Text 7

compaction – ущільнення

diagenetic – діагенетичний

bulk – величина, маса

intercalate – перешаровувати

infer – робити висновок

expel – витісняти

overburden – нанос; четвертинні відкладення, покривний шар

Compaction is a diagenetic process that begins on burial and may continue during burial to depths of 9 km (30,000 ft) or more. Compaction increases the bulk density of a rock, increases its competence, and reduces porosity.

Sands compact with relatively little loss of porosity or permeability, but other diagenetic processes may considerably reduce porosity and permeability with authigenic minerals in the pore spaces.

Mudstones compact with serious and permanent loss of porosity and permeability.

Carbonates compact to varying degrees depending on the proportion of plastic material. Most appear to compact by solution processes rather than by mechanical compaction.

Compaction can proceed only with the compression and expulsion of a commensurate proportion of the pore fluids, which move to positions of smaller energy. If mudstones alternate with more permeable beds, the more compactible mudstones expel fluids both upwards and downwards to the more permeable beds. The downward energy gradient in the lower part of the mudstone makes the mudstone a perfect barrier to the upward migration of fluids. Lateral migration takes place in the relatively permeable, intercalated beds.

Pore-fluid pressures in mudstones cannot be measured, but they are inferred to be greater than normal hydrostatic in a compacting mudstone because this is a necessary condition for flow. In a thick mudstone loaded faster than the corresponding rate at which the fluids can be expelled, compaction is retarded and the mudstone retains the mechanical properties it had at shallower depth; and the pore pressures are correspondingly elevated above the normal hydrostatic. The pore fluids are bearing part of the overburden load.

Text 8

adequately – відповідно

entity – об’єктивне існування

wildcat – розвідувальна свердловина

Velocity – швидкість

emerge – показуватися, з’являтися

appraisal – оцінка, експертиза

deviate – відхилятися, відступати

evolve – розвиватися, розгортати

completion – завершення свердловини, розкриття покладу

sequence – послідовність, результат

An oil and gas field is a petroleum accumulation that has been discovered and found to contain enough petroleum of sufficient quality to be worth more in the market place than the total cost of getting it there. It remains an oil or gas field until the value of the production in the market place no longer exceeds adequately the cost of getting it there. It is then abandoned. It is not therefore a purely geological entity, but one that involves engineering, transport, marketing, finance and economics. During its life it involves people with a great range of skills in the various activities required. These people require the same varieties of goods and services as people in other activities, and so oil and gas fields include houses, shops, schools, recreation, and the power to run these. The only difference between an oil-field community and others is, perhaps, that the average age is rather younger.

Oil and gas fields, like people, come in all shapes and sizes, some onshore, some offshore. In plan, they may be long and narrow, or nearly circular; in section, they can be thick or thin, deep or shallow. In size, they can be very large or rather small – but they must all contain some minimum quantity of recoverable oil or gas of marketable quantity. This quantity may be quite small near markets or near other fields, or rather large if offshore at some distance from markets. There are very many accumulations that are too small to become fields.

An oil or gas field begins as an anomaly on a map, revealed by regional geological and geophysical surveys. The anomaly is investigated with more detailed mapping, and if this shows features that could trap petroleum, the company must decide whether it is sufficiently promising to drill and if they can obtain the finance to drill it (or seek partners). The drilling site is chosen on the basis of a detailed seismic reflection survey. Such is the precision of seismic surveys that there will be great confidence in the geometry of the anomaly and in the general nature of the sedimentary rocks in it, and, if there is some local knowledge, also the ages of the rocks. There may even be direct indications of gas (“bright spots”, “flat spots”), but the survey will not generally provide any information on whether oil was actually generated and came to accumulate there. That can only be found out by drilling.

The first well drilled to a prospect is an exploration well, sometimes called a wildcat. It is designed to acquire information, not necessarily to produce what it finds on a permanent basis. This exploration borehole will be designed and drilled to the greatest depth of practical interest, and the geological results – the stratigraphy, ages of the rocks, dips, sonic velocity characteristics, maturation levels of organic matter in the rocks, and fossils recovered in cores and samples – all contribute to a refinement of the original model that was based on the seismic surveys. More seismic work may be done. If petroleum is not found, the information gained may lead to the drilling of further exploratory wells in the area. If petroleum is found, it will be tested and analyzed, and the pressures and flow rates will be measured.

On the basis of the refined model, appraisal wells will be sited and drilled to establish as quickly and as economically as possible the minimum size of the accumulation. This is not just a matter of drilling elsewhere on the anomaly but rather of deciding how big such an accumulation must be if it is to be commercial, and drilling near the limits so indicated. The appraisal wells may therefore be some kilometers from the discovery well.

With the drilling of the appraisal wells, the nature of the accumulation begins to emerge. With each well, the information gained is used to refine the model of the accumulation until the point is reached when it can be stated with some confidence that the accumulation is sufficiently large and contains sufficient recoverable oil or gas to make the development of the field commercially sensible.

In a sample onshore structure, these appraisal wells may be designed so that they can produce petroleum eventually; but their real purpose is to obtain more information. Offshore, the appraisal wells will not be capable of more than limited testing of reservoirs because the economics of offshore production requires production platforms that will be used first to drill a number of deviated wells to develop part of the field, then to produce them to a central facility for preliminary processing (such as gas separation) and transport through pipelines to the shore facilities. More than 20 wells are usually drilled and produced to one platform, so the appraisal program must be sufficiently comprehensive offshore to permit planning of these wells and the platforms.

It is in the nature of petroleum fields that the decision to develop is made without full knowledge of the accumulation. Full information is available only when it is no longer needed. The appraisal phase ends with the decision either to abandon the prospect as non-commercial, or to develop it as an oil or gas field.

After the decision to develop is made, development wells are drilled for the production of oil (or gas, as the case may be); and their siting is such that each reservoir will be developed as economically as possible (which is not usually the same as cheaply as possible) on the model that has evolved with the added information, which may include additional seismic data as well as the borehole data. This is an exercise in engineering economics based on geology that must also take transport of the production into account. It will normally take at least five years from discovery to put a new field onto production, and during this time there is no income from the field to offset the expenses incurred in its development.

The rate at which a new field can be produced economically is part of the nature of an oil field.