A job in mud logging: is it worth it? A s k a group of mud loggers to comment on their job, and you will get a variety of answers. Some will probably say that mud logging is a boring, repetitive job and a dead end career. Others will claim that mud loggers are underpaid, overqualified geologists. On the other hand, some will tell you that i t means employment, steady income, plenty of time off and in some cases extensive travel. You will probably also be told that, although the money was poor to start with, it improved with time and that, above all, it was one of the few geologically orientated jobs available after spending three years obtaining a geology degree.

M u d logging companies, of which there are some half-dozen or so with offices in the UK, are probably the biggest single employer of graduate geologists. Employment is usually obtained in one of two ways - either as a result of ‘milk-round’ interviews at uni- versity or by writing to the various companies on spec. The latter approach now tends to be the more common, as there is a relatively rapid turnover of personnel. An average mud logging career probably lasts between one and three years, though in recent years this has tended to be longer.

A training course of one to two weeks in the office usually precedes one or more training ‘hitches’ on a rig. A typical starting salary would be in the region of f3500-4000, on top of which, once training has finished, a daily rig bonus of around 510 is paid for every day spent on the rig. (These salaries are gener- ally somewhat higher whilst working abroad.)

O n c e the training period is over a mud logger is assigned to a permanent crew working on one particu- lar rig. This may be an overseas location or closer to home in the North Sea. The working schedule will either be two weeks on the rig followed by two weeks off, or two weeks on followed by one week off. This depends upon the number of people assigned to the crew. Whilst on the rig, the mud logger’s work is in a self-contained, pressurised laboratory containing re- cording equipment and microscopes, etc. These laboratories vary in size, level of sophistication and capability between companies and within companies.

T h e job itself is not quite as abstract as the name suggests. As wells are drilled, a fluid known as ‘mud’ is pumped down the drillpipe (Fig. 1). The mud has various functions, the most important of which are to cool and lubricate the bit, carry cuttings to the surface and hold back the formation pressure. Mud types vary enormously depending on a whole range of variables, but the most basic type is a mixture of bentonite and water. The mud passes down the drillpipe to the bottom of the hole and out through small holes in the drilling bit before returning to the surface up the annulus between the drillpipe and the hole. As the mud is pumped, the drillstring is rotated by a large electrically driven motor at the surface. This causes the drilling bit at the end of the drillstring to rotate and grind the formation into small chips of rock known as cuttings. These cuttings are trans-

Fig. 1. Schematic diagram of the mud circulation system.

ported to the surface carried in suspension by the mud. The viscosity of the mud which allows the removal of cuttings to the surface is one of the most important properties of mud.

O n c e the mud arrives at the surface it is passed over a set of vibrating mesh screens called ‘shale shakers’. These agitate the mud and allow it to pass through the screens whilst the cuttings, unable to pass through the mesh, are piled up as they drop off the end of the screens. The mud logger then collects samples from this accumulation at specified intervals and places them in bags to form a complete lithologi- cal record of the well. If these samples are to be meaningful, it is essential to know what depth each collected sample comes from. Obviously there is a delay between when the formation is cut and when the cuttings reach the surface. This delay is known as the ‘lag time’ and is calculated by the mud logger. It varies from a couple of minutes to several hours depending upon the depth of the well, the diameter of

GEOLOGY TODAY Mar-Apr 1985159

the hole and the rate at which mud is being pumped down (and hence returning from) the hole. By noting the time at which a particular depth interval is drilled, a mud logger knows the time at which it will reach the surface and can go to the shakers at this time to collect the sample.

