Field and Laboratory Results of Carbon Dioxide and Nitrogen in Well Stimulation
Authors: Neill G.H. , Dobbs J.B. , Pruitt G.T. , Crawford H.R. 
Published: December 2007

Abstract
Carbon dioxide and nitrogen have both proven to be useful aids in will stimulation. Laboratory data are presented showing the effect of carbon dioxide on foaming agents, corrosion, reaction rate of hydrochloric acid, fluid-loss additives and clay swelling. Carbon dioxide is generally beneficial for all of these except the fluid-loss additives. The corrosion rate of carbonated water is very low compared to inhibited hydrochloric acid. A chart of the viscosity of carbon dioxide is presented. If is estimated that carbon dioxide can reduce friction loss of oil-base fluids by 29 to 60 per cent. Individual field results and conclusions from other summaries are presented. Both nitrogen and carbon dioxide are effective in removal of stimulation fluids. Carbon dioxide has proven useful in removing water or emulsion blocks.

Introduction
The use of nitrogen and carbon dioxide in well stimulations has grown rapidly in the past two years. The uses and advantages of these gases have been described previously for well stimulation, testing and cementing programs. Because of the differences in physical and chemical properties between nitrogen and carbon dioxide, one gas is usually better suited than the other for a specific application. Generally speaking, nitrogen is superior in low injection rate applications and when precise volume control is critical. Carbon dioxide, on the other hand, is better adaptable to high rate fracturing and acid treatments.
Gases were introduced to the oil and gas industry primarily as an aid to recovery of stimulation fluids. This application still accounts for the major usage of nitrogen and carbon dioxide. Special applications, however, which utilize specific properties of the gases, are being discovered continually. The development of these methods is opening the door to better controls over well performance.

Effect of a Foaming Additive with Nitrogen and Carbon Dioxide
By the nature of the solubility-pressure relationship of carbon dioxide, an induced solution-gas-drive mechanism is created when the pressure is lowered and the gas comes out of solution. To demonstrate this effect the apparatus shown in Fig. 1 was constructed. The 160-cc cell was filled to 100 cc with the fluid to be tested, a gas pressure (nitrogen or CO2) of about 800 psi was applied and allowed to come to equilibrium. The valve was then opened and the amount of liquid carried over was measured in a graduated cylinder. These tests were also conducted using various amounts of foaming additive to see if the additive would enhance the recovery. The results of these tests are given in Table 1. As expected, the recovery of fluids was substantially greater when using CO2 than when using nitrogen. For example, at 80F the recovery with CO2, and no foaming additive was 40 per cent, while with nitrogen it was essentially zero. The addition of the foaming agent increased the recovery substantially. With CO2 the recovery increased from 40 per cent to 70 per cent when 0.2 per cent foaming additive was used.