In Havlena and Odeh's material balance, m is defined as which ratio?

• A. Volume of gas cap divided by volume of oil
• B. Volume of oil divided by volume of gas cap
• C. Volume of gas cap
• D. Ratio of gas cap pressure to oil pressure

Answer: (A)

In Havlena and Odeh's material balance, you treat the reservoir as two interacting volumes: the gas cap and the oil region. The parameter m is defined as the volume of the gas cap divided by the volume of the oil region. This ratio captures how much gas-cap volume exists per unit of oil volume, which from a modeling standpoint sets how pressure decline translates into changes in both gas and oil volumes. Using this ratio helps linearize or normalize the balance relationships, making it easier to relate produced gas and expanded oil through a single scaling factor that depends on reservoir geometry. The other options don’t reflect the relationship between the two phases—the gas cap volume alone ignores the oil volume, the oil volume alone misses the gas cap’s influence, and a ratio of pressures isn’t about volumes.