During hydraulic fracturing, reliable characterization and modeling of friction pressures in casing as a function of flow rate, friction reducer (FR) type, FR dosage, and casing ID is critical to optimizing frac designs. However, measuring bottomhole pressures during hydraulic fracturing is challenging and often, casing friction pressure estimates are made with wellhead pressures along with step-down tests. These estimations can be inaccurate due to difficulties associated with decoupling casing friction pressures from perforation and near-wellbore pressure drops. A novel casing friction pressure model for FR is presented in this study and validated with field data.

The new multivariable friction model is based on lab FR rheology measurements and pipe velocity profile derivation of FR fluids. Step-down tests were implemented with 6 different FRs at various dosage rates to obtain friction pressures for a range of conditions. The friction pressures were estimated along the lateral of the well using innovative bottomhole gauges installed at two distinct locations on the casing string and were compared to our model to validate the accuracy.

Casing friction pressures between the wellhead gauge and the two BHGs were measured during various stages of a frac job. Six FRs were tested at concentrations ranging from 0.4-1 gpt at flow rates of 10 to 100 bpm. For each of these conditions, friction pressure data were collected for flow through various IDs in the 3-string casing design. With lab measured rheological properties and field data, the casing friction pressure model was successfully calibrated for each FR. The validated casing friction model was utilized with step-down tests to calibrate perforation friction pressure equation utilizing the kinetic energy erosion model and near-wellbore friction pressure equation. The three calibrated equations for friction components along with ISIP successfully estimated wellhead pressures during pumping for an entire stage as well as stages from toe to heel of 2-mile lateral. Some significant findings discovered from this study: less severe mechanical degradation effects observed in the field compared to lab flow loop measurements, casing friction pressure reductions plateau around 1-1.5 gpt of FR, and high-viscosity friction reducers typically possess higher casing friction pressures due to its higher viscosity.

The novelty of this study is the newly developed casing friction model for frac fluids with FRs. The model results in three main benefits. First, it allows for accurate estimation of FR performance in terms of casing friction pressures to optimize FR selection and dosages. Second, more precise estimation of casing friction pressures allows for evaluation of perforation and near-wellbore friction pressures as well as net pressures during pumping from just wellhead pressures coupled with a step-down test. Third, technical and economic feasibility studies can be performed to evaluate changes in lateral length and casing size designs which mainly impacts casing friction pressures.

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