Material Balance Equations in Oil or Combination Reservoirs

When discovered, a reservoir may contain oil, gas, and water that can be intermingled or segregated into zones. As described earlier, recovery may be caused by solution gas drive, water drive, gas cap drive, or a combination of these mechanisms. A general material balance equation should be capable of handling any type of fluid distribution and any drive mechanism. From the compressibilities given in the first section, water and formation compressibilities are less significant where there is appreciable gas saturation such as in gas reservoirs, gas cap reservoirs, and in undersaturated reservoirs below the bubble point.

Because of this and because of the complications they would introduce in already complex equations, water and formation compressibilities are generally neglected, except in undersaturated reservoirs producing above the bubble point. Gas in solution in the formation is small and also generally neglected. One general material balance equation, the Schilthuis equation, is a volumetric balance stating that the sum of the volume changes in oil, gas, and water must be zero because the reservoir volume is constant.

Although connate water and formation compressibilities are quite small, relative to the compressibility of reservoir fluids above their bubble points they are significant and account for an appreciable fraction of the production above the bubble point. In cases when compressibilities can be important, Equation 5-150 can be written as:

Material balance equations for various drive mechanisms and different initial conditions are summarized in Table 5-31. Note that in this table B is expressed in ft3/scf, and the conversion factor, 0.1781, is the reciprocal of 5.61% ft5 per barrel.