Most data used by exploration and development teams to describe the earth hundreds or thousands of meters below the surface are from indirect methods such as wireline or while-drilling logging tools. Although both sophisticated and invaluable to operator planning, logging measurements are subject to analysis and calibration and are limited as to how finely they can divide and measure the earth. 

The only way to directly measure the earth's subsurface is by analyzing samples, or cores, that engineers extract from the formation using special bits or wireline-conveyed coring tools. Once the cores are cut, they are captured and retrieved to the surface.

These cores are transported intact to laboratories located around the world to provide ground truth for calibration of well logs and to reveal variations in reservoir properties that might be undetectable through downhole logging measurements alone. Analysts use cores to characterize pore systems in the rock and model reservoir behavior to optimize production based on the analysis of core porosity, permeability, fluid saturation, grain density, lithology, and texture. 

Conventional cores, also known as whole cores, are continuous sections of rock extracted from the formation during otherwise standard drilling operations. The coring bit is hollow so that as it cuts through the formation it creates and captures a solid cylinder of rock that can be brought to the surface as a single piece. 

Sidewall cores (SWCs) are plugs of rock cut from the wellbore wall. These cores are usually acquired by wireline-conveyed tools; SWC operations are less expensive and time-consuming than those of conventional coring and can recover cores from multiple zones of interest in a single wireline run. Because SWCs are typically obtained after logging tools have been run, geologists can use log measurements to pick the depths at which the SWCs should be taken. 

An article in the May 2015 issue of Oilfield Review “Defining Coring: Getting to the Core of the Matter” focuses on the basics of coring and describes the tools and methods operators use to retrieve cores from the Earth's subsurface.

The Defining Series provides E&P professionals with concise, authoritative, up-to-date summaries of a wide range of industry topics.