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Hello everybody, I am a reservoir engineering student. I want to know what is the best practical method for calculating the average reservoir pressure for material balance equation in an oil reservoir. I have data for individual wells from build up tests. I know that theoratically we use the drainage volume weighted average, but in practice this is very difficult to perform. Also, how do we account for water injectors? Thank you in advance.
In the practice the pore volume of each well's drainage area is difficult to determine (as you pointed out). It's common to use then a flow rate weighted average as pointed out in Ahmed's book "Reservoir Eng Handbook, 3rd edition"p. 418. Take a look at this reference.
What about injection wells? Do we take them into account? And if so, do we assign a negative sign to their rates? Although I think this doesn't make sense.
I think that for material balance that takes into account water injection you should consider some waterflood calculation method. The best is to start with BUckley-Leverett and then to dig into craigs waterflood method (I think all is very well described in craigs SPE monograph).
Average pressure in a formation for material balance you can extrapolate to the average formation depth, by using hydrostatic gradient. However, if you perform simple material balance calculation (for example, Muskat), and then use the same basic input data for reservoir model with for example 10x10x5 grid, you'll get different solutions, because numerical reservoir model uses different pressures at each layer