Another day at the office: Working as a wellsite geologist in the North Sea: a short summary

Gary Paton (UK)

There are many types of employment that geology may offer, but one in particular that I would like to describe is the wellsite geologist. I have been employed in this position for approximately four years in the Norwegian Sector of the North Sea. This article aims to describe briefly what is involved in working as such a geologist.

What does a wellsite geologist do?

Needless to say, the wellsite geologist works at a wellsite in that he or she is involved in the drilling of a well. To summarise, his main priority is to identify the various formations that are drilled. To enable such a task, it is required to identify particular rock types from such data as rock cuttings and logging measurements made during drilling.

The job carries a certain amount of responsibility, as it is a requirement to keep other drilling personnel constantly updated on where the well is with respect to the original well plan and when a particular depth interval should not be drilled further. To do the job, the wellsite geologist needs to liaise and collaborate with a range of specialists at the wellsite, that is, he is only one component of an effective team.

Why become a wellsite geologist?

I must admit, when I was an undergraduate, becoming a wellsite geologist in the oil and gas industry was not one of my initial objectives. However, the need for a steady employment with a reasonable salary soon led to my initial employment within the exploration office of a major oil company and, several years later, I find myself at the wellsite working for a major consultancy.

There is a common saying, “every well is different” and this is appropriately correct. Indeed, every well has its own particular challenges and problems. Therefore, the variation in work is one of the more attractive issues of the job and I am actually also able to utilise what I learned at university, that is, from structural geology to sedimentary petrology. This is indeed a luxury that sadly few graduate geologists experience these days. Needless to say, oil and gas are natural resources that continue to be in huge demand in an ever expanding and consumer orientated society.

Therefore, for the time being, the process of drilling into sedimentary basins around the world is as intense and continuous as ever and the wellsite geologist is in continual demand. It must be stated that, as a result of past hectic changes in the price of oil, recruitment into the oil and gas industry has been extremely variable. For example, very little if any recruitment occurred in the late 1990s. This ‘gap’ is now becoming obvious as the elder, experienced generation approach retirement and there are not sufficiently experienced personnel to fall in behind them. Therefore, for the time being, continual employment within this profession is quite likely.

A brief geological overview of the North Sea

The geological structure of a particular field area in the North Sea is something that the wellsite geologist becomes extremely familiar with, as one would become familiar with a particular town or city after living there for a certain period of time. Needless to say, an intimate and detailed geological knowledge of the North Sea has developed over the years from the extensive seismic surveying and the number of drilled wells. Wellsite geologists tend to stay within several fields, usually in relatively close proximity to each other, thereby accumulating an intimate knowledge of the formations that are usually drilled. This is extremely useful, as there is always an unwelcome surprise when the drill path ends up not where it is intended.

Without going too much into detail (as there is huge amount of literature available on the subject), to all intents and purposes, oil and gas exploration in the North Sea is centred along the arms of a failed rift system or aulacogen, in a typical Y-shaped rift junction configuration, This rift basin configuration consists of the Viking Graben, the Central Graben and Moray Firth Graben (Fig. 1).

Fig.1. Simple structural map of the North Sea (courtesy of the Geological Society).

This rift system was actually initiated during the Early to Middle Jurassic (185 to 180 millions of years ago), which originated as a zone of thermal upwelling in the mantle, creating ‘doming’ and ‘uplift’ in the crust, followed by extensive erosion. This resulted in a typical structural feature across the North Sea – the BCU or Basal Cretaceous Unconformity. By the Late Jurassic (about 160 to 150 millions of years ago), extensive rifting and normal faulting followed, which created a series of structural topographies known as ‘horsts’ and ‘grabens’ (Fig. 2).

Fig. 2. Simplified diagram to illustrate the horst and graben structure that occurs during rifting.

A similar process is occurring today in East Africa along the rift valley, where the continental crust is being dragged apart, resulting in a series of these graben and horst structures. This crustal extension eventually ceased in the Early Cretaceous. The remainder of the Cretaceous and the Cenozoic has seen minimal further tectonic activity in the North Sea with thick accumulations of sediments.

