A Stimulation Technique Using Only Nitrogen
Authors: Freeman Earl Ray , Abel James C. , Kim Chin Man , Heinrich Carl 
Published: December 2007

Summary
Nitrogen alone has demonstrated success as a fracturing fluid in reservoirs normally found sensitive to liquid systems. It has proved useful in shales of the Ohio Valley and West Virginia areas, and in similar lithology of the Fort Worth basin located in north central Texas. The fracture efficiency of nitrogen, as related to leakoff and flow capacity testing with no propping agent, has been investigated to analyze the effectiveness of nitrogen stimulation. Also, field data are presented that demonstrate successful results of the nitrogen technique in both oil and gas reservoirs. From the laboratory studies and field results, several conclusions were drawn concerning nitrogen stimulation. Of primary interest is that most of the width reduction in an unpropped fracture will occur in the early stage of production, which indicates a sharp decline of the well tow rate after a relatively short period.

Introduction
Nitrogen has been applied for many years in the stimulation process to enhance recovery of oil and gas. The properties of nitrogen, including its relative inertness, low solubility, and compressibility as well as other advantages, are well-documented. Although gaseous nitrogen was introduced initially in the oil field to assist in drilling operations, it was eventually employed to complement and energize liquid or gelled treating fluids.
In recent years, nitrogen has been used increasingly as a major part of foam fracturing systems. These foams can be prepared with a variety of base liquids and typically contain 60 to 90% nitrogen. Foam treatments are particularly beneficial in low-pressure gas wells and in water-sensitive reservoirs such as the shale formations described in this paper. Indeed, it appears that foam is one of the most suitable fluids for the stimulation of such formations.
However, in certain lithological areas of the Devonian shale formation, namely Washington County (OH), little success with foam has been encountered. Thus, the use of nitrogen alone has been pursued as a less-damaging method of treatment. This stimulation technique, applied to more than 1,000 wells since 1979 has been found to have considerable merit. This paper discusses treatment design, case histories, and laboratory evidence that corroborate present success and encourage further interest in the use of nitrogen alone as a stimulation technique in these formations.

Stimulation History
Formations having a high shale content and natural fractures, such as those found in the Ohio Valley and the Fort Worth basin, have been difficult to stimulate. These formations are characterized by low flowing pressures and low permeability. The presence of these natural fractures is crucial to the economics of the well because they provide the needed drainage system for production. Furthermore, these formations are water-sensitive and have a tendency to clean up slowly following a stimulation treatment. For the ideal movement of fluids, such as is necessary for well productivity, the primary concern is to have a continuously fractured reservoir.
Shale specimens also absorb moisture and upon drying become very brittle and unstable. This, too, is a determining factor in fracture-fluid selection.
JPT
P. 2165^