The gas cap of the St. John Oil Field had a bulk volume of 17, 000 ac-ft when th

Question

The gas cap of the St. John Oil Field had a bulk volume of 17, 000 ac-ft when the reservoir pressure had declined to 634 psig. Core analysis shows an average porosity of 18% and an average interstitial water of 24%. It is desired to increase the recovery of oil from the field by repress ring the gas cap to 1100 psig. Assuming that no additional gas dissolves in the oil during repress ring, calculate the SCF required. The deviation factors for both the reservoir gas and the injected gas are 0.86 at 634 psig and 0.78 at 1100 psig, both at 130 degree F. If the injected gas has a deviation factor of 0.94 at 634 psig and 0.88 at 1100 psig and the reservoir gas deviation factors match those presented in (a), recalculate the injected gas required. Is the assumption that no additional solution gas will enter the reservoir oil a valid one? Considering the possibility of some additional solution gas and the production of oil during the time of injection, will the figure of part (a) be maximum or minimum? Explain. Explain why the gas deviation factors are higher (closer to unity) for the injected gas in part (b) than for the reservoir gas.

Explanation / Answer

a) SCF for the reservoir,

Vs = Va * Fp * Tm (Fpv)2

for Va = 1 cubic feet

   (Fpv)2 , Super Compressibility Factor = 1

  Fp = (Line Gauge Pressure + Atmospheric Pressure)/Base Pressure

= 634+50/ 14.73 = 46.43

Tm =   [459.67 + Standard Temperature(deg F)]/[459.67 + Line Temperature (deg F)]

= 459.67 +134 / 459.67 +134 = 1

so , Vs = Fp = 46.43 scf

c) any fluid and and gas can enter in the reservoir oil by percolation or on the basis of rock properties. if rock will be permeable than gas can be enter in the reservoir otherwise not. entering of the fluid or gas in the resrvoir also depend on the physicochemical properties of fluid.

d) again we wil consider the properties of rocks for the additional solution in the reservoir, like permebility and porosity of rock. if upper layer of the reservoir will be permeable and rocks of upper layer rocks having a porosity then there is a lot of chances adding the solution in the reservoir gas during the injection. weather conditions is also important for the additional of solution because of weathering and erosion through weather conditions create the fractures in the rock which are the way to the reservoir gas for a external solution.

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