For a borehole to be properly logged, the driller and supervisor need to know its exact depth at all times. This is necessary for the calculation of drilling charges, and while designing the borehole. First, make a note of the length of the drill bit and of any other tools that may be used to drill the hole. Put the bit on the ground and make a chalk mark, ‘0,’ on the first drill pipe against a suitable fixed point on the rig and at a known height above ground level, such as the drilling table (which centralizes the drill pipes in the hole). From then on, marks can be made on the drill pipe at regular intervals – say, every half meter – to record the depth of drilling and to assist in the logging of penetration rates.

Formation samples need to be obtained as drilling proceeds: the usual sampling interval is one meter. These are obviously highly disturbed samples, having been sheared or broken from their parent formation, so should not be used to infer characteristics such as bedding, texture, porosity, or permeability. There will be a slight delay as formation fragments are lifted to the surface by the circulating mud, but a rough estimate of the up-hole velocity should enable one to calculate the actual depth at which cuttings were derived. Keep in mind that if mud viscosity is too high, or if formation collapse occurs (viscosity too low), some fragments could return to the borehole, with the potential of causing confusion. Cuttings obtained from the shallow mud channel near the borehole should be washed in water to remove mud, and laid out in order (by the depth at which each was acquired) on the ground or in a sample box with separate compartments for each sample. They can then be logged by the supervisor or site geologist and bagged if required. Samples should, of course, be labeled correctly with all information relevant to the job in hand.

The main attributes of a borehole log are accuracy and consistency; a good set of logs can be a useful resource when planning future drilling programs. Drillers must keep their own logs and notes and, as is often stipulated in contracts, these should be accurate; however, in practice, they cannot always be relied upon, especially if the supervisor is absent from the site for a period. All geological samples and water strikes should be logged by the drillers and the supervisor, as this important information will be required for designing the borehole and the equipment to be installed.

Full borehole logging may also include geophysical logging, which is normally carried out only after a well has been completed. Annexs gives a typical example of a drilling log sheet, which is applicable for both mud and air drilling, and which should be kept by the supervisor. The driller’s log should also include information on drilling or other work time, standing (waiting) time, and downtime (breakdowns).

Logging Photo Credit: Tsatsu Promise

– Geophysical logging

Information about structural features and geological formations in a borehole can be remotely obtained by geophysical borehole logging techniques. The object of well logging is to measure the properties of the undisturbed rocks and fluids they contain. Geophysical logs can provide information on lithology, the amount of water in a formation, formation density, zones of water inflow, water quality, and other in situ parameters that cannot be derived from highly disturbed drilling samples. A suite of geophysical log data, including deep-penetration methods, will more or less complete the technical description of a borehole, but geophysical logging is a specialized field best left to geophysical contractors or hydrogeological consultants.

Log sheet Sample

A logging unit consists of a power supply, a receiver/data processing unit, and a cable on a powered winch that lowers special sensor probes (‘sondes’) into the borehole to measure various properties. The cable contains multi-conductors that transmit signals to the receiver console. Data, processed by computer, can be shown as a geophysical record on a graphic display, which should consist of a number of different structural, formation, and fluid logs. Specialized software packages enable
manipulation, interpretation, and comparison of data. Multiple-sonde geophysical (‘suite’) logging can provide a substantial amount of information about the sub-surface conditions in and around a borehole.