Oil & Gas Focus Group April 2012 GAS DENSITY MEASUREMENT TIME FOR A CHANGE?

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1 Presented by: David Mills Oil & Gas Systems Limited Gemini House The Business Park Ely Cambs CB7 4EA tel Slide 1

2 Current methodology and problems This presentation has resulted from carrying out uncertainty calculations for numerous gas metering systems especially those for the North Sea. For a typical gas metering system the actual volume flow measurement is likely to use:- orifice plate (DP) measurement ultrasonic flow meter (USM) turbine meter (TM). In each of these cases the other measurements will be:- temperature (T) pressure (P) gas composition (mole%) Where the final parameter required is density (ρ) Slide 2

3 The overall system uncertainty is a function of the uncertainties of the individual measurement devices making up that system. Pressure, temperature, differential pressure and flow measurement uncertainties can be (reasonably) confidently assessed. Gas density is a much more interesting question! Gas density can be measured, or it can be calculated from knowledge of the gas characteristics. Measurement normally uses a transducer such as the MicroMotion 7812 (previously Solartron) density transducer. Calculation uses a chromatograph to evaluate the percentage of each of the various gases that go to make up the flowing gas. Density can then be calculated from the gas composition, the flowing pressure and the flowing temperature. Slide 3

4 Basic schoolboy science taught that the density of a gas is directly proportional to the absolute pressure and inversely proportional to the absolute temperature. This comes from the ideal gas laws known as Boyle s law (for pressure), dating from 1662, and Charles s law (for temperature) dating from the 1780s. This gives a simple equation for density:- ρ T, P = k * m. w. * P T abs abs We know that this is a simplification and should be:- ρ T, P = m. w.* P Z * R* T abs abs Slide 4

5 ρ T,P k m.w. P abs Z R T abs Density at operating temperature and pressure Constant Molecular weight Operating pressure (absolute) Compressibility Universal gas constant Operating temperature (absolute) Slide 5

6 All calculation methods use an equation of state. This is just shorthand for a thermodynamic equation describing the state of matter under a given set of physical conditions. We have already seen that pressure, temperature and gas composition can be determined with a sufficiently low uncertainties. The calculation method for Z is the problem! Slide 6

7 A considerable number of equations of state have been developed over the years and used to solve this problem. These include:- NX-19 Redlich-Kwong Peng-Robinson. More recently AGA-8 has been adopted fairly universally for the calculation of vapour phase density for natural gas and has been adopted in ISO Natural Gas Calculation of compression factor. Slide 7

8 The applicable limits are defined as:- pressure 0 MPa 12 temperature 263 K 338 superior cal value 30 MJ/m 3 45 relative density methane 70.0 mole% nitrogen 0.0 mole% 20.0 carbon dioxide 0.0 mole% 20.0 ethane 0.0 mole% 10.0 propane 0.0 mole% 3.5 butanes 0.0 mole% 1.5 pentanes 0.0 mole% 0.5 hexanes 0.0 mole% 0.1 heptanes 0.0 mole% 0.05 octanes+ 0.0 mole% 0.05 hydrogen 0.0 mole% 10.0 carbon monoxide 0.0 mole% 3.0 helium 0.0 mole% 0.5 water 0.0 mole% If the gas and the operating conditions are ALL within these limits the uncertainty is predicted to be ±0.1%. Slide 8

9 Paragraph gives a wider range of applications, but in order to evaluate uncertainty, reference has to be made to Annex E where uncertainty data is given separately for gases with high nitrogen, carbon dioxide, ethane or propane. A summary table does give expected uncertainties for combinations of these gases with higher content than for the basic pipeline quality gas, but there is no data available for gases containing higher amounts of anything heavier than propane. Slide 9

10 Probably the most important statement in the standard is:- The method applies only to mixtures in the single-phase gaseous state at the conditions of temperature and pressure of interest When you consider this statement in relation to gases containing high percentages of ethane, propane, butanes and pentanes, and particularly at low temperatures, it is apparent that ISO does not provide information to enable uncertainty to be evaluated. Slide 10

11 The table below summarises the limits which should be applied when using IS and also shows four different recent cases of North Sea Gases where uncertainty evaluations had to be carried out. Case 1 Case 2 Case 3 Case 4 ISO limits CO <%< 20.0 N <%< 20.0 C <%< C <%< 10.0 C <%< 3.5 C <%< 1.5 C <%< 0.5 C <%< 0.2 P <barg 119 from T <K< 338 Slide 11

