Bedrock Drill Core and Cuttings

Introduction

Our daily lives are affected by the geology around us, from the water we drink and use to irrigate farms to the oil and natural gas that heat our homes, power our vehicles, and ensure our ability to manufacture consumer goods such as plastics, cosmetics, clothing, and chemicals. Rocks, also, are a vital resource. Some can be turned into the fine sand or coarse crushed rock used for road construction and urban development. Others contain the essential ingredients that we use in our daily lives, such as salt, coal, and the critical elements that make technology — solar panels and computer systems, for example — work. To know where to find these resources, private and government enterprises need to understand the properties and placement of rocks deep underground. They obtain this information by digging, or drilling, below Earth’s surface, sometimes to depths of thousands of feet. This Public Information Circular describes how rock material, also known as bedrock, from Kansas is collected from underground in the form of solid rock cores or granular cuttings, how this material is preserved at the Kansas Geological Survey, and how to access these rock samples and information about them.

What Are Core and Cuttings?        

Physical samples of bedrock can be collected from deep beneath Earth’s surface in two forms: core and cuttings. Core is a long, continuous, cylindrical sample of solid rock that is collected by driving or drilling a coring drill bit (a specially designed hollow pipe) into the ground. These materials are often referred to as drill core and can be thousands of feet in length and have various diameters (typically two to four inches). Cuttings are rock fragments that are brought to the surface via drilling fluids during the drilling process. They are like the sawdust you might produce when drilling into wood with a power drill. Cuttings and core are typically collected at specific intervals underground so geologists can see snapshots of the types of rock that occur at specific depths.

How Are Core and Cuttings of Bedrock Collected?

Core and cuttings of hard bedrock are collected by use of a drilling rig (fig. 1). A drilling rig supplies the power needed to rotate long lengths of steel pipe and drive it hundreds or thousands of feet into the ground. Multiple lengths of steel pipe are threaded together to create what is referred to as a “drill stem.” The bottom section is attached to a drill bit that cuts the rock as the drill stem rotates. Depending on what type of rock samples are needed, one of two forms of drill bit will be attached to the drill stem: a coring bit, which has a round opening in the middle for core collection, or a rotary bit that cuts and crushes the rock for cuttings collection. To prevent overheating the drill bit and to control pressure in the well (to prevent a blowout), a lubricant (water, drilling mud, or air) is pumped down the center of the drill stem under pressure. The pressurized injection results in upward fluid circulation on the outside of the drill stem, which also brings cuttings to the surface (fig. 2). This process is similar to blowing bubbles in a glass of water through a straw. Cuttings are caught in a sieve, washed, and then bagged for later delivery to the KGS cuttings repository in Wichita.

For the recovery of cylindrical core samples, drillers use an inner tube. Inner tubes — also referred to as core barrels — are dropped down the inside of the drill stem when the drill stem is at rest (that is, not rotating). Once the inner tube is inserted into the drill stem, a clamp locks the inner tube into place and drilling continues. A ball-bearing assembly (swivel) prevents unwanted rotation of the inner tube while the rotating drill stem cuts downward into the bedrock. Once the inner tube is full of core, a “fishing line,” which is a heavy gauge wire with a grapple on its end, is dropped down the interior of the drill stem in preparation for hoisting the inner tube to the surface. The fishing line attaches to a special knob, called a pulling arm, at the top of the inner tube. As drillers remove the slack in the fishing line and pull upward on the arm, the inner tube clamps are disengaged, allowing the tube to be pulled to the surface. A compressor spring (called a “core catcher”) installed at the base of the inner tube prevents the cylindrical core from falling out of the tube as it is hoisted. Core is then removed from the inner tube and placed in storage containers, and the process is repeated until the desired depth is reached. To prevent groundwater contamination and collapse of the drill hole by soil or rock at the surface, an outer casing is driven or drilled into the ground to a desired depth early in the drilling process. Cored wells are put into service — as an oil or gas well, a monitoring well, or an injection well for non-potable wastewater — or are plugged according to appropriate regulations.