Core samples

by Tom Sturman, with edits by Luke Stoeckel

The most detailed and most valuable source of data from any well is a core sample. Cylindrical in shape (Figure 2, below), core samples allow a close inspection of the subsurface strata almost exactly as it would appear in-situ, and allows for visual inspection of the formations physical properties. In most developed countries, a prerequisite for drilling any well is that a core sample of a specified length be given to the government for geological research, therefore companies will often utilise the presence of the coring equipment in place to save money by obtaining further core samples at the same time and to aid their own research.
[Figure 2: Core samples in tray on a drill site]
 Core samples are especially important as they allow for direct measurement of the subsurface physical properties. By conducting lab experiments we are able to determine the porosity and permeability of a formation, as well as the saturation levels of any hydrocarbons or water present. We are also able to determine the point at which the formation will fracture (this can also be obtained by conducting a fracture test) and the exact point of formation boundary interfaces. Additionally, core samples also allow thin sections to be created an analysed in order to visually determine the minerals and any small flow barriers that are present. Furthermore, via physical inspection of the core we are able to determine the grain size, sorting and rounding, as well as any physical or biogenic structure present such as ripples or fossils. Physical core analyses are recorded on a core logging sheet (Figure 3, below).

[Figure 3: Core sample logging sheet]

Currently there are two main methods for obtaining core; conventional coring and sidewall coring. Conventional coring involves using a specially designed drill bit with a hollow centre (Figure 4, below) which drills ahead while allowing a cylinder of formation to be retained in near perfect condition within a separate core barrel above the bit. When the desired length of core is obtained, the drill string is removed and the core collected. Side wall coring on the other hand involves sending a specialised coring tool (Figures 5 and 6, below) down hole and collects many small cores, often only a few inches in length and approximately 1” in diameter. One of two tools is used; either a wireline sidewall core gun or a rotary sidewall coring tool. The wireline sidewall coring gun uses small explosives to shoot hollow ‘bullets’ into the well bore wall which slice around a core of formation, when the tool is removed the bullets along with the core samples are pulled to the surface; there are 12, 24 or 30 bullets per tool (Figure 7, below). Rather than explosives, the rotary sidewall method uses a small hollow diamond tipped drill bit to cut into the sidewall of the formation, popping a small length of core loose and collecting it within the tool itself. The rotary sidewall coring tool is able to store up to 50 individual samples, and a coring summary is made detailing the time and exact depth (driller’s depth) at which the samples were taken.
[Figure 4: Typical coring bits]
[Drilling Engineering 2011 Lecture “Drill Bits”]
[Figure 5, Left: Sidewall coring tool with hollow bullet]
[Figure 6, Right: Rotary sidewall coring tool]
[Drilling Engineering 2011 Lecture “Drill Bits”]

[Figure 7: Recovery sequences of hollow bullet sidewall coring tool]
[Drilling Engineering 2011 Lecture “Drill Bits”]

The greatest downfall of coring is undoubtedly the extremely high cost of coring and core retrieval. As previously mentioned, coring requires a specially designed bit and core collection barrel above (conventional) or a coring tool, and thus before coring can commence the entire drill string must be pulled out of hole and the new bits/tools attached. This process takes a great deal of time, costing more money and naturally the deeper the well the greater the cost of coring resulting in a smaller likelihood that core samples will be taken. Another problem with coring is the damage that the samples caused by their acquisition and retrieval. Sidewall coring causes the sample to be fractured and slightly compacted by the bullet impact/drilling action; while the majority of full cores will also expand and fracture as they are brought to the surface, due to a pressure loss. There is a possibility that the core samples may be invaded by drilling fluid making it difficult to evaluate the fluids within the formation accurately. Whilst these above mentioned characteristics of coring negatively affect a core sample’s ability to accurately depict a formations properties, if the core is analysed appropriately and in correlation with other data (i.e. wireline logs, drilling fluid chemistry etc.) then it can prove to be an invaluable evaluation tool.