12 Cases 1 and 3 are clearly outside the basic limits stated in ISO-12213, for operating pressure. Case 2 and 4 are marginally outside the operating pressure range, but it would not be reasonable to attribute 0.1% uncertainty to these cases as the pressure is only marginally above the limit. Cases 1, 2 and 3 are also outside the limits stated in ISO Annex E as a result of the combinations of gases. Slide 12

13 A possible way forward The AGA-8 DC92 equations are only applicable in the true vapour phase, which means that uncertainties increase as the operating envelope approaches the liquid phase and cannot even be quantified outside a narrow range of operating conditions. In order to overcome this limitation and provide an equation of state that could be used for any phase state, the GERG 2004 equations were developed. This has advanced to GERG 2008, which now covers n-nonane, n-decane and hydrogen sulphide as well as the original GERG 2004 gases which were methane, nitrogen, carbon dioxide, ethane, propane, n-butane, i-butane, n- pentane, i-pentane, n-hexane, n-heptane, n-octane, hydrogen, oxygen, carbon monoxide, water, helium, and argon. Slide 13

14 Density calculated using the methodology of GERG-2008 has been compared with a database of more than 36,000 data points which have an accuracy of better than 0.1%. (GERG TM7 1996) From these results it has been determined that the uncertainty of the equation in gas phase density is less than 0.1% in density over the temperature range from 250 K to 450 K at pressures up to 35 MPa. This uncertainty estimate is valid for various types of natural gases, including, for example, natural gases rich in nitrogen, rich in carbon dioxide, rich in ethane, rich in hydrogen (natural gas hydrogen mixtures), and natural gases containing comparatively high or considerable amounts of propane and heavier alkanes, carbon monoxide, or oxygen. This applies to almost every gas mixture given in GERG TM7. Slide 14

15 GERG TM7 also includes reference data for rich natural gases as given in the next table. The green highlighted data indicates component concentrations which are above the limits for ISO where uncertainty can be assessed when using GERG The uncertainty assessment using GERG 2008 for gases in this table is 0.1% to 0.3% between 280K and 350K and up to 30MPa. Slide 15

16 TM7 Methane Nitrogen Carbon dioxide Ethane Propane Butanes Pentanes hexane RNG RNG RNG A RNG RNG RNG RNG B RNG RNG RNG RNG RNG RNG RNG7-A RNG7-B Slide 16

17 Case 1 Case 2 Case 3 Case 4 GERG 2008 limits note 1 Carbon dioxide mol% 0.0 <%> 8.0 Nitrogen mol% 0.0 <%> 20.0 Methane mol% 0.0 <%> Note 2 Ethane mol% 0.0 <%> 18.0 Propane mol% 0.0 <%> 14.0 Butanes mol% 0.0 <%> 6.0 Pentanes mol% 0.0 <%> 0.5 n-hexane mol% 0.0 <%> 0.2 Pressure barg <299 barg Temperature o C 280 <K< 350 Notes:- 1 limits approximated to maximum values from TM7 table above 2 methane limit is taken as 100% as the binary mixing of methane separately with the other components is modelled with low uncertainty. Slide 17

18 It seems we have a possible way forward for some of the tricky gases. More work needs to be done in quantifying uncertainties for a wider range of gases. It seems that adopting GERG 2008 as the standard method for calculating gas density would be a significant step forward. This would need the flow computer manufacturers to implement this method in their firmware not an easy task! Since this presentation was first prepared the draft DECC Guidance Notes for Petroleum Measurement Issue 8 has been issued:- Slide 18

19 Extract from the draft copy of DECC Guidance Notes for Petroleum Measurement Issue 8 Where the composition lies outwith the expanded limits of AGA8, DECC may require that a new equation of state is derived. New or upgraded systems will be expected to take account of ISO (Table 1) for the treatment of other components outwith the normal AGA 8 component list. Slide 19

20 As discussed earlier it looks as if GERG 2008 is already capable of overcoming the modelling problems associated with some, although probably not all, of these gases. Further theoretical work on assessing the uncertainties associated with gases tricky gases should enable GERG 2008 methodology to be used directly, without the need for developing modelling methods on a case by case basis as seems to happening at the moment. As a final point, ISO part 2, which is intended to make GERG 2008 equation of state an international standard is with working group 13. Current expectations are that will be at least 12 months from now as it is still in the draft stage. Now would seem to be a good time to work towards the use of GERG 2008 for North Sea gas metering. Slide 20