Units and Nomenclature

Part 1, Chapter 1: Foundations and the Subsurface

Learning objectives

Recognize the common oilfield and SI units for the key reservoir quantities
Convert between them and avoid unit-mismatch errors
Develop a feel for typical reservoir values and their qualitative classes
Read the standard nomenclature used throughout the course

A Field That Speaks Two Languages

Reservoir engineering grew up in the oil patch, so it still speaks in barrels, pounds per square inch, and millidarcies, even as the science around it uses SI. You must be fluent in both and comfortable converting, because a unit slip can turn a good decision into a costly mistake. The classic quantities are permeability (millidarcies, mD), pressure (psi or bar), volumes (barrels or cubic meters), flow rate (barrels per day), viscosity (centipoise, cp), and density, which is often quoted as API gravity, where $\text{API} = \frac{141.5}{\text{SG}} - 131.5$ , so lighter oil carries a higher API number.

Build a Feel for the Numbers

Knowing the conversion is only half of it; the other half is knowing what a number means. Is 5 mD a good permeability? (It is poor, nearly tight rock.) Is a 20 cp oil heavy? (It is only medium; heavy oils run to thousands of cp.) The widget below lets you sweep each quantity across the range a real reservoir spans and names the qualitative class, so the vocabulary turns into intuition. Drag the value and watch where it lands and what it is called.

The Standard Nomenclature

A few symbols recur throughout the course. STOIIP (stock-tank oil initially in place) and GIIP (gas initially in place) are the volumes in the ground. N/G is net-to-gross, the fraction of the rock that is reservoir quality. The saturations $S_w$ , $S_o$ , and $S_g$ are the pore fractions filled by water, oil, and gas, and they sum to one: $S_w + S_o + S_g = 1$ . Porosity is $\phi$ and permeability is $k$ . We define each carefully as it arises, but it helps to recognize them now.