As part of drilling operations, geological information about formation is required to evaluate a reservoir. Coring is a process of taking a sample of reservoir rock for further analysis, to obtain information on reservoir porosity (fluid-carrying capacity), permeability (the ability of fluids to flow) and hydrocarbon and water saturation. Knowing these properties is required to estimate hydrocarbon production and how variation in these parameters will affect production and ultimate recovery.
Coring services consist of the following activities: 1) acquiring core samples of cylindrical shape, 2) preparation for shipment, which includes split into core plugs of up to 3” long each, core plugs stabilization and 3) packing for transportation. Sometimes a coring company will be in charge of transportation to a laboratory as well.
There are four distinct methods that are used to collect core samples:
- Conventional coring - A core bit (similar to the drill bit, but with hollow/empty center) is placed at the bottom of the drill string, with a core barrel located immediately above the coring bit. The barrel (fiberglass or aluminum) is where the core sample is retained. By rotating the drill string with the coring assembly, a sample rock enters the core barrel where it is protected until it reaches the surface. Typically, inner and outer core barrels, a core catcher and a pressure-relief valve are part of the core barrel assembly. A key advantage of conventional coring is that it allows a larger diameter core sample to be obtained in a single coring operation. The major disadvantage of this technique is the cost because the drill pipe must be removed and replaced with special coring assembly. It works in the same order when coring job is done, i.e. all coring equipment is recovered and drilling tools are run back. As a result, core sampling becomes very costly, when high daily rig and services rates are incurred.
- Diamond Coring - is a conventional coring method whereby a diamond-head core bit is used. Although costing up to 20x more than the conventional core bit, the benefits of using a diamond core bit may outweigh the costs and provide significant benefits due to higher penetration rates when coring is dense or hard formations.
- Wire-line coring - is a similar conventional coring process, but coring equipment with additional tools is run on a wire-line inside the drill pipe. Hence, drilling can suspend and start again immediately after the coring job has been completed, making it significantly more cost-effective. This coring technique is used mostly for a wildcat/exploration well, when the depth of coring is not known before a well is drilled.
- Sidewall Coring tool allows collecting core samples after the well has been drilled and logged. Using wire-line, the tool can be positioned at the exact depth to obtain samples horizontally. While this may be a cost-effective solution in certain cases, conventional coring is still the first choice, as it provides information on the complete geological composition and sequencing of the reservoir rock. Each having its application and limitations, a sidewall coring tool could be rotary, percussion and mechanical. During sidewall coring, several (up to 110) small core samples are obtained, 1” in diameter and 2”-3” long.