A way of measuring well conditions downhole by studying samples of reservoir rocks, core analysis gives the most accurate insight into the porosity and permeability, among other characteristics, of the well.

Core SamplesSource: Intertek Exploration and Production

A core is a sample of rock in the shape of a cylinder. Taken from the side of a drilled oil or gas well, a core is then dissected into multiple core plugs, or small cylindrical samples measuring about 1 inch in diameter and 3 inches long. These core plugs are then dried and measured.

In order to complete a core sample, drilling must be halted at the top of the subsurface of the reservoir. The drillstring is removed from the wellbore, the drillbit removed and a rotary coring bit is attached in its place. Similar to a drillbit, the rotary coring bit consists of solid metal with diamonds or tungsten for cutting at the reservoir rock; but unlike a drillbit, a rotary coring bit has a hollow center.

Rotary Coring BitSource: Integrated Ocean Drilling Program

On a rotary coring bit, the cutting apparatus surrounds a hollow center, called the core barrel, where the core sample is stored. This core barrel is made up of an inner and outer barrel separated by ball bearings, which allow the inner barrel to remain stationary and retain the core sample while the outer barrel is rotated by the drillstring and cuts the core.

The core catcher is located within the core barrel. The core catcher has finger-like apparatuses that move the core sample farther into the barrel and keep it from falling back into the well.

After the core sample has been cut from the well, the drillstring is raised, and the rotary coring bit, barrel and catcher are removed -- and the core sample is retrieved. The drillbit is reattached, and drilling can commence again.

When performing coring operations, instead of having the pieces of broken rock removed from the well via drilling fluids, the rock is kept intact and raised to the surface for study. Because coring requires the suspension of drilling, the process is quite expensive and usually only performed at the reservoir interval.

Types of Cores

There are several types of cores that can be recovered from the well, including full-diameter cores, oriented cores, native state cores and sidewall cores.

Ranging in size from approximately 1.75 to 5.25 inches in diameter and about 400 feet to 20 feet in length, a full-diameter core is the typical coring sample that is taken from a well. Because reservoir rocks that are highly fractured, and thus very porous, are not retained in the core barrel, loss of core can be a very strong sign for the reservoir rock.

Oriented cores are marked with a groove along the length of the sample to indicate the geographic or magnetic north. In an effort to contain all the fluids in the core under reservoir conditions,native state cores are bagged, or encircled by a rubber sleeve as the sample is drilled.

A less expensive and less time-consuming option to obtain a core sampling,sidewall coring involves retrieving several small core samples measuring approximately 1 inch in diameter and 1.75 inches long.

One type of sidewall coring requires a percussion sidewall coring tool to be lowered into the wellbore. The instrument contains 30 small coring tubes called bullets that have explosive charges behind them. The detonation of these bullets launches them into the sides of the wellbore to take the samples. The bullets are connected to the instrument via wires, and when the percussion sidewall coring tool is raised, the samples follow.

Lessening the amount of change enacted on the sample, another method of obtaining sidewall core samples is to lower a rotary sidewall coring instrument into the well. This device includes a small bit that rotates to drill the sample, which then falls into the tools. The rotary sidewall coring instrument can obtain a number of different samples throughout the well, which are separated from each other via discs within the tool.

Core Measurements

Core analysis is used to define not only the porosity and permeability of the reservoir rock, but also to unearth the fluid saturation and grain density. All of these measurements help geologists, engineers and drillers better understand the conditions of the well and its potential productivity.

Examining Core SamplesSource: US Department of Energy, N.E.T.L.

A porosimeter is the instrument used to determine porosity, or the amount of space within the rock not occupied by solids, in the core plug. Porosity measures the capacity of the reservoir rock to hold fluids.

The ease by which a fluid can flow through the reservoir rock, permeability is measured on the core plug via a permeameter. Measured in darcies by studying the speed air or nitrogen passes through the dried core plug, permeability is usually connected to porosity. In other words, porosity and permeability typically go hand-in-hand.

Moving beyond the basic petrophysical properties of the core, special core analysis usually involves measuring the reservoir characteristics to determine permeability, wettability and capillary pressure, and electrical characteristics to understand resistivity, formation factor and cation-exchange capacity. Petrographic studies and sieve analysis are also types of special core analysis that can be undertaken.

Sources

Nontechnical Guide to Petroleum Geology...