Carbon Dioxide - A Multipurpose Additive for Effective Well Stimulation
Authors: Crawford H.R. , Neill G.H. , Bucy B.J. , Crawford P.B. 
Published: December 2007

CRAWFORD, H.R., THE WESTERN CO. DALLAS, TEX. JUNIOR MEMBER AIME NEILL, G.H., THE WESTERN CO. FORT WORTH, TEX. BUCY, B.J., THE WESTERN CO. FORT WORTH, TEX. MEMBERS AIME CRAWFORD, P.B., TEXAS A and M COLLEGE COLLEGE STATION, TEX. MEMBER AIME

Abstract
This paper presents the physical and chemical properties of carbon dioxide whic h form the theoretical basis for its use in well stimulation. The means of applying this unique chemical in fracturing and acidizing treatments are described, and some results of its use in the field are given.

Introduction
Carbon dioxide is normally associated with carbonation of soft drinks and the extinguishing of fires. Now, however, its unique physical and chemical characteristics have been utilized as a multipurpose additive for well stimulation fluids.
Some of the advantages which can be expected from the use of carbon dioxide are as follows:
eliminates swabbing in most cases; provides rapid clean-up which helps remove muds, silts, etc.; removes or prevents water and emulsion blocks; retards acid reaction with formation; helps prevent clay swelling and precipitation of iron and aluminum hydroxides; reduces friction loss of oil-base fluids; and increases permeability of carbonate formations.
This paper presents the physical and chemical properties of carbon dioxide which form the theoretical basis for its use in well stimulation. The means of applying this unique chemical in fracturing and acidizing treatments are described and some results of its use are given.

Physical Properties
Carbon dioxide (CO2) is familiar in all of its physical forms-gas, liquid and solid (dry ice). Its low boiling point is one of the highly desirable qualities utilized in fracturing and acidizing operations.
Fig. 1 is a pressure-enthalpy-temperature chart for CO2. The right-hand side represents the gaseous region, the upper left-hand is the liquid region and the lower left represents solid CO2. The area under the curve represents the two-phase region. The upper portion is liquid-gas and the section below 75 psia represents solid-gas equilibria.
This chart shows that, for a pressure of 300 psi (a normal pressure maintained when transporting liquid CO2), the temperature of the liquid will be about 0 F. At atmospheric conditions CO2 exists as a colorless, odorless gas which occupies about 8.57 cu ft/lb.It may also be seen from Fig. 1 that, at temperatures above 87.8 F, pure CO2 will exist as a gas regardless of the pressure applied. Fig. 1 also shows that at pressures below 75 psia liquid CO2 cannot exist and only solid or gaseous CO2, is possible. The solid (dry ice) sublimes directly to gaseous CO2.Fig. 2 shows the specific gravity of saturated liquid CO2. For example, at 0 F the specific gravity of liquid CO2 is about 1.0, or the same as water.
Fig. 3 shows the specific heat of saturated liquid CO2 as a function of temperature. For example, at 0 F the specific heat is about 0.5 Btu/lb F, and at 75 F the specific heat is about 1.0 Btu/lb F. This chart is useful when calculating the temperature of a combined stream of water or oil with carbon dioxide. Fig, 4 gives the compressibility factor z for gaseous carbon dioxide. This is used to calculate gas densities from the equation(1)
For carbon dioxide,
PV = 0.243 zMT where P = pressure, psiaV = volume, cu ftM = weight, lb, andT = temperature, deg. R.
Solubility of CO2 in water (from Dodds, et al ), as a function of pressure and temperature, is given in Fig. 5. The solubility of the gas in brines is less than in fresh water. This solubility may be obtained by multiplying the solubility in fresh water obtained from Fig. 5 by the correction factor obtained from Fig. 6.The solubility of CO2 in oils is not so well known, but Holm' and Beeson and Ortloff have published some recent data. These are given in Fig. 7. The volumetric behavior of CO2 dissolved in oil is presented in Fig. 8.
JPT
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