During the Late Jurassic, the extensive rifting resulted in significantly deep half-graben depocentres, where deep marine sediments were deposited in anoxic conditions. This led to extensive deposition of organic rich shales, such as the Kimmeridge Clay Formation (known as the Draupne Formation in Norway), which, as a result of continuing rifting and subsidence, became exposed to high thermal gradients. Subsequently, a process known as thermal maturation occurred, when the organic material in these shales underwent alteration initially to kerogen and eventually to oil and gas. These resultant hydrocarbons eventually migrated to a host reservoir rock, typically a Lower to Middle Jurassic sandstone interval commonly found within the tilted blocks.

To prevent the oil and gas migrating further and eventually seeping to the surface, a seal is required. Fortunately, such a feature exists, provided by the thick and extensive Upper Cretaceous to Palaeogene mudstone overburden. Therefore, in essence, the North Sea has all the typical features that, when combined, provide an effective hydrocarbon play – source, maturation, migration, reservoir and seal.

Furthermore, a typical well profile involves drilling through a thick, relatively continuous Palaeogene to Cretaceous claystone overburden. This is followed by penetrating the BCU and arriving in a particular Middle to Lower Jurassic reservoir interval within the many normal faulted blocks as components of a half graben structure (Fig. 3). Although this is a gross oversimplification with regard to the North Sea, the principle remains the same.

Fig. 3. Cross section through the North Sea. It can be seen that a well path typically has an upper section through the overburden of Neogene to Upper Cretaceous mudstone intervals, before entering an eroded horst block with Middle to Lower Jurassic sandstone reservoirs (courtesy of the Geological Society).

Investigation, analysis and interpretation

To become a wellsite geologist, one has to become skilled in recognising particular formations as they are drilled and monitoring the well path in comparison to a particular well plan. This is vitally important. The well is constructed in a telescopic arrangement consisting of different sections. The top hole section is initially drilled with a 36 inch diameter bit and this is fitted with a particular size of casing, in the example in Fig. 10, a 28 inch diameter bit and cemented in place.

Subsequently, at each depth, the drilled hole with appropriate casing is sequentially reduced in size. Ultimately, the section that is drilled through the reservoir interval will be either an 8½ inch or 7 inch section (Fig. 4).

Fig. 4. Simplified well path profile indicating alternate section intervals with depth.

In essence, each casing point is chosen to ensure that particular intervals of the well are isolated for various reasons, one of which is to allow particular drilling fluid densities (that is, mud weight) to be used in different well sections. Obviously, drilling different formations with different pore pressures is simply not viable using the same mud weight. Therefore, section total depths (TD) will be decided in the planning stages and it is imperative that the wellsite geologist can determine the particular formation accurately to decide when to call TD and set section casing. To drill any further, or to stop drilling early, can have serious repercussions regarding well stability and integrity.

To enable effective formation identification, various types of information are available to the wellsite geologist, including:

Rock cuttings. As the well section is drilled, drilling mud is constantly pumped down in the inside of the drill string and is ejected out of nozzles located on the face of the drill bit. This subsequently washes out and carries the recently drilled loose rock material up to the surface through the annulus (the gap between the drill string and borehole wall) as ‘rock cuttings’ (Fig. 5).

Fig. 5. Schematic illustrating movement of cuttings through the action of mud movement up the annulus (courtesy of HRH Geological Services).

The drilling mud at the surface is then conditioned and cleaned by using huge shakers, which effectively ‘sieve’ the cuttings from the mud (Fig. 6).

Fig. 6. Collecting the rock cuttings at the shakers.

The cuttings are then collected and labelled from particular lag depths, that is, the calculated depth from where the cuttings have originated. They are cleaned, put on a small inspection tray and examined by the wellsite geologist to determine what type of lithology the cuttings are and what formation they are derived from at that particular lag depth (Fig. 7).

Fig. 7. Wellsite geologist examining rock cuttings under the microscope.

Needless to say, this requires a practised eye that results from considerable experience. The point is to identify the fresh arrival of a new lithology – what is in the sample tray may have actually originated from further up in the well (Fig. 8).

Fig. 8. Typical rock cuttings, indicating a complex range of lithology types (courtesy of HRH Geological Services).

The action of a rapidly rotating steel pipe does tend to aggravate formation and cause infill, especially if it is a loose formation, that is, a fissile shale. As previously stated, a wary eye and experience is necessary.

