P/z Abnormally Pressured Gas Reservoirs

Introduction

The P/z vs G_p plot is a graphical method commonly used to obtain an OGIP in a gas reservoir. In normally pressured reservoirs with a pressure gradient between 0.43 and 0.50 psi/ft, this approach works very well.

However, in certain petroleum producing areas, pressure gradients can be much higher at values of 0.85 psi/ft. In such scenarios, the gas compressibility can actually approach formation comrpessibility resulting in a non-linear behavoir of the P/ z vs G_p as shown below.

Discussion

Harville and Hawkins (1969) attributed the concave-downward shape of the P/z vs Gp curve to both pore collapse ad formation collapse. Based on work on North Ossum Field (Lousiana), They stated that:

• The initial slope is a result of the continous increase in net overburden pressure (as pore pressure declines with production) which causes rock failure (I.e. Rock Collpase) which manifests as continous decrease in formation compressibility.

• This above process continues until formation comperssibility eventually reaches a “normal
  value” marking the beginning of the second slope. At this point, the reservoir performance
  becomes similar to that for a constant-volume, normally pressured, gas reservoir system.

Ramagost and Farshad stated that neglicting rock and water compressibiity in the conventional P/z extrapolation can lead to significant error in OGIP estimates

Linearization Approaches

As discussed in conventional material balance, the basic gas material balance equations are given below:

The base material balance equation used for analyses in conventional reservoirs is shown below:

and:

If we define total sytem compressibility as shown below,

We then have the following which creates a linear relationship accounting for compressibility effects

Plotting the above results in a straight-line with an x-intercept at OGIP. However, c_t is unknown and will require an interactive approach to plot the y-data.

See Also:

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References:

• Tarek Ahmed, Nathan Meehan, Advanced Reservoir Management and Engineering, Gulf Professional Publishing