Flowing Material Balance

Introduction

The flowing material balance (FMB) is a concept introduced by Agarwal-Gardner [1999]. They showed that plots of dimensional wellbore pressure vs dimensionless cumulative production (scaled to Area) will result in in separate curves for different re/rw. During PSS, they asymptotically merged into a single line with an x-intercept of (1/2) Pi.

The FMB (or normalized rate / normalized cum ) approach applies to both oil and gas reservoirs, and works for constant- or variable-rate systems. Its advantage is its flexibility and ease of use. Its primary drawback is that the resulting analysis plot is not as intuitive as that of the flowing p/z. It has since then been adapted for many novel scenarios in the oil and gas literature.

Single Gas FMB

The classic form of the FMB for gas wells is shown below, and will result in a straight line intercepting OGIP

Where the x variable is

and the y variable is:

Single Oil FMB

For oil, the single phase form is:

Although this form is heavily documented, it has some severe assumptions such as constant fluid properties (I.e non-pressure dependent). This equation does not account for any multiphase effects such as solution gas, water production, etc.

References:

• Applications of Flowing P/z* Material Balance for Dry Coalbed-Methane Reservoirs. K. Morad, C. R. Clarkson, SPE 114995, 2008

• Analysing Well Production Data Using Combined Type Curve and Decline Curve Analysis Concepts, Agarwal, R.G., Gardner, D.C., Kleinsteiber, S.W. and Fussell, D.D., SPE-57916-PA. 1999

• IHS Harmony Enterprise Help Manual, Revised June 26, 2018