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The final exam will be focused on the topics covered since the last test. However you need to know the basic principles that were introduced during the earlier part of the class. Be sure to study the first two review pages.
This review page is not a substitute for the text book. It emphasizes topics that I expanded on in lecture.

Third Review
Methods of Exploration:
1. Introduction
2. Geophysical Methods of Exploration.
3. Well Drilling and Completion.
Reading: Ch. 3, pp. 37-55

4. Formation Evaluation
Sedimentary Basins and Distribution of Petroleum :


The exploration process

The exploration effort in any given area builds on the work that had been done previously by others. Therefore the specific sequence of steps varies depending on how much information is already available. Below I list a typical process for a frontier basin.

1. Regional Studies. Objective: to identify a sedimentary basin with good hydrocarbon potential (good source and reservoir units). These are studies at the scale of a sedimentary basin.

 Data  Sources
 Surface Geology  Geological survey, other companies
 Results of old wells  National oil company, other companies, geological literature
 Existing seismic data  Other oil companies, data repositories, seismic contractors
 Regional Stratigraphic and geologic history  Geologic journals, university thesis, company reports
 Location of oil and gas seeps  All the above sources

2. Land acquisition

After a prospective basin is identified the company must acquire the right to explore (and exploit) the hydrocarbons in a given area. Different countries have different rules about the ownership of the subsurface. In the US the surface owner also owns the minerals rights (except off shore), in many other countries the state owns the subsurface mineral rights and they lease them to the explorationist in exchange for cash or for a percentage of the oil production if there is a discovery. Often the exploration rights are assigned on the basis of a competitive bidding process. In some other countries only the national oil company is allowed to carry out exploration, but often they establish partnerships with private companies to share the risk (and hopefully also the profits).

This is a complicated political, economic, and legal issue that often determines whether exploration can be carried out in a basin. Oil exploration also impacts the local population (human or otherwise), so the explorationist must be aware of any potential conflicts. Recent examples of this are:

-Debate in US congress over opening the Arctic National Wildlife Refuge to exploration (Alaska North Slope)
-Uwa indian community threatens to commit mass suicide if Occidental Petroleum drills in their ancestral land (Northeastern Andes, Colombia)
-Talisman Energy (Canada) is criticized for investing in oil exploration in Sudan where a civil war is in progress.
 
3. Exploration

Once the exploration rights are secured, a first pass of data is acquired over the lease area. This includes:

-Regional seismic grid (lines are long but widely spaced)
-Detailed surface geological maps
-Geochemical sampling to evaluate source rock maturity and richness
-Age dating to refine knowledge of geological and thermal history
Goals: to identify plays and prospects.
 
What is a petroleum play?
Conceptual model of a system of possible petroleum accumulations in a basin taking into account both the hydrocarbon source and the style of trap that may be present.
What is a prospect?
A specific trap that has been identified but not tested by the drill yet.
 

 

Frequently a second pass of seismic data (shorter, closely spaced lines) is acquired to fully constrain the geometry of each prospect. Products:
-Structural map on the reservoir units
-Isopach maps of the reservoir
-Maps on basement structure and any other important structural feature
-Map of source rock .
 
The goal is to determine:
-Trap geometry and type.
-Closure
-Migration pathway
-Size of the trap
 
Other important studies are:
-Models of thermal maturity
-Structural analysis
-Geochemical modeling
This help determine the timing of hydrocarbon generation relative to the age of the trap, and they type and amount of hydrocarbons expected.
 
4. Risk Assessment (Prospect evaluation)

Once the prospects have been identified and successfully mapped a business decision must be made: Does it make sense to drill or not? To answer this question one must determine:

- What are the potential oil or gas reserves?
-What is the risk of the prospect?
-Cost of drilling?
-What infrastructure is needed? Cost?
-Is there a market for the hydrocarbons?
-How will they be transported to this market?
 
Companies often have to compare prospects from different basins in different parts of the world in order to decide how to spend their exploration budget. It is the geologist's job to promote his/her prospects within the company or to other potential partners.
 
5. Drilling an exploratory well

The objective of an exploratory well is not only to test a specific prospect but also to learn as much as possible about the petroleum geology of the area. Historically the success rate of exploratory wells is about 1 in 10. For this reason it is important to collect as much data as possible from the well, even a dry well is very valuable in guiding future exploration. This means:

6. Well completion and testing

The decision of whether to complete a well (to set permanent casing) is the second turning point in the history of a prospect. Completing a well is expensive, but the only way to know how much a well can produce is by carrying out production tests. Surprisingly, after collecting all the data possible from an open hole, it is still not known if a commercial HC accumulation was drilled.

7. If the well produces oil or gas

If the well produces significant HC, more wells are drilled to define the extent of the field. Once a commercial deposit has been demonstrated to exist, and a way to transport the hydrocarbons to market has been established, the field goes into development and production.

8. If the well produces mostly water,

Then the well is plugged and abandoned and a "post mortem" study is carried out to determine what went wrong, and exploration of other prospects continues aided by the data from the dry hole.

Can you describe the series of steps that lead to the discovery of an oil field?


Surface Geology, Geological Cross-Sections

Surface geological mapping is the oldest and cheapest exploration tool. A geological map contains a wealth of information about the stratigraphy, structure, and geological history of an area.

Examine the relationship between the map and the block diagram. If I gave you a geological map, could you make a block diagram? Could you write the geological history of the area?

Do you understand the relationship between a geological map and a cross section ? Could you draw a cross section to illustrate the subsurface structure implied by the map relationships?

