Measurement of CO2 Flow in Its Supercritical and Gaseous Phases
- F.G. Oakes (Amoco Production Co.) | J.T. White (Amoco Production Co.)
- Document ID
- Society of Petroleum Engineers
- SPE Production Engineering
- Publication Date
- August 1987
- Document Type
- Journal Paper
- 209 - 217
- 1987. Society of Petroleum Engineers
- 4.2 Pipelines, Flowlines and Risers, 4.1.3 Dehydration, 4.2.3 Materials and Corrosion, 4.3.4 Scale
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Production of CO2 and its injection for tertiary recovery of hydrocarbons has produced a need for an accurate method, adaptable to microcomputer technology, for orifice measurement of CO2 fluid flow. A method meeting that need has been developed and successfully implemented. The accuracy of the method for orifice measurement of both gaseous and supercritical CO2 has been verified at the temperature and pressure conditions experienced in field operations. Details of the measurement method and its verification are presented.
The flow rate of any single-phase fluid through an orifice is proportional to the density (and related to the viscosity and ratio of specific heats) of the flowing fluid and the pressure differential across the orifice plate. Mathematical methods are available and applicable for calculation of all three of these physical properties as a function of pressure and temperature. While the work presented here is directed to the measurement of pure CO2, the method is applicable to measurement of any single-phase fluid, provided suitable models are available for calculation of the values of the three physical properties.
Until recently, CO2 in the oil field was nothing more than a contaminant in produced hydrocarbon streams. It hindered accurate measurement, lowered Btu content, and contributed to corrosion problems. With the advent of large-scale programs that use supercritical fluids for hydrocarbon extraction, however, CO2 changed from a nuisance to an important fluid. To supply CO2 needs for tertiary operations in west Texas, Amoco Production Co. operates a producing CO2 field and dehydration/compression plant near Clayton, NM. CO2 produced from wells in the field has a purity exceeding 99.9%.
Measurement of the CO2 as a gas is required at the individual wells. Measurement as a supercritical fluid is required at the pipeline inlets and at various points along the pipeline where deliveries are made. Tertiary injection operations also require measurement of supercritical CO2.
Measurement of hydrocarbons, both liquids and gases (vapors), is routine throughout the oil and gas industry. Measurement of fluids other than hydrocarbons and water is not unknown, but is far from routine. The need for measurement of any supercritical fluid under normal field operations was unknown until fairly recently.
Thermophysical-property models are used routinely for prediction of the various properties of fluids as a function of pressure and temperature. The measurement method developed and presented in this paper implements the predictive capabilities of thermophysical-property models in microcomputers for the measurement of pure CO2. To our knowledge, this implementation is unique in the industry.
Orifice Mass Flow Measurement
The equation for the calculation of the true instantaneous mass flow rate of a fluid through an orifice applies the discharge coefficient, the gas expansion factor, and the thermal expansion factor to the equation for the theoretical mass flow. This equation is applicable to all single-phase fluid flow.1 In customary units the equation is
In SI units Eq. 1 is
Calculation of an instantaneous mass flow rate with Eq. 1 requires that three physical parameters be measured at or near the orifice: (1) static pressure, either up- or downstream of the orifice; (2) the differential pressure across the orifice; and (3) the temperature of the flowing gas. In addition, three physical properties of the flowing fluid must be known; density, viscosity, and ratio of the specific heat at constant pressure to the specific heat at constant volume. Densitometers are available for direct measurement of density. These devices are expensive, however, and at best are difficult to calibrate. Also, their accuracy and reliability in supercritical CO2 is subject to debate.
Direct measurement of the viscosity and specific heat ratio is not feasible. Unless the values for these properties are available, approximations must be accepted for them. The assumption of values near the middle of the "window" of normal operating values for temperature and pressure can introduce measurement errors in supercritical CO2 approaching 0.4%. The error becomes greatest as the static pressure approaches the critical pressure, at elevated temperatures, and at low differential pressures. In an actual operating environment, this error is not likely to be random. Instead, it will most often appear as a bias because operating conditions are not likely to be uniform throughout the entire window of temperature and pressure values.
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