This paper describes a series of tests designed to evaluate how perforating variables such as penetration, number of charges, phasing, and orientation affect fracture initiation. Average Permian basin conditions were used in this laboratory test study. In this basin, the vertical in-situ stress is significantly larger than the horizontal stresses. This stress regime increases the chances of axial fracture initiation and high near-wellbore fracture tortuosity. Optimizing these perforating variables can result in lower breakdown and treatment pressures, a reduction of near-wellbore fracture tortuosity, and, ultimately, more clusters with productive hydraulic fractures.
Over a dozen perforating and hydraulic fracturing tests were performed in a polyaxial stress frame to simulate a horizontal well in the minimum stress direction in the field. A special concrete blend mixture with a compressive strength of approximately 8,000 psi was used to make the 18×18×18-inch blocks. A 2.9-in. wellbore, 2.38-in. casing, 1.56-in. gun, and shaped charges with 1-, 2-, and 6-in. penetrations were used to scale the field conditions. Gun phasing included spiral, in-plane, and oriented. The casing was cemented under stress equivalent to field conditions of 1,000-psi overbalance. The fracturing fluid viscosity and pumping rate were designed using dimensional analysis and numerical simulations to replicate a typical hydraulic fracturing operation in the field to produce similar fracture opening width and near-wellbore behavior. This led to fracturing with glycerin at 10 cm3/min. The test results clearly show that different perforating strategies affect fracture initiation geometry and pressures: 1) In-plane perforating enhances transverse fracture initiation. 2) Spiral perforating resulted in multiple transverse fractures. 3) Oriented perforating in the vertical direction produced axial fracture initiation with medium breakdown pressure and a medium propagation pressure. 4) Oriented perforating in the horizontal direction produced transverse fracture initiation with high breakdown pressure and low propagation pressure. 5) A perforated openhole test representing poor cement conditions showed low fracturing pressures with mixed axial and transverse fractures. 6) Larger penetration helped reduce breakdown pressure.
This study shows how operators can use laboratory testing to optimize their unconventional perforating strategy, reducing fracturing pressures and near-wellbore tortuosity, which can lead to more efficient fracturing operations. Very few published hydraulic fracturing laboratory studies included properly scaled fracturing parameters, casing cemented under stress, and perforations made by real shaped charges.