Material balance evaluation can be conducted at every stage of reservoir development, including production rejuvenation using horizontal wells. Material balance evaluation for a reservoir with water influx usually begins with the calculation of water influx quantities using aquifer models (the aquifer model approach). The calculated influx quantities are substituted into material balance equations that are arranged in straight-line formats. Should a linear slope be found, the original-hydrocarbons-in-place is the y-intercept. The key to a successful evaluation using the aquifer model approach lies in the selection of a representative aquifer model. Selecting a representative aquifer model is difficult due to reservoir uncertainties such as rock properties, and the size, geometry, and continuity of the aquifer. Also, influx calculations using the aquifer model approach are time-consuming because they usually require a trial and error method of solution.

This paper presents a material balance equation in a form that allows for calculating water influx without the use of an aquifer model. This approach has many applications and is particularly useful for production rejuvenation projects, such as the drilling of horizontal wells.

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