As an alternative to conventional proppant pack placement, propped pillar-fracturing promises more effective and conductive fractures by enabling hydrocarbons to flow through open channels. Recent experimental and numerical studies confirmed that viscous fingering phenomena can be used to develop a proppant pillar-fracture type placement: High-viscosity, proppant-laden fluid is placed, and then a low-viscosity clean fluid is pumped to carve pathways through the proppant-laden fluid in a dynamic, continuous process. However, the created channel pattern was found to be significantly dependent on fracture geometry and treatment design parameters such as injection rate, fluid pulsing time, and fluid viscosity ratio.
The objective of this study is to extend the numerical investigation and normalize it to develop a treatment design methodology for constructing proppant pillars throughout the created fracture. A computational fluid dynamics (CFD) model was constructed using commercial CFD software, simulating the flow of fluids inside the fracture and the resulting proppant pillar generation. The study focused on the effects of surface injection rate, pulsing time, and viscosity ratio between the two injected fluids, to develop correlations between all these parameters and the created fracture geometry.
Based on numerical results, the viscosity ratio chosen to achieve the proppant pillars allows for the use of conventional crosslinked fluid without the need for a hindered settling agent. The optimum channel pattern has small channel sizes, remains open under closure stress, creates more channels distributed throughout the entire fracture area, and maintains good communication between unpropped areas. A new dimensionless term, Dimensionless Stage Volume (VSD), is presented to describe the channel pattern inside the fracture. The settling of proppant occurs after the pumping of the job has ended, and prior to fracture closure, yielding wider pillar widths compared to current pillar-fracturing treatment techniques. A combination of conventional proppant and ultra-lightweight proppant can be continuously pumped as part of the treatment schedule. The application of lightweight buoyant proppant positioned within the created channels supports and braces against possible fracture closure between proppant pillars.