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Unit Outline

Outline Introduction 1 The chemistry

of petroleum –what ispetroleum?

2 Keyingredients forpetroleumaccumulation

3 Exploring foroil and gas 3.1

Detection,explorationandevaluation

3.1Detection,

explorationandevaluation(continued)

4 Petroleumproduction

5 Safety and theenvironment

6 Oil and gasreserves

7 Non-conventionalsources ofpetroleum

8 Unit summary Glossary References andAcknowledgements

3 Exploring for oil and gas3.1 Detection, exploration and evaluation (continued)3.1.3 Seismic data and interpretationSeismic surveying is by far the most widely used and important method of gaining an impression ofthe subsurface. Seismic surveys can be acquired at sea as well as on land. The marine method is themost common in petroleum exploration and is shown schematically in Figure 6, although the sameprinciples apply to any seismic reflection survey.

Figure 6: Marine seismic acquisition – pulses of sound energy penetrate the subsurface and arereflected back towards the hydrophones from rock interfaces.

Compressed air guns towed behind a boat discharge a high-pressure pulse of air just beneath thewater surface. The place of detonation is called the shot point and each shot point is given a uniquenumber so that it can be located on the processed seismic survey. The sound waves (effectively thesame as seismic P-waves produced by earthquakes) pass through the water column and into theunderlying rock layers. Some waves travel down until they reach a layer with distinctively differentseismic properties, from which they may be reflected in roughly the same way that light reflects off amirror. For this reason such layers are called seismic reflectors.

The reflected waves rebound and travel back to the surface receivers (or hydrophones), reachingthem at a different time from any waves that have travelled there directly. Their exact time of travelwill depend on the speed that sound travels through the rock: its seismic velocity. Other waves maypass through the first layer and travel deeper to a second or third prominent reflector. If these areeventually reflected back to the hydrophones they will arrive later than waves reflected from upperhorizons.

The hydrophones therefore detect ‘bundles’ of seismic waves arriving at different times because theyhave travelled by different routes through the rock sequence. Computer processing allows theamalgamation of recordings from all the shot points, filtering out unwanted signals of various sorts.

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amalgamation of recordings from all the shot points, filtering out unwanted signals of various sorts.The final result is a two-dimensional (2-D) seismic section. By using closely spaced survey lines orhydrophones arranged in a grid it is possible to produce 3-D seismic datasets. These are usuallyinterpreted on a PC workstation and colours are normally used to enhance the image and aidinterpretation. The data can be viewed in any orientation in order to create a 3-D visualisation ofselected horizons (Figure 7).

Figure 7: A 3-D view of the Palaeocene reservoir in the Nelson Field, North Sea. The image isderived from a ‘cube’ of closely spaced 3-D seismic data, onto which the paths of the productionwells are superimposed. Bright colours in this perspective view relate to depths to a particular

reflecting boundary. Reds and greens are structurally highest, where petroleum may be trapped.Seismic data of all forms (2-D or 3-D) are displayed with the horizontal axis indicating geographicorientation and distance, whereas the vertical axis is calibrated in time. The time, measured inseconds, records how long it took the seismic wave to travel from shot to reflector and then back tothe hydrophone, so it is described as two-way travel time (TWT). Further processing and theincorporation of seismic velocity data allows TWT to be converted into depth. Depth-convertedseismic data is the mainstay of exploration since it provides a meaningful basis for all subsequentinterpretation.

What would happen to seismic waves if there was a strongly reflective layer, such as anigneous sill or salt body, in the shallow subsurface?

Now read the answer

Interpreting seismic sections is something of a ‘black art’, requiring both experience and a certainamount of interpretative flair. At the outset, interpretation involves tracing continuous reflectors on 2-Dsections in order to build up a plausible structural representation of the subsurface. In the context ofan initial exploration programme to find possible traps this is often sufficient.

Look at the 2-D seismic section in Figure 8. Even though it was produced to explore for coalseams it contains lots of information that might help the petroleum explorationist. What kindsof trap shown in Figure 4 might be present in that section?

Figure 8: An example of a seismic section. The (vertical) arrival time axis in milliseconds (ms) isroughly equivalent to increasing depth. Towards the top of the section a pair of dark lines

indicate major coal seams. They are displaced by a fault near the centre of the traverse (markedby the dashed red line). Many other features show up, including greater complexity in the deeperpart of the section, and towards the left of the section deep, more steeply dipping reflectors are

part of the section, and towards the left of the section deep, more steeply dipping reflectors aretruncated by the simpler ones at shallower depths: this is an unconformity (solid blue line).

Now read the answer

Good quality 3-D seismic data provides sufficiently fine resolution for exhaustive processing andanalysis to help in managing and developing known oilfields (Box 2).

Box 2: Applications of 3-D seismic dataModern 3-D seismic data can be used for many purposes other than simply defining trap geometry.Sometimes it is possible to identify the presence of petroleum directly, particularly dispersed gaswhich tends to dissipate seismic waves and produce an ill-defined ‘shadow zone’ above a leakingtrap. The changes in acoustic properties across a gas–water or gas–oil contact may also bedetected as a horizontal reflector that conforms to the geometry of the trap.

More commonly, however, seismic data are used to map rock characteristics at a variety of scales.Starting with the recognition of distinctive reflector geometries and seismic sequences, and then byapplying a range of seismic techniques, depositional environments can be mapped over a verywide area. As drilling progresses and data on rock properties (such as seismic velocity anddensity) become available, increasingly sophisticated reservoir descriptions can be developed.These commonly include an assessment of lithology, the amount of petroleum that is present, fluidtype and porosity.

