PNG 301
Introduction to Petroleum and Natural Gas Engineering

5.0: Lesson Overview


In this lesson, we will discuss Reservoir Engineering for natural gas reservoirs. The tasks performed by reservoir engineers working on gas fields are much the same as those performed for oilfields. These tasks include estimating the original-gas-in-place, or OGIP, analyzing current production rate and pressure trends from the wells and the reservoir, forecasting future performance from these trends, and determining the economic recovery and the Estimated Ultimate Recovery (EUR) of a well, reservoir, or field.

The main difference between natural gas as a produced phase and oil as a produced phase is the highly compressible nature of gas. In many of the methods we discussed for oil reservoirs, we assumed that the oil phase compressibility obeyed the definition of compressibility for a slightly compressible liquid, Equation 3.47:

c o = 1 V o d V o dp ] T=constant
Equation 3.47

Natural gas is a Real Gas, and as such, it will obey the Real Gas Law, Equation 3.27:

The fact that natural gas is a real gas and obeys the Real Gas Law has many significant ramifications to the reservoir engineering tasks that we have discussed. In this lesson, we will use the Real Gas Law to develop our equations for the stabilized inflow performance relationships, the transient diffusivity equation, material balance equation, and decline curve analysis.

Equation 3.27

Learning Objectives

By the end of this lesson, you should be able to:

  • discuss the difference between compressible fluids (gases) and slightly compressible fluids (crude oils);
  • discuss two methods of calculating the Original-Gas-in-Place, OGIP, in a gas accumulation;
  • calculate the OGIP of a natural gas reservoir using the Volumetric Method and Material Balance Method given appropriate data;
  • list and describe the four drive mechanisms associated with gas production;
  • calculate the stabilized production rates from a gas production well given the current flow regime and given appropriate data;
  • list the three formulations used in the stabilized production rates from a gas production well and the pressure ranges where these formulations are appropriate;
  • forecast future production from a natural gas reservoir or production well using the Material Balance Method and Decline Curve Analysis given appropriate data; and
  • estimate the economically recoverable gas and Estimated Ultimate Recovery (EUR) from a natural gas reservoir

Lesson 5 Checklist

To Read Read the Lesson 5 online material Click the Introduction link below to continue reading the lesson 5 material
To Do Lesson 5 Problem Set Submit your solutions to the Lesson 5 Problem Set assignment in Canvas

Please refer to the Calendar in Canvas for specific time frames and due dates.


If you have questions, please feel free to post them to the Course Q&A Discussion Board in Canvas. While you are there, feel free to post your own responses if you, too, are able to help a classmate.