A common interval for sampling is every 20 feet (about 6m), although as the well approaches a possible reservoir horizon this interval may be every 10 feet (about 3m). After bagging up the required amount of cuttings, a small sample is taken into the mud logging unit, washed and observed under the microscope. This is then described, and the details, along with a graphical representation of the lithology, are plotted on the ‘mud log’, which is a continuous record of the well drawn up as it progresses. Alongside the lith- ology, in a separate column, the ‘rate of penetration’ is graphically recorded. This is generally a five or ten foot average of the speed (feet per minute) at which the well was drilled. Two other columns record the amount of gas liberated from the formation as the well is drilled and a chromatographic breakdown of this gas, which may be anything from methane to pentane. Various other parameters are also recorded by the mud loggers, although these tend to be related more to the drilling operations of the well than to the geological aspects. One of these parameters is the level of mud in the ‘pits’ (mud tanks). This is a very important safety consideration; observing any in- crease in the pits is the first indication of high formation pressure and a potentially dangerous situ- ation. The more sophisticated, computerised mud logging units, called data units, store a considerable amount of data on magnetic tape or disc and can

provide an on-line engineering service to the drilling engineers.

S i n c e the mud logger is generally supervised by the operator’s geologist, it can be seen that from a geological point of view much of the job is often little more than data collection and sample preparation. However, the logger must be able to maintain the equipment in the logging unit, which often involves making temporary repairs with a limited number of spare parts.

w h e t h e r or not becoming a mud logger is a good move is a matter of some debate. At one time, a degree followed by few years spent with a mud logging outfit was a ticket to a job with an oil company. Today this is rarely the case; competition for jobs has increased, and many oil companies now demand higher degrees as a minimum qualification. Nevertheless, a couple of years with a mud logging company can have advantages. It is an ideal job in which to gain valuable offshore experience. It allows time in which to decide if a career in the oil industry is the right choice. It opens doors to many other offshore job opportunities with other service com- panies. Finally, with the increased competition for places on further degree courses, the experience gained may provide the necessary stepping-stone to obtaining a place. In conclusion, a job in mud logging has points for and against. How you weigh up these factors depends upon what you want to get out of it. Ultimately, mud logging is exactly what you make of it.

MARTIN HOUSTON Exploration Geologist, British Gas

Museum file 2: Hancock Museum Geological Collections Address: Barras Bridge, Newcastle upon Tyne NE2

4PT. Telephone: (0632) 322359.

Administration: University of Newcastle upon Tyne (from 1974). Previously the Natural History Society of Northumberland, Durham and New- castle upon Tyne (founded 1829).

Admission: 30p adults; 15p children.

Times of opening: Monday to Saturday 10 a.m. to 5 p.m.; Sunday 2 p.m. to 5 p.m. (April to September).

Foundation: As collections of Marmaduke Tunstall (17441790) of Welbeck Street, London. Tun- stall Museum moved to Wycliffe on Tees, York- shire, 1780-1781; sold to George Allan ( 1 7 3 6 1800) of Blackwell Grange, Darlington, 1791; opened to public 1792. Purchased by Newcastle Literary and Philosophical Society 1822 (whose collections had been growing since its foundation in 1793). Museum of the Natural History Society of Northumberland, Durham and Newcastle upon Tyne 1829. As Hancock Museum 1891. (For details see Goddard, T .R. , 1929, History of

the Natural History Society of Northumberland, Durham and Newcastle upon Tyne (1829-1929).)

Foundation of geological collections: Private museum of Marmaduke Tunstall (1744-1790) and Newcastle Literary and Philosophical So- ciety (1793).

Number of specimens in the collections: 50000 approximately.

Number of status specimens: Types 226; figured 628; cited 123.

Principal collections: T. Atthey (1814-1880); T .P . Barkas (1819-1891); W. Dinning (A. 1860); J. Duff (fl. 1870); J. Dunn (1865-1937); R. Howse (1820-1901); W. Hutton (1797-1860); J.W. Kirkby (1834-1902); A. Logan (b. 1937); Romo- nov Tsar Nicholas 1 (coll. 1830); T. Sopwith (1803-1897); T.S. Westoll (b. 1912); H.T.M. Witham (1779-1844); G.A. Lebour (1847-1918); A.G. Long (b. 1915); plus a further 500 collec- tions.

Published catalogues: Carboniferous Amphibians (Boyd, M. J.F. & Turner, S . , 1980. Transactions

60IGEOLOGY TODAY Mar-Apr 1985