Nitrogen Gas and Sand: A New Technique for Stimulation of Devonian Shale
Authors: Gottschling J.C. , Royce T.N. 
Published: December 2007

Summary
A process has been developed and demonstrated in 17 field tests in which sand proppant is added to a nitrogen gas stimulation treatment. Excepting minor problems with five treatments, all recent applications have been operationally successful. The early production results from two offset wells show decline curve improvement from the nitrogen-gas/sand treatment. Theories based on observed data from well treatments have been developed. Displacement of proppant without the use of water or other liquids is a significant advantage both environmentally and economically. Additionally, the process causes no formation damage, and the well cleanup is rapid. The current major disadvantage is the small amounts of proppant displaced, although improved process design already has shown an increase in sand quantities. A wide variety of stimulation conditions involving flow rates, total nitrogen, number of perforations, and well depths have been tried.

Introduction
The use of nitrogen gas alone as a stimulation treatment has been practiced since 1978. Preliminary investigations into the specifics of this process have been published by several authors. Nitrogen gas has been the most widely used stimulation treatment on the Devonian shale for several reasons. The advantages of using nitrogen gas on the shale are as follows.Nitrogen gas, being inert, is nondamaging to the shale. Problems with clay swelling, clay migration, or oil and water emulsions are eliminated.Nitrogen gas stimulated wells will clean up quickly in most cases. Controlled cleanup will cause minimal permeability damage from fractured formation fines moving to the wellbore.There is no liquid invasion of the microfractures in the shale, thus preventing possible reduction of the long-term productivity of the well.The cost for a nitrogen gas treatment is usually less than for other stimulation methods, which allows a quicker return on invested capital.
These advantages provide a strong case for nitrogen gas stimulation; however, there is one disadvantage that has caused limited acceptance of the technique by some. The lack of any type proppant being used with a nitrogen gas stimulation treatment has provided the strongest argument against it. provided the strongest argument against it. The contention that fracture conductivity cannot be maintained without proppant is well known. Production results showing rapid decline curves Production results showing rapid decline curves are given as corroboration to the lack of fracture conductivity after the nitrogen gas fracture. It has been demonstrated that some natural propping does occur under certain conditions. However, any additional placement of a proppant would improve the expected results from a nitrogen gas fracture. This placement of proppant and the process description thereof are the main concerns of this paper. The three main considerations when adding proppant to a nitrogen gas fracture are (1) proppant proppant to a nitrogen gas fracture are (1) proppant transport capability of nitrogen gas; (2) created fracture geometry capacity to accept proppant; and (3) a method to add proppant to a high-velocity nitrogen gas stream under pressure. The approach selected to investigate these considerations was to perform an experimental treatment on a gas well completed in a shale interval. Before this treatment, preliminary testing was used to determine proppant transport ability of nitrogen gas. The test was set up as shown in Fig. 1. Nitrogen gas was pumped down the 2 3/8-in. [6.0-cm] tubing at velocities ranging from 27 to 120 ft/sec [8.23 to 36.58 m/s] and out the three perforations. Then, 20/40-mesh sand was added to the nitrogen gas stream at the various nitrogen rates. The sand was transported out the perforations at all rates. At 27 ft/sec [8.23 m/s], sand was transported out the bottom perforation only. At 120 ft/sec [36.58 m/s], sand was carried out all three perforations. Visual inspection inside the tubing after pumping stopped showed minimal sand left inside the tubing below the bottom perforation. From this test, it was concluded perforation. From this test, it was concluded that nitrogen gas could be used to transport proppant and, if a created fracture was wide proppant and, if a created fracture was wide enough, the proppant could be displaced into it. The test to determine the creation of adequate fracture width was designed to be an experimental treatment on a well completed in a gas-bearing shale. The treatment was simply to pump a pad of nitrogen gas followed by more nitrogen gas carrying 20/40-mesh sand. Total job size was 1,700 bbl [270.3 m3], with 60% being pad and 40% carrying sand at an average concentration of 0.3 lbm/gal [0.14 kg/m3]. Maximum rate achieved was 20 bbl/min [3.2 m3/s].
JPT
p. 901