Interpretation of 3-D seismic data is an enormously varied and rapidly developing area ofpetroleum exploration that is beyond the scope of this unit.

Seismic technology has been transformed since the 1980s. Today, 3-D seismic, rather than single2-D sections, are routinely used for exploration purposes in offshore environments because thedata can now be acquired quickly and cheaply. New processing techniques and improvedcomputerised visualisation tools add clarity to the data, helping to provide an unparalleledimpression of the subsurface. The emphasis in exploration is to reduce the risk of drilling a dryhole and wasting a great deal of investment. This can only be achieved by integrating all theappropriate types of data, and with thoughtful analysis.

3.1.4 Exploration drillingWhen seismic data highlight a suitable prospect, the next step is to drill into the reservoir in order toestablish whether or not petroleum is trapped, and, if it is, to establish how large the accumulationmight be. There are several types of drilling rig, ranging from relatively small ones as deployed onland (Figure 9a) that can be dismantled and transported by truck or helicopter, to large offshore units(Figures 9b–d) that are capable of working in a range of water depths and sea conditions. Anoffshore jack-up rig is a barge with lattice steel legs that can be raised and lowered (Figure 9b). It istowed into position by tugs and its legs are lowered to the seabed before the barge is raised 10–30 mout of the water to create a stable drilling platform. They usually operate in water depths up to 200 m.

Drilling in greater water depths requires a floating unit and the semi-submersible rig is the mostcommon and versatile type (Figure 9c). The working platform is supported on vertical columns thatare attached to submerged pontoons. Once in position, the rig is anchored to the seabed and thepontoons are flooded with water to submerge them beneath wave level. The lower the pontoons arebeneath the water, the less likely they are to be affected by wave action. This makes them stable inrough seas. Some semi-submersible rigs have computer-controlled positioning propellers, rather thananchors, to keep them in position and they can be used in water depths down to 1000 m or more.

Figure 9: Mobile drilling units can operate on land (a) or in a variety of water depths. Jack-up rigsat rear and front right in (b) are used in water up to 200 m deep, whilst semi-submersible rigs

foreground in (b) and (c) and drill ships (d) can operate in much deeper water.Drill ships resemble conventional ships and they can move easily around the world (Figure 9d). Theytoo have dynamic positioning that allows them to stay on location with remarkable accuracy in all butthe most severe storms. Since they are not ballasted they can be unstable in high seas but theiradvantage is that they can drill in water depths in excess of 2000 m.

What type of rig would be used to drill in the Amazonian rainforest and what preparationwould be required before drilling commenced?

Now read the answer

Drilling for oil and gas is a sophisticated and very expensive process. Wells often penetrate over 3000m into sedimentary rock; the deepest exceed 6500 m. At such depths the fluid pressures in the rockformations are so high that a dense drilling mud is continuously pumped into the borehole to counter-balance the pressure. This significantly reduces the possibility of an uncontrolled surge of petroleumto the surface, a situation that is graphically described as a ‘blowout’. The enduring image of rigworkers celebrating beneath a gushing fountain of crude oil in the pioneer days of exploration distortsreality, since blowouts and the release of associated toxic gases such as hydrogen sulphide (H2S)are very dangerous. Every modern well is fitted with hydraulic rams that instantly isolate the boreholeif excess pressures cause the well to flow. The other useful functions of drilling mud are to lubricateand cool the drill bit, to circulate rock fragments (cuttings) back to the surface and, in some cases, topower a turbine that rotates the drill bit.

3.1.5 Well evaluationTo some extent, well evaluation is similar to evaluation of coalfields. Traditionally an exploration wellis evaluated at discrete stages by withdrawing the drill bit, lowering instruments (colloquially knownas ‘tools’) down the hole on a steel cable and then hauling them slowly back to surface. This processis known in the petroleum industry as wireline logging. As the tools are withdrawn they record theproperties of the rocks that surround the well and the fluids in them. Nowadays this approach issupplemented by measurements that are made while drilling is in progress, which has theadvantages of providing near instantaneous data and incurs none of the expense of halting thedrilling process.

The rock properties that are of interest include those used for identifying lithologies and small-scalestructural or sedimentological features. Other tools help estimate porosity, permeability, pressure andfluid content. None provide a completely definitive description of the borehole wall, but incombination the data acquired by wireline logging provide sufficient information to determine whetherfurther evaluation is justified.

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The most useful geological data are derived from pieces of rock recovered from specific depthintervals. They range in size from small fragments of rock (drill cuttings) produced as the drill bit cutsinto rock, to thumb-size and larger (5–15 cm diameter) cores of solid rock that are retrieved withspecial tools. These provide the basis for a detailed description of the reservoir, although cores mayalso be taken in mudstones to gain biostratigraphic and/or geochemical information.

Some exploration wells, particularly those that encounter significant volumes of petroleum, justify anextensive evaluation programme that is designed to recover fluid samples from selected intervalsdown the well. The fluids (oil, gas and water) are captured in situ at reservoir temperature andpressure, and then brought to the surface in a small sealed chamber for analysis. Less commonly, thefluids may be sampled by allowing them to flow to the surface. Such well testing may continue forseveral days. During that time it is possible to draw some preliminary conclusions about the nature ofthe reservoir, flow rates and the commercial potential of the petroleum accumulation.

Activity 3Exploration is an expensive activity that can quickly lead to ‘gambler's ruin’ – betting moremoney than you win – unless there is a proper understanding of risk and potential reward. Atthe outset it is vital to decide where not to explore. List some of the fundamental geological,technical and commercial factors that you might use to reject certain parts of the world fromexploration.

Now read the answer

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