Science & Technology: PETROLEUM RESERVOIR PERFORMANCE OVERVIEW

PETROLEUM RESERVOIR PERFORMANCE OVERVIEW

HYDROCARBON RESERVOIR

In this article we are going to examine the engineering work, required to analyze, and evaluate the characteristics of a petroleum reservoir. in talking about hydrocarbon reservoirs, this book explain some terms and definitions that will help us understand the scientific principles of how reservoirs are produced and then describe the fluid systems and their characteristics. After that we are going to examine the different types of reservoir recoveries and the drives that are used to bring hydrocarbons to the surface, we will end by defining oil reservoirs, describing how they are calculated and explaining their significance in predicting future production.

Let’s begin by describing what a reservoir is, a reservoir holds fluid that is trapped underground in porous permeable Rock.

Reservoir

The porous rock is rock with microscopic holes where fluid can accumulate, and permeable Rock is rock with channels that connect the holes allowing the accumulated fluid to flow, as you know a fluid is defined as a substance that changes its shape to whatever container it's in.

The porous rock

The permeable rock

Accumulated fluid to flow in rock

When we talk about a fluid in a reservoir, it can be either a gas or a liquid , both meet the definition of a fluid. there are three basic fluids that accumulate in porous permeable rocks, they are water, oil and gas.

Water, oil and gas in Reservoir

Obviously the two fluids we are interested in are the hydrocarbon gases and the hydrocarbon oils, but unfortunately they are found in commercial quantities in only about 1% of 1% of underground reservoirs throughout the world. It is the task of the Geoscientists and the Petroleum engineers to find that 1% of 1% as efficiently and as cheaply as possible to do that, they must have a thorough understanding of the Earth's formation, its structures, and the history of its geological features, and how they changed over time. They must also be able to apply sound scientific principles pertaining to heat pressure, and the flow of fluid within a reservoir. Before proceeding, it is imperative that we define some terms and definitions that are fundamental to comprehending the scientific principles, operating in a producing hydrocarbon zone. We refer to these terms as reservoir fluid definitions.

The Geoscientists and the Petroleum engineer’s task

Earth's formation, structures, and the history of geological features

Reservoir fluid definitions

RESERVOIR FLUID TERM

Fluid

Let's start with the term fluid, a fluid is a substance that flows and yields to any force tending to change its shape, this means that it takes the shape of its container both liquids and gases, are fluids petroleum gas is defined as a substance in a gaseous state at normal atmospheric conditions, usually defined as sixty 60 degrees Fahrenheit or 25 degrees Celsius and 14.7 PSI pounds per square inch.

Density of Hydrocarbon Fluids

Now we need to know how to measure the density of hydrocarbon fluids, and understand the significance of these measurements. Remember density is measured by dividing the weight by its volume. To begin the measurement of density its defined by the American Petroleum Institute (API) and Fixes the specific gravity for water at one (1) gram per centimeter cubed. This standard allows us to compare the Relative weight of crude oil to the weight of an equal volume of water. in calculating API gravity in the laboratory, we use the formula API equals 141.5 divided by the specific gravity minus 131.5. for instance if a cubic centimeter of water is one (1) we subtract 131.5 from 141.5 because API gravity is expressed in degrees, we therefore say that the API for water is ten (10) degrees. The numbers greater than API gravity 10 degrees indicate that the hydrocarbons are lighter than water, and numbers lower mean that the hydrocarbon is heavier. most oils have a higher API gravity, meaning they are lighter than water and that is why oil usually floats on top of water there are however a few oils that are heavier than water.

Density of hydrocarbon fluids

(API) density measurement

API density formula

API gravity more than 10 degrees

API gravity more or lower than 10 degrees

Solution Gas

Now let’s we look at solution gas. Solution gas is gas that has been converted into a liquid after pressure has been applied.

Solution gas

For example you’ll find solution gas in any kept bottled soft drink called soda pop in the US or fizzy drinks in the UK, these popular drinks contain solution gas. Once you remove the top and the pressure is released, the gas solution bubbles to the top. It reverts back to a gaseous state and escapes the same thing happens in a reservoir. Under pressure and trapped in a confined space hydrocarbon gas converts into solution gas and is dissolved in with the larger liquefied hydrocarbon molecules we call oil. When we drill an oil well, we release some of the more pressure and solution gas will move to the surface changing its state to a gas.

Soft drink contain solution gas

Solution gas in reservoir

Critical Saturation

Another term is critical saturation, this refers to the minimum saturation of oil gas and water in a reservoir, where the molecules are connected in such a way that they create a continuous medium. This medium allows the oil or water to flow to the surface.

Critical saturation in reservoir

In Petroleum Origin there is a hypothetical well and asked if after drilling a well whether only water would flow or whether both oil and water would flow at that time I explained that for oil to flow it has to make up at least 25 % of the fluid otherwise only water would flow this exact percentage where oil will flow is known as critical saturation.

Oil or water flow in Reservoir

Since every reservoir is a little different we need to determine critical saturation for our particular reservoir, we do that by measuring the API gravity and viscosity of oil samples in the laboratory.

API gravity and viscosity of oil samples

The Bubble Point Pressure

The bubble point Pressure is the exact pressure point where in the solution gas phase reverts back to a gas phase as pressure is released like opening a soft drink the bubble appears in the gas solution it is this pressure point that is known as the bubble point pressure. theoretically if there's no gas in a reservoir they say that this reservoir is above the bubble point pressure the reverse of the bubble point pressure where gas goes back into a liquid is called the dew point a detailed study of bubble point pressure and dew point pressure for different oil types are measured in laboratories this is referred to as PVT studies.