A Comparison of Hydraulic and Propellant Fracture Propagation in a Shale Gas Reservoir

Abstract

Abstract This paper describes laboratory and field experiments that were conducted to compare hydraulic and propellant fracturing techniques in the Mancos Shale of Colorado. The Mancos is a Cretaceous shale approximately 2,000 feet thick(1). Although mainly considered a source rock for other formations, the Mancos is also productive in certain areas. To better understand how stimulation technologies might be used to commercially develop the shale, four unique data sets were compared including results from both laboratory and field hydraulic and propellant fracturing. The results of this work are presented and include the following conclusions: the propellant fractures in the block test were constrained by relative strengths of the strata; the layering of the Mancos shale resulted in better height containment than expected in field test stimulations, although the mechanism of containment in the field tests is still not fully understood; and both propellant and hydraulic fracturing can provide stimulation benefits, but only when applied in appropriate situations. Introduction The majority of all oil and gas wells drilled in North America are completed with some form of stimulation. Stimulations are used to bypass drilling-induced damage and increase effective wellbore radii, thus, increasing well productivity. Stimulation types include hydraulic fracturing, acid fracturing, matrix acidizing and propellant fracturing. The majority of stimulations performed worldwide are hydraulic fracture stimulations. Hydraulic stimulations were first experimented with in the 1940s and the technologies, such as fluid chemistries and proppant design, have been significantly improved(2). However, hydraulic stimulations are not necessarily optimal in all situations; they require the introduction of foreign fluids into the reservoir which can often cause adverse chemical reactions. Additionally, there is a substantial added cost to hydraulically stimulating wells, which has significantly increased in the past few years. Propellant fracturing, a relatively new technology, is an alternative to hydraulic fracturing. It can be fairly inexpensive and does not involve pumping fluids into the reservoir. The process uses a combined solid fuel and oxidizer to create the rapid expansion of gas to rapidly generate pressure and drive fractures out into the rock. Unlike explosives, which were used as one of the earliest forms of stimulation, propellant fracturing causes little plastic deformation of the near wellbore region, which can be adverse to flow(3). A significant disadvantage to propellant fracturing is that it does not carry proppant into the fracture. Instead, propellant fracturing relies upon shear slippage or spalling to prevent the fracture from fully closing back on itself, leaving a conductive path back to the wellbore. Little research has been published on propellant stimulations due to their somewhat limited applications. This paper describes a unique research project in which propellant and hydraulic stimulations were tested in the same reservoir in both laboratory and field tests. The research was conducted at the Colorado School of Mines, Golden, Colorado, between 2005 and 2007. Laboratory experiments were conducted on campus as well as at the TerraTek laboratory facilities in Salt Lake City, Utah, and learnings were applied to field tests in western Colorado.