When the BOP has been re-installed and pressure tested a 12 1/4″ hole is drilled through the oil bearing reservoir. Whilst drilling through this formation oil will be visible on the cuttings being brought to surface by the drilling fluid. If gas is present in the formation it will also be brought to surface by the drilling fluid and detected by gas detectors placed above the mud flowline connected to the top of the BOP stack. If oil or gas is detected the formation will be evaluated more fully.
The drill string is pulled out and tools which can measure for instance: the electrical resistance of the fluids in the rock (indicating the presence of water or hydrocarbons); the bulk density of the rock (indicating the porosity of the rocks); or the natural radioactive emissions from the rock (indicating the presence of non-porous shales or porous sands) are run in hole. These tools are run on conductive cable called electric wireline , so that the measurements can be transmitted and plotted (against depth) almost immediately at surface. These plots are called Petrophysical logs and the tools are therefore called wireline logging tools .
In some cases, it may be desireable to retrieve a large cylindrical sample of the rock known as a core . In order to do this the conventional bit must be pulled from the bore hole when the conventional drill bit is about to enter the oil-bearing sand. A donut shaped bit is then attached a special large diameter pipe known as a core barrel is run in hole on the drill pipe.
This coring assembly allows the core to be cut from the rock and retrieved. Porosity and permeability measurements can be conducted on this core sample in the laboratory. In some cases tools will be run in the hole which will allow the hydrocarbons in the sand to fl ow to surface in a controlled manner. These tools allow the fluid to flow in much the same way as it would when the well is on production. Since the produced fl uid is allowed to fl ow through the drillstring or, as it is sometimes called, the drilling string, this test is termed a drill-stem test or DST .
If all the indications from these tests are good then the oil company will decide to complete the well . If the tests are negative or show only slight indications of oil, the well will be abandoned.
Institute of Petroleum Engineering, Heriot-Watt University