FAQs - OilWat®/GasWat®
Questions answered in this document:
Q. Is training available?
A. Yes. Training is held in our Dallas and Houston offices, about 2-4 times a year. It is free for clients who are current on support. Training dates are announced periodically on our web site. On-site training at client's offices is available for a fee.
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Q. In what type of reservoirs can OilWat/GasWat be used?
A. OilWat/GasWat v.8.0/8.01 is capable of performing material balance calculations for the full range of reservoir fluids like:
- Blackoil
- Volatile Oil and Retrograde Gas-condensate
- Wet Gas
- Dry Gas
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Q. What are the material balance formulations available in OilWat/GasWat?
A. Both conventional material balance equation (CMBE) and a new generalized material balance (GMBE) can be used to calculate OGIP/OOIP. This new GMBE can be applied to the full range of reservoirs including volatile oil and gas condensate.
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Q. What is compositional modeling and when would I use it?
A. An important limitation of the CMBE is that it ignores the presence of volatilized oil, which is analogous to but distinctly different from dissolved gas. Dissolved gas is the surface gas contained in the free reservoir oil phase. In contrast, volatilized oil is the stock tank oil contained in the free reservoir gas phase. The compositional model, which uses GMBE, accounts for that volatilized oil. This model handles fluid PVT properties in two ways-- they can be either calculated or entered by the users. Fluid composition must be entered to calculate PVT properties. The calculated PVT properties are a result of the choice of different reservoir depletion/production methods. Each method will yield different properties for cases where the reservoir is in two-phase region. Another key feature of the compositional model is the ability to incorporate field separator conditions. The user can enter up to 4 field separators.
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Q. What is the significance of Rv?
A. The volatile oil-gas ratio (Rv), effectively describes the amount of volatilized oil in the reservoir gas phase and is typically expressed in units of STB/MMSCF. Cook et al. referred to Rv as the " liquid content of the gas" and Coats referred to it as the" oil vapor in the gas". This Rv is a function of the reservoir fluid composition. The volatile oil-gas ratio can cause significant improvements in predictions especially for gas condensate and volatile oil reservoirs since for these reservoirs, there is a significant amount of oil present in the gas phase.
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Q. I have a database with production data. Can I use it?
A. You cannot use a custom database directly with the OilWat/GasWat 8.0/8.01. However OilWat/GasWat 8.0/8.01 is able to read files generated by Microsoft Excel, or files with columnar text data. You can create a new OilWat/GasWat 8.0/8.01 database by selecting the database tab from "Open/Create case" dialog which will save all the information in a Microsoft Access database.
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Q. Can I have pressure and cumulative production history on separate tables?
A. Yes. The current version has the option to have pressure and production history in separate tables. Useful when pressure and production data was recorded in different time intervals.
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Q. Can I input production data on a per well basis?
A. Yes, OilWat/GasWat 8.0/8.01 will allow you to enter pressure and production history on a per well basis and you can add, delete or modify well data.
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Q. What types of compressibility values are required in the "Basic Input Parameters" table?
A. The compressibility values used in the "Basic Input Parameters" table is the total compressibility i,e, hydrocarbon plus formation compressibility together. Water compressibility is entered separately.
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Q. What method should I use to model a particular reservoir?
A. Use the following table as an approximate guide:
|
Type |
Black & Heavy oils |
Volatile oil |
Retrograde Condensate gas |
Wet Gas |
Dry Gas |
|
Rv at saturation point, STB/MMSCF |
<10 |
100-200 |
50-250 |
20-100 |
Negligible |
|
Initial producing GOR, scf/STB |
<1800 |
1500-3200 |
>3200 |
> 15,000 |
>100,000 |
|
Initial stock tank API for liquid |
< 45 |
> 40 |
> 40 |
< 70 |
No liquid |
|
Stock tank liquid color |
Dark |
Colored |
Light color (red / orange/green) |
Near-transparent and colorless |
No liquid |
|
Heptanes + mole percent |
>20% |
20 - 13 |
< 12.5 |
< 4 |
< 0.7 |
|
Field GOR vs. time |
Increasing GOR (till Pb), then decreasing. |
Similar to blackoil |
GOR steadily increases |
GOR is constant |
Not applicable |
|
Field stock tank liquid gravity vs. time |
Decreasing, then increasing |
Constant above dew point, then increasing |
Increase more pronounced than in volatile oils |
Constant |
Not applicable |
|
OilWat/GasWat model to use |
Non-compositional oil or compositional |
Compositional |
Compositional |
Compositional (preferred) or non-compositional gas |
Non-compositional gas. Compositional model for OGIP calcs only (no prediction). |
Also, you may not be able to use a method because of insufficient data available. All of the above assumes a single block of reservoir. For non-compositional gas cases, user can also use the multi-compartment option.
Q. Why do you suggest the use of a non-compositional method for dry gas reservoirs instead of compositional model?
A. It is recommended that the non-compositional gas reservoir option be used, since it is pretty accurate for dry gas reservoirs. It will also let you use different OGIP calculation methods (like P/Z , Roach or Cole methods) which are not available in compositional modeling. In fact the only useful property generated by the compositional model for dry gas reservoirs would be the specific gravity (based on the composition) - and this is allowed through the non-compositional gas reservoir model also. Note that you can model the calculation of OGIP via the compositional model for dry gas reservoirs - only the pressure prediction part is not allowed.