Logging while Drilling (LWD). This is essentially a series of measurements that are constantly made from downhole sensors located in the Bottom Hole Assembly or ‘BHA’ located between the drill bit and drill string. The information is sent from the BHA to the surface by telemetry, that is, a series of incremental changes in mud pressure or pulses. These pulses are detected by a pressure transducer on the drill floor and then decoded to give a series of ‘curves’ or ‘logs’ displayed in ‘real-time’ for the geologist (Fig. 9).

Fig. 9 Logging while drilling (LWD) (courtesy of Statoil).

Without going into too much intricate detail, these ‘curves’ or ‘logs’ consist of the following:

Gamma Log. The amount of gamma rays emitted from particular formations, which is dependent upon lithology type. Clay minerals have a susceptibility to accommodate particular radioactive isotopes in their atomic structure, that is, K⁴⁰ and they will emit a greater amount of gamma rays than, for example, a relatively clean sandstone. Therefore, it can be initially interpreted where the distribution of sandstone, siltstone and mudstone occurs along the well path (Fig. 10).

Fig. 10. The log indicates how various lithologies have been interpreted by the wellsite geologist according to particular changes in the curves. Further information is given by the cuttings (courtesy of Statoil).

Neutron Porosity Log. The formation is bombarded by neutrons from a radioactive source. These neutrons are primarily absorbed by hydrogen (H₂) a component of water. Obviously, for water to be present within a rock, there must be porosity. Therefore, the neutron porosity log is merely an indirect measurement of porosity by virtue of the hydrogen content present within a rock formation. In the example, it can be seen where porosity varies in response to lithology (Fig. 10).

Density Log. The formation is bombarded by gamma rays from a radioactive source, which interact with the formation. The subsequent count rate and energy of the returning gamma rays are recorded at the detectors on the density tool. The idea is that dense formations have atoms of higher atomic density and, therefore, more electrons for the gamma rays to collide with than less dense formations. This efficiency at gamma ray absorption is reflected in the returning counts to the tool. In the example, it can be seen where density varies, especially in response to such lithologies as coal, which have a very low density (Fig. 10).

Resistivity Log. A current is induced in the adjacent formation from which the conductivity of the formation fluid can be measured and subsequently fluid type can be inferred, that is, water, gas or oil. Oil and gas are complete insulators in contrast to formation water which will be conductive. Therefore, higher resistivity measurements will indicate hydrocarbons. In the example given (Fig. 10), it can be observed how the resistivity gradually decreases as the well path passed through a gas interval followed by eventually water.

Calliper Log. An ultrasonic scanner measures the borehole diameter to indicate where washout or clay swelling has occurred, indicated by an increase or decrease in calliper measurement respectively.

In addition, other indications of a change in formation can be actually obtained from the drilling parameters. Drilling from a soft to a harder formation or vice versa will result in a ‘drilling break’, whereby the rate of drilling or ‘ROP’ will either increase or decrease. A change in torque will also be observed as the drill bit bites into a more or less competent rock type.

Therefore, it can be seen, with all this information assembled together, how the wellsite geologist can identify, interpret and describe the particular formation interval.

In mature exploration areas with abundant seismic coverage, the anticipated depths to formations can be predicted using depth-time conversion algorithms. An interpretation of the seismic data will only give an approximate anticipated depth to particular formation intervals based on the time taken for seismic waves to be reflected from the formation boundary and back, that is, two way time. Therefore, there is often the occasion when, at the wellsite, the geologist can expect to encounter particular formations at approximate anticipated depths, that is, if the seismic interpretation is correct.

Occasionally, especially in exploration wells or when the horizontal section of the well needs to be “landed” with a certain reservoir interval, biostratigraphy is required. The purpose of a biostratigrapher is to identify particular microfossils and palynomorphs (organic walled microfossils, that is, dinoflagellate cysts, marine algae, pollen, spores and so on) to distinguish a particular biozone. Subsequently, the biozone will correspond to the particular reservoir interval or formation. In the oil industry, it is common to find individuals who specialise in particular microfossils or palynomorphs from a particular geological period. I tend to work with biostratigraphers who specialise in Lower to Middle Jurassic benthic agglutinated foraminifera and palynomorphs. These guys are serious experts and I have often been grateful to their assistance when drilling through a complex fault zone directly beneath a variable unconformity.

