Field Applications of Cryogenic Nitrogen as a Hydraulic Fracturing Fluid
Authors: McDaniel B.W. , Grundmann Steven R. , Kendrick William D. , Wilson Dennis R. , Jordan Scott W. 
Published: December 2007

Abstract
New techniques allow liquid nitrogen to be safely delivered to a moderate-depth formation at typical fracturing rates and at cryogenic temperatures (−320 to −232 F) while protecting the casing from damage. This process provides a high degree of thermal shock to the reservoir rock, creating adequate physical alteration of the fracture walls to prevent closure of hydraulically and/or thermally induced fractures. Additionally, thermal stress-induced microfractures that are orthogonal to the fracture plane will also occur. In general fracturing applications, severe thermal shock could seldom be achieved other than in a few applications of fracturing geothermal (nonhydrocarbon-bearing) reservoirs. The results of this field project indicate that the use of cryogenic nitrogen in refracture applications appears to be successful in reducing the damage from gel filter-cake residue of earlier fracturing treatments. There has been no evidence of any casing damage. This process has not been used on a nonfractured hydrocarbon zone.

Introduction
Thermal shock has been previously applied as a means to alter the physical conditions of reservoir rock and to stimulate hydrocarbon production. Before the development of techniques described in this paper, the maximum cooling effects that could be achieved were limited to those that could be obtained by pumping chilled brines &gt;20 F) or liquid CO2 &gt;0 F). Liquid nitrogen has a boiling point of −320 F at atmospheric pressure. Carbon steel alloys normally used for surface iron manifolding, wellhead configurations. and wellbore tubulars cannot withstand even very short-term exposure to cryogenic temperatures.
For the new procedure, construction of special (all stainless-steel) surface piping, manifolding, and wellhead components prevented thermal contraction problems, and the use of free-hanging fiberglass tubing afforded protection to the casing from thermal shock damage. Four coalbed methane (CBM) wells (Wells A, B, C, and D) and a tight sandstone reservoir (Well E) were successfully fracture-stimulated with the use of cryogenic-nitrogen treatments. Standard oilfield nitrogen pumping units were modified to deliver either high-pressure liquid nitrogen or vaporized (warm) nitrogen gas. A technique for downhole diversion from one zone to another was also developed so that a second stage could treat a different part of the reservoir. Refracture treatments have been performed on five wells using these new techniques. Initial postfracture response was very good in all five wells, but only two wells appeared to provide long-term production enhancement. This process has not yet been applied as an initial stimulation treatment on a new well, because the operator was not drilling any new CBM wells at the time of the project. To protect the secrecy of the cryogenic-nitrogen treatment, the operator did not want to test the process on wells that it owned partly with other parties. Consequently. only wells that were wholly owned by the operator were chosen for the project.

Observed Thermal Effects on Coal
While a search was undertaken for a chemical. additive, or fluid that might have an advantageous effect on gas production from tight, low-rate CBM wells, coal samples were subjected to contact with and submerged in liquid nitrogen. The effects observed were that audible cracking sounds were heard while the samples were cooling down and warming back up. Measurements of the samples indicated significant shrinkage while cold. When competent coal samples came in contact with liquid nitrogen, the samples fractured and separated into smaller cubical units. Each time a coal sample contacted liquid nitrogen. the sample would break into smaller cubical units; repeated contact caused coal samples to continue to break into smaller cubical units. These observations were made at atmospheric pressure.
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