In Lesson 3, we went over the basic rock, fluid, and rock-fluid interaction properties used by petroleum engineers on a daily basis. These form the building blocks for reservoir engineering calculations and forecasting procedures. In this lesson, we will discuss how these properties are used by reservoir engineers to predict how oil wells and oil fields behave.

For reservoir engineers, two main concerns are the estimation of the in-place fluids and the estimation of the rates and volumes of fluids produced from the production wells and from the field. For in-place fluid calculations, the Volumetric Method, which is based on static geological data, and the Material Balance Method, which is based on dynamic production and pressure data, are used. Both methods are commonly used in the oil and gas industry today.

For well performance, we will use Darcy’s Law in our analyses. In Lesson 3, we briefly discussed Darcy’s Law for fluid flow through porous media. The multi-phase version of Darcy’s Law (**Equation 3.85**), written for phase “$p$ ” $\left(p=o,\text{}w,\text{or}g\right)$ is:

Where $\frac{\partial {p}_{p}}{\partial l}$ is the pressure drop in the $\u201cI\u201d$ direction, which causes fluid flow. Darcy’s Law governs flow both at the well locations and in the interior of the reservoir. Consequently, this equation will be a fundamental tool for evaluating the performance of individual wells and the reservoir in its entirety.

Finally, we will discuss the application of material balance methods in the reservoir. Material balance is a tool used in many engineering disciplines; however, in this lesson, we will apply it to crude oil reservoirs. Put simply, material balance states that “matter can be neither created nor destroyed.” For our purposes, this implies that any change in mass in the reservoir must equal the mass being removed through the wells. As discussed earlier, material balance can be used for the estimation of the STOOIP (Stock Tank Oil Originally In-Place). In addition, it can be used for estimating the reservoir and field performance. We will discuss several material balance methods for crude oil reservoirs. In the next lesson, we will apply material balance to natural gas reservoirs.