In this modern, hi-tech age, extra assistance to formation interpretation and well path monitoring is given by the modern PC. For example, we are now able to plot the actual drilled wellpath against the particular wellplan through particular formations as they are interpreted on the seismic (Fig 11).

Fig. 11. Modern software enables the well path to be plotted against the actual well path with additional seismic horizons superimposed (courtesy of Statoil).

We also have fast and immediate access to nearby offset wells to enable and assist rapid formation identification using a correlation panel (Fig. 12). Long gone are the days of coloured pencils and countless rolls of paper print outs.

Fig. 12. Plot showing how various offset wells can be assembled in a correlation panel to assist in formation identification (courtesy of Statoil).

One particular aspect of the job, especially with respect to exploration wells, is the requirement for coring. This operation is required to extract actual reservoir rock to obtain the necessary sedimentological and petrophysical information. This involves drilling an interval within the reservoir itself using a core bit drilling assembly, thereby extracting a section of rock to be brought to the surface for further analysis. To enable this, the coring assembly is fitted with a particular bit type, with a hollow insert to allow the passage of the rock core into an aluminium sleeve within the assembly.

Once the core interval has been drilled, the process of bringing the rock core back to the surface takes a considerable amount of time to allow for gradual depressurisation of the core. Bear in mind that reservoir pressures may be between 350 to 400 bar on average. At the surface, the core is cut into one metre sections and described with respect to lithology and fluid type (Fig. 13).

Fig. 13. Core showing variable sedimentary type and structures. Smaller plugs have been extracted to determine actual petrophysical parameters such as porosity, permeability, cementation type and so on.

Additional surprises also occur, including the occasional fossil (Fig. 14).

Fig. 14. Piece of core with pyritised ammonite.

It is indeed a humbling experience initially to see a core section of rock, profusely dripping oil everywhere, realising the age and depth from where it originated.

Essential job requirements

Prior well site experience is essential before becoming a wellsite geologist. Indeed, many consultant wellsite geologists have spent years working as mudloggers or LWD engineers before reaching this level. An ability to get on with people and work in a team is also necessary. You are required to consistently liaise, communicate and report information to various people at the wellsite and in the drilling department onshore. Everyone has a job to do and they rely on you as much as you must rely on them. A critical and investigative eye is a necessity, for example, to quickly distinguish if the well path is being drilled appropriately or not. A mistake or lack of observation during drilling can be, as well as being obscenely expensive, a serious safety issue.

Advantages and disadvantages

Without a doubt, being employed as a geologist in a challenging exploration well is an extremely rewarding experience, requiring fast and effective decisions to be made when required. However, there are times where, for particular reasons, drilling has to be discontinued that is, technical problems, changing BHA component and so on and a lengthy period of waiting will ensure. Obviously, when working offshore, a simple case of travelling back to shore is simply not practical and boredom can be an issue at times. Spending several days on a high-inclined wellpath when drilling through monotonous mudstone intervals in a mature field for several days is hardly an exciting experience.

Another disadvantage is being away from your family for significant periods of time, although the North Sea sector, especially in the Norwegian sector, allows for a very reasonable rotation. A typical work rotation for a wellsite geologist usually consists of two weeks work with three to four weeks off, a luxury in the modern working environment of long hours and the minimum of staff. Additionally, as well as the excellent standard of food on Norwegian offshore installations, I also have breathtaking views of the sea (Fig. 15), as well as the occasional visit by wildlife (Fig. 16). It’s quite a thought to realise that some people pay for such privileges.

This article is a cursory summary of the work required as a wellsite geologist. Obviously, the job varies considerably depending on particular well types and geographical locations and is indeed significantly more complicated than what has been outlined in this article. The reader is encouraged to use the huge amount of literature for further information concerning this type of employment.

References

Petroleum Geology of the North Sea, Glennie, K.W. (1998).

Petroleum Geoscience, Gluyas, J. & Swarbrick, R (2004).

Elements of Petroleum Geology, Selley, R.C. (1998).

Captions