Experimental Investigation of Fracturing With Propellant in a Large Sandstone Block
Authors: Wilson Adam 
Published: June 2018

This article, written by Special Publications Editor Adam Wilson, contains highlights of paper OTC 27563, “Experimental Investigation of Propellant Fracturing in a Large Sandstone Block,” by Sahil Malhotra, SPE, Peggy Rijken, and Alicia Sanchez, Chevron, prepared for the 2017 Offshore Technology Conference, Houston, 1–4 May. The paper has not been peer reviewed. Copyright 2017 Offshore Technology Conference. Reproduced by permission.

Propellants have been used in oil and gas wells to assist with perforating and creating near-wellbore stimulation. Propellants are electrically ignited at the perforated interval. Upon ignition, they rapidly create a large amount of gas, and the pressurization leads to breakdown of the formation. Hypotheses suggest that the pressurization leads to the creation of multiple fractures. This paper describes an experimental study with a new propellant and aims to understand the pattern of fracture creation with these propellants.

Introduction

The fracture patterns created with propellant can include biwing fractures (in the direction perpendicular to the minimum principal stress), multiple/radial fractures, and explosive fractures, depending on the pressurization rate, as demonstrated in multiple small-scale experiments.

A large-scale block test had been conducted previously to observe fracture patterns with the propellant Arcite 368M. The test resulted in the creation of a dominant biwing fracture in the direction perpendicular to the minimum principal stress. This paper presents an experimental study aimed at comparing the performance of a second-generation propellant with that of Arcite 368M. The goal is to determine if the propellant provides a higher pressurization rate and if multiple fractures can be obtained with the second-generation propellant.

Laboratory Setup and Experimental Procedure

A 30×30×54-in. Colton sandstone block was prepared with a 2-in.-diameter wellbore and loaded into a triaxial test frame. The openhole section of the borehole was etched with five small V-notches (2-in. long), spaced 60° azimuthally and 2 in. longitudinally. A small propellant assembly was centralized in the wellbore adjacent to the openhole section. The propellant assembly, integrated with an ignition system, was sealed with a water- and pressure-tight covering. The propellant charge was 1 in. in diameter and 0.5 in. long. Multiple yard tests in which propellant charges were ignited in a confined chamber were conducted to determine the appropriate size of the propellant. The size of the propellant charge was reduced until the peak pressure was less than 10,000 psi, which had been determined to be the safe limit for the experiment to be performed in the triaxial test frame.

The wellbore was instrumented with a fast pressure transducer (recording at 10 kHz) to record the pressure transient caused by the propellant burn and subsequent fracturing events. In addition, the block was instrumented with six pressure/timing probes (also recording at 10 kHz). The purpose of the probes was to indicate arrival timing of the fracture tip, thus enabling determination of the fracture growth rate and symmetry.