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Q. Sometimes, I do not get much difference in results among the 4 depletion drive cases within compositional models. Why does this happen?
A. This behavior is seen in cases where the production history entered in the Schedule Data is mostly outside the 2-phase region. In such cases, calculations are defaulted internally to the constant composition case. You may see a small difference in results because of the slightly differing behavior in the small production region inside the phase envelope. For cases where you start production inside the 2-phase region and have a significant history, you will begin to see a much more pronounced difference in the results among the 4 depletion methods. Always be aware of the field conditions and that changes in the separator data can cause significant differences in results.
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Q. Do you have any example cases?
A. The program is shipped with numerous examples, many of them from SPE papers and textbooks. These are located under the EXAMPLES subdirectory of OilWatTM/GasWatTM. Chances are very high that you will be able to find a case that resembles your problem.
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Q. How are the examples which were included with OilWat/GasWat organized.
A. Non-compositional oil cases are named example1, example2, etc. This corresponds to the prior version's ".OWV" cases. Non-compositional gas cases are names Gexample1, Gexample2, …, and correspond to prior version's ".GWV" cases. Multi-compartment non-compositional gas cases have names GWIZex1.MB8. All non-compositional examples are found in the "Oil_Gas Examples" directory. All compositional cases are all included in the "Compositional Examples" subdirectory and have names like "composition1.mb8".
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Q. I have only gas component data to C7+. How do I enter this for compositional runs?
A. If you have only data till C7+, enter all the data for the pure components (C1, C2, C3, … till C6). Then, leave C7, C8 and C9 blank (zeros). Enter the data you would have entered for C7+ in the C10+ section. Note that these names (C7+ or C10+) are completely arbitrary. OilWat/GasWat allows a more general formulation because, oftentimes for heavier oils, data entry till C6, followed by C7+ will probably not be sufficient since users have data for heavier ends (C7, C8, C9, C10+ etc) also available for improved accuracy.
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Q. What kind of data can I import from PVTLIBTM?
A. You can import data for non -compositional runs from PVTLIBTM version 4.0. Non-compositional data import is not allowed because OilWatTM/GasWatTM is more accurate. OilWatTM/GasWatTM considers Rv, which PVTLIBTM does not account for. Most of the underlying calculations for EOS etc were derived from PVTLIBTM.
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Q. Can I import / export data from tables to Microsoft Excel?
A. Yes, use the Edit box to do so. You can then re-plot the values according to your needs. This option is available for any Schedule data or PVT data table in OilWat/GasWat 8.0/8.01.
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Q. Many times negative values of OOIP / OGIP or aquifer constants are calculated. What is wrong?
A. Usually, calculation of negative values implies that the method is not applicable to your system. This lack of applicability results in a poor fit that is reflected as abnormal values. All this does not imply an error in the program.
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Q. The legend section in the graphs sometimes overlaps the plotted lines. Is there a way to change this?
A. Yes. Each graph can be custom-tailored to your needs. Use the "Graph Setup" option in the toolbar.
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Q. Can older version files be read?
A. Version 8 can read version 7 files (*.owv, *.gwv formats). It cannot read earlier formats.
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Q. What methods are available for future prediction?
A. OilWat/GasWat uses two methods for future predictions:
- Tarner method-considers two phases (oil and gas)
- Simulation method- considers three phases (oil, gas and water)
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Q. Additional notes
A.
- Tehrani method for calculation of aquifer effects is more accurate than the Havlena-Odeh method. Also, linear increments yield better answers compared to step pressure methods
- Fetkovich method is not included in OilWat/GasWat. The methods included in the program are more accurate. (see discussion in textbook by Dake, Chapter 9). Note that Fetkovich had developed his method to simplify calculations involved with other, more accurate methods.
- PR method gives user more options to calibrate the compositional results than the RKS option. If no calibration is needed, both methods will give better answers than the convergence pressure method.
- For non-compositional oil cases, it is strongly recommended that the user input Bob and Pb values in the PVT section. This significantly improves the accuracy of the calculated PVT properties.
- It is recommended that the user take advantage of the option to recalculate bubble point (via the "Use variable bubble point" checkbox.). This has been shown to also significantly improve accuracy of the results.
- Multi-compartment gas reservoir modeling (G-WIZ) option is valid for non-compositional gas runs only.
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Q. How long can I use the application on a trial basis?
A. With a trial license you can use our fully functional software for 15 days.
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Q. What is the best method to register my product?
A. The best way to register your application is, what's best for you. Our phones have basic coverage from 8:00am to 5:00pm, US Central time. You are welcome to leave a voice mail, or you may send e-mail to, register.psg@ihsenergy.com. What ever way you choose, please be prepared to give us your 4 digit user-id and 3 digit registration number.
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Q. I recently switched from Windows 95/98 to Windows NT/2000, and my bitlock application no longer works, what happened?
A. Windows NT/2000 has a different internal architecture than Windows 95/98/ME, and requires a special driver to be installed before accessing the bitlock. We have included bitlock drivers and the utility required to install them with your application product in a self-extracting zip file named "Rainbow.exe".
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Q. Can I move the application after installation?
A. You can move your application after installation, only if you have a bitlock or you unregister and uninstall the application. To receive a new installation password, you must present the unregistration number.
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