured bottomhole pressures for all single layer models regardless if actual or default mechanical properties were used. The overestimation reflects a threefold increase in pore volume due to the single layer values. For the finer resolution 1-ft model, the simulation pressure was significantly below measured pressure values using default mechanical properties. However, using actual mechanical properties in the 1-ft resolution model resulted in an increase in the FCD due to the decrease in fracture toughness and stress gradient input values. As a result, a very good match was obtained between simulation and actual pressures; indicating the 1-ft model with the measured mechanical properties is a good representation of the actual reservoir system.