Material Balance of A Dry Gas Reservoir

 A reservoir that primarily stores gas is known as a dry gas reservoir. Furthermore, there is no liquid condensate that emerges at the surface; all that is created is gas. Once, we require to take account the reservoir's starting and present states in order to generate the material balance equation for a dry gas reservoir. It is also necessary to able to depict a dry gas reservoir's phase diagram. Evaluate the following states of a wet gas reservoir:


                                               Figure-1 Phase Diagram of Dry Gas Reservoir


Figure-2 States of Gas Reservoir


    It can be seen from te Figure-2 above, at the initial and final pressure, a dry gas reservoir only contains gas and as the fluid goes to surface, only gas is produced at the seperator. In the Figure-1, it shows number "1"  indicates the initial state, number "2" is the final state and "S" means the seperator condition. Also, we consider material balance equation because we know the reservoir states and pressure decline path of dry gas reservoir on the phase diagram above. So, we can begin to remove the terms that do not apply and the results are shown below:

  • W_p = 0 : assume negligible water production
  • N_i = 0: no oil is initially in place
  • G_I = 0: assume no gas injected into the reservoir
  • R_s =undefined: no solution gas evolving out of the oil
  • R_v = undefined: no condensate dropping out of the gas
  • B_o = undefined: no oil expanding to surface
By eliminating the  terms, the material balance equation for a gas condensate reservoir becomes the following: 


 We initially have gas in the reservoir, thus G_{fg}_i is defined. Whether how far the pressure declines, we always have single phase gas in the reservoir. Therefore, two phase gas formation volume factor B_{tg} gets changed to the single phase gas formation volume factor B_g. On the phase diagram, as the fluid goes to the surface, only a single fluid is produced. Therefore R_v is undefined because liquid never releases the gas. Also, B_o and R_s is undefined because oil never expands to the surface.  IF one supposes a volumetric reservoir, the equation shows which terms are eliminated as follows:


  • Assume volumetric reservoir: (B_w-B_{wi}) =0 , c_f = 0, and W_e =0 
After removing terms with the volumetric approximation, the material balance equation for a volumetric gas condensate reservoir is the following:

Overall, material balance equation combined with our knowledge of phase behavior results in a simple expression to describe the production of a dry gas reservoir and we have applied the single macroscopic material balance equation to describe several different reservoir types.

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