Experimental Investigation of Propellant Fracturing in a Large Sandstone Block

Abstract

Propellants have been utilized in oil and gas wells to assist with perforating and creating near wellbore stimulation. Propellants are electrically ignited in the wellbore at the perforated interval. Upon ignition they rapidly create a large amount of gas and the pressurization leads to breakdown of the formation. It has been postulated that the pressurization leads to creation of multiple fractures in the formation. This paper describes an experimental study with a new propellant and aims to understand the pattern of fracture creation with these propellants. The results are also compared to an older generation of propellant tested by Wieland et al. 2006 (SPE 102907). A large scale laboratory test was performed in a sandstone block (30″ × 30″ × 54″) with a 2″ diameter vertical centralized wellbore extending the full block height. The block was loaded in a poly-axial stress frame. A proppellant cartridge was positioned in the center of the wellbore. Small holes were drilled in the rock to intersect the expected primary fracture and instrumented with high resolution pressure gauges to enable fracture timing and growth rate analysis. Anisotropic stresses representative of field conditions were applied on the block and the wellbore was pressurized prior to ignition. The propellant ignition produced an initial peak pressure of 5,790psi in 1.4 milliseconds followed by saw tooth pattern of pressure increase to a maximum pressure of 6,600psi before decaying due to fracture growth and gas leak-off. The block was removed from the test frame and cut vertically and horizontally to examine the fracture pattern generated by the propellant. A dominant planar fracture was observed on either side of the wellbore which propagated in the direction perpendicular to the minimum horizontal stress direction. It was verified that the propellant had much higher burn rate than the propellant tested by Wieland et al. (2006). The large block test provides critical insights and data which can serve as important inputs to calibrate physics based models utilized for modeling propellant ignition and stimulation. The results help in understanding the benefits and limitations of using propellants for stimulation.