Effects of Proppant Selection on Shale Fracture Treatments

This article, written by John Terracina, manager of fracturing technology at Momentive, contains highlights of paper SPE-135502-MS, Proppant Selection and Its Effect on the Results of Fracturing Treatments Performed in Shale Formations, by J.M. Terracina, SPE, J.M. Turner, SPE, D.H. Collins, SPE, and S.E. Spillars, SPE, of Hexion, prepared for the 2010 SPE Annual Technical Conference and Exhibition in Florence, Italy, 19-22 September. In October 2010, Hexion merged with Momentive. The oil and gas industry has strived to provide methods to test the proppants used in shale formation fracturing, but they still do not adequately address many of the factors that impact their effectiveness. There are many factors that occur downhole that need to be considered, such as: Proppant fines generation and migration in the fracture Proppant resistance to cyclic stress changes Effective conductivity vs. reference or baseline conductivity Proppant flowback and pack rearrangement in the fracture Proppant embedment in the fracture face Downhole proppant scaling The hypothesis of this study was that because of the formation characteristics in the three areas studied, curable resin-coated sand (CRCS) with its grain-to-grain bonding technology should provide higher downhole fracture conductivity leading to increased postfracture treatment well production. To verify the hypothesis, laboratory tests outside the traditional long-term baseline conductivity were conducted with proppants and formation core samples from each area. The objective was to more accurately simulate proppant performance under specific downhole conditions of temperature, pressure, fluid, and rock properties pertaining to each area. Proppant Selection Factors Studied Proppant fines generation and migration, as well as proppant flowback, were studied in the Fayetteville Shale of Arkansas. Proppant fines and embedment were studied in the Bakken Shale of North Dakota. And finally, proppant pack cyclic stress, embedment, and scaling were examined in the Haynesville Shale of Louisiana. Proppant Fines Generation and Migration Proppant fines are the small particles that break off from the proppant grain when subjected to fracture closure stress. The small broken pieces reduce pack porosity and permeability, and cause major degradation in the conductivity of proppant packs. When proppant fines migrate down the proppant pack toward the wellbore, they accumulate and reduce flow capacity. Proppant Pack Cyclic Stress When comparing proppants, one factor often overlooked is a proppant’s performance under closure stress changes. The forces of cyclic stress exerted on proppants downhole can cause them to fail. Events often occur multiple times throughout the life of a well, such as shut-ins because of workovers or connections made to a pipeline; in some cases, a well could be shut in because of pipeline capacity. These events lead to cyclic changes in fracture closure stress. This varying amount of pressure and stress can cause the proppants to shift or rearrange, resulting in a decrease in fracture width as well as additional proppant fines and proppant flowback.