During conventional coring operations, the operator first drills the well down to a zone of interest using a conventional drill bit and drillstring. A wellsite geologist closely monitors drilling progress to decide when to begin coring operations. The timing of this decision is critical because if the coring begins too soon, the operator will waste rig time obtaining unneeded core above the zone of interest; if coring begins too late, the drill will have already penetrated the zone and possibly miss the most crucial section of the formation.
Correlations with offset well logs usually provide the first indication that the drill bit is nearing the coring point. By charting the formation type, drilling rate and amount of gas extracted from the mud during drilling, the geologist can create a mud log that may be compared with logs from offset wells. Some zones have been cored simply on the basis of a drilling breakâ€”an increase in drilling rate, which is often accompanied by an increase in gas or evidence of oil in the formation cuttings. Modern logging-while-drilling technology, however, can deliver resistivity-at-the-bit measurements in real time to help operators determine when the bit is approaching the zone of interest.
Once the geologist gives the order to begin coring, the driller pulls the drill bit out of the hole, and the drilling crew exchanges the drilling BHA for a coring bit and core barrel. The hollow coring bit grinds away the rock, leaving a cylindrical core of rock at its center. This core is retained inside the core barrel, which is mounted just above the bit. The core barrel consists of an inner and outer barrel and a core catcher. These barrels are attached to a swivel that enables the inner barrel to remain stationary while the outer barrel rotates with the coring bit. Drilling fluid can circulate between the inner and outer barrels. The catcher keeps the core from slipping out through the hollow bit when the coring BHA is retrieved to the surface. Cores typically range in diameter from 4.45 to 13.34 cm [1.75 to 5.25 in.] and are usually cut in 9-m [30-ft] increments, corresponding to the length of the core barrel or its liner, which in turn, is consistent with the length of standard drillpipe.
When the core barrel is full, the drilling crew pulls the drillstring to the surface and retrieves the core barrels. A core recovery specialist lays the barrel liner on the pipe rack. The liner, with core inside, is then scribed with depth markings and orientation lines. The metal liner is usually cut into segments and sealed at each end for shipping to a core analysis laboratory.
Conventional coring operations often provide the best rock samples for testing, analyzing and evaluating reservoirs. However, the time required to cut and recover whole cores can impact drilling efficiency. Depending on coring objectives and cost limitations, some E&P ventures may deem conventional coring nonessential. In such cases, the operator may turn to an alternate method for sampling downhole formations.
Sidewall cores (SWCs), plugs of rock taken from the wellbore wall, may offer a cost-effective alternative to conventional cores. The SWCs are usually acquired by wireline tools, and a single wireline descent can recover SWCs from multiple zones of interest.
After the driller reaches a casing point or drills to total depth (TD), the drillpipe is pulled out of the hole and the well is logged before casing is set. Sidewall cores typically are obtained after logs have been run, usually near the conclusion of an openhole wireline logging job. This gives geologists time to pick core depths after consulting the logs to identify zones that merit sampling. Wireline gamma ray or spontaneous potential logs are used to correlate between openhole log depths and core depths. Sidewall coring devices are controlled from the surface logging unit and can extract samples from the side of a wellbore at up to 90 selected depths.