Subsurface Geological Maps

Subsurface structure contour maps are one of the most common ways to represent geological structure in petroleum exploration. you must learn to read and understand contour maps. Structure contour maps are very similar to topographic contour maps. You must always be aware of the contour interval and the reference datum used when making the map.

Rules of contouring:

1.Contours cannot cross (except for overhangs, and thrust faults)
2. A contour must pass between pair of points that are higher and lower than the contour
3. A contour is repeated to show slope reversal
4. Every contour line must close, or go to the edge of the map

 

 

Isopach maps: Contours of equal stratigraphic thickness. These maps are often used to find the thickest part of the reservoir. Other variables that are commonly contoured are net pay, porosity, oil saturation, pressure, etc.

Example 1. Hibernia Field, off shore Newfoundland (Canada)
 
Structure contour map on the top of the Avalon reservoir.
What is the overall structure?
What kind of faults are these?
Why do faults appear as broad gray areas?
Why are the faults on the map not straight?
What determines where the oil/water contact is?
Which is the high and low side of each fault?
Are the faults seals? Is the structure cut into several independent compartments?
What is the vertical offset (or throw) on the G-55 fault?
 
 This is an East West cross section through the map above.
Make sure you understand the relationship between the structures on the map and on the cross section.
Why was well G-55A dry?
What do you make of the fact the the oil/water contact is at -2600m on the fault block with wells I-46 and J-34 (see map) and at about 4100m on the fault block of well O-35?
 
 This is a seismic line through the Hibernia structure. The area shown in the cross-section above is the left part of the line.
What is the relationship between the Murre fault and the anticline?
Can you see the minor faults on the seismic line?
Notice the unconformity above the Avalon reservoir.What does it tell you about the age of the structure?
Example 2. Wilson Creek Anticline, Uinta Basin, NW Colorado
 
 This is an anticline formed over a thrust fault that cuts basement. It is located southeast of the Uinta Mts in Colorado.
Compare the map and cross section. Pay special attention to the way the different fault blocks are shown.
Notice that part of each footwall block is hidden underneath the hanging wall.
Do you see any undrilled prospects on the map?
  Notice that if the Husky well were deeper, it would drill through the Kd-Pmi section a second time. Repetitions like this are typical of thrust faulted areas.

Example 3. Contour map of a stratigraphic trap
   This is an example of a contour map of the Burbank field in Oklahoma. The oil is found in Mississippian deltaic sandstones that are surrounded by shale. It is a strictly stratigraphic trap. It was discovered by chance while drilling on some minor surface anticlines. Notice that the structure contours do not have any closure in the area of the field. The convoluted contours are typical of an area with minor relief that is mapped with a tight contour interval (100 ft in this case).


2. Geophysical Methods of Exploration.

Seismic Reflection Surveying.

Seismic surveying involves three distinct stages of work:

 

 Seismic reflection surveys input a sound wave at the surface and record the echoes, or reflections that bounce back from the earth's layers. These reflections are used to create an image of the subsurface structure.

Seismic reflection method uses P-waves.
What is the difference between P- and S- waves?
 

 The seismic (or sonic) velocity of rocks depends mostly on their density, which generally increases with compaction due to burial. Seismic velocity increases with depth. Salt, limestone and dolomite are generally faster than sandstone and shale.

A reflection is produced when the wave front encounters a sharp velocity contrast. Most lithological boundaries represent a velocity contrast.
Why is the seismic velocity such an important variable in seismic reflection profiling?
 

 In this figure the earth is represented as a pile of rock layers. Each layer has a characteristic acoustic impedance (density * sonic velocity). The contrast in acoustic impedance at each boundary produces a reflection coefficient. This coefficient determines what fractions of the energy are reflected up or transmitted downwards. A high reflection coefficient produces a high amplitude reflection on the seismic record. The seismic trace is the product of the series of reflection coefficients times the input signal.

Make sure you understand this figure

   During seismic reflection profiling the input pulse may be a blast of dynamite, a pressure wave from an air gun (in water), or a series of vibrations from a vibroseis truck.
   In this example shows the response of a three layer sedimentary sequence to a vibroseis input signal. The reflections from the first layer arrive after some delay (time for the wave to travel down to the reflector and back up tot he surface). After some more time the reflections from the second layer arrive, and then from the third layer. Because the three reflected waveforms overlap in time the trace recorded in the field is the sum of the three. In order to recover the earth's signal, it is necessary to subtract the signal from the vibroseis sweep from the field signal (last trace on figure).
 

 The earth acts like a filter which absorbs some frequencies preferentially. Seismic data usually contains frequencies ranging from 8 to 60 Hertz. The low frequencies dominate the deep part of a seismic profile. This means that the vertical resolution of seismic data is limited. Only layers on 10's to 100's of feet thick can be resolved. The seismic signal is the product of the interference of many thin layers that are not individually resolved.

How does the vertical resolution of seismic data compare to that of well logs?

 

 The geometry of a reflected ray is such that the travel-times from a horizontal reflector increase with horizontal distance away from the shot. Normally a long array of geophones are laid out on the ground to receive the reflections. The increase in travel-time with distance away from shot is known as "move-out" and it follows a hyperbolic function.

Why is move-out usefull?

   The geometry shown above means that the uncorrected seismic records from a single shot recorded by 25 geophones spread out on each side of the shot would look like this figure. Flat reflectors appear curved as hyperbolas. With increasing seismic velocity (increasing depth) the hyperbolas become flatter. This effect must be corrected out during processing. Accurate correction requires knowledge of the seismic velocity .
 

 During a seismic survey many input signals (shots) are recorded at uniform intