Coring and Core Analysis (SideWall Coring (Coring After Drilling))

SideWall Coring (Coring After Drilling)

Sidewall Coring

Wireline Core Gun

The chronological sample taker or core sample tool (CST) is a multishot gun that is lowered into the borehole on a wireline logging cable. At the appropriate sample depth, a 1-inch hollow bullet is fired horizontally into the borehole wall, and a core up to 2 inches long recovered ( Figure 1 , Sample taking operation of the sidewall gun). On a single run into the hole, combined guns can shoot up to 72 sidewall cores.

In a very irregular, washed-out, overgauge hole, cores may be much shorter, or may even consist entirely of filter cake or drilling fluid-contaminated sediments. In very hard rock, the bullets may break on impact, fail to penetrate, or, if recovered, may contain only fractured or crushed material.

Though, for a number of obvious reasons, sidewall cores are inferior to bottomhole cores, they are far more commonly cut. One reason is that they are cheap to obtain, requiring minimum rig time. More importantly, they are taken after the well has been drilled and logged and, therefore, are planned with the benefit of hindsight.

 

Rotary Sidewall Corer

This tool is an attempt to combine the advantages of sidewall coring with conventional coring technology. The inner barrel is forced out at an angle through the side of the outer barrel, and into the borehole wall by the hydraulic action of pumped drilling fluid. Pumping is then stopped, and the inner barrel is retrieved on a wireline; the outer barrel is moved to a new location in the hole, and a new inner barrel pumped into place for the next core.

The rotary sidewall cores are 1 inch in diameter and up to 1 foot long. They are of only slightly better quality than wireline gun cores and require more expensive equipment and rig time to obtain. This type of coring is not commonly performed.

Sidewall Core Slicer

The tricone tool that is used is run on a wireline and can cut long (up to 3 ft) triangular cores from the borehole ( Figure 2 , The sidewall core slicer (tricone) tool).

The tool is lowered to the zone of interest, at which point a pad is extended against one side of the borehole wall, forcing the cutting tool against the opposite wall. Two diamond-tipped, circular saw blades mounted at 600 to each other move out of the tool and up its length, cutting a triangular slice of formation, approximately 1 inch on each side. After cutting, the pad is retracted into the tool and the core falls into a core catcher below. Up to four cores can be cut on a single run in the hole. Any irregularity in the borehole will prevent tool contact, resulting in a fragmentary core.

Such core slices are very useful geologically. They provide extensive stratigraphic information rapidly and at a relatively low cost. Unfortunately, tricores can only be obtained successfully in very well-consolidated formations. Softer rocks tend to disintegrate with saw vibration.

 

Core Point Selection

Sidewall core points are selected at intervals that have been indicated to be important by cuttings or wireline logs. A second crucial selection criteria is the likelihood of successful recovery of the core.

If the borehole is deeply washed out and overgauge, the normal sidewall bullet will fail to penetrate the borehole wall ( Figure 1 , Selection of the correct sidewall bullets and fasteners for formation type and hole size). Where the borehole is this much oversized, extra long wire fasteners are used to extend the bullet's penetration range. Similarly, special hardened bullets are used to penetrate and core unusually hard formations. However, long fasteners and hardened bullets used in soft formations and in-gauge holes can result in excessively deep bullet penetration and failure to retrieve the core. Previous experience with both the interval of interest and the success rate of the wireline company being used should be a factor in deciding the number and type of shots required, including some accounting for misfires.

Core Recovery

On retrieval from the borehole, the sidewall core gun is taken to the wireline service company work area. Before any other work is attempted, the logging engineer should remove the explosive charges from all bullets that failed to fire.

The successful bullets are then removed from the gun by cutting the wire fasteners. Extra care should be taken to keep the bullets in correct order after they have been removed from the gun! Each core is then extracted from the bullet into a labeled glass jar ( Figure 1 , Extraction of the sidewall core from the hollow bullet after removal from the gun).

After the core is cut, the key to successful recovery is preparation. This means efficient and speedy completion of the various stages of recovery, boxing, sampling, geological evaluation, and shipping. If two geologists are present at the wellsite, e.g., an oil company geologist and a mud logging geologist, duties should be allocated between them so that they can work sequentially on each aspect of the job without getting in each other's way ( Figure 2 , Sequential core processing by two geologists working together). It is critical that the whole process be carried out in a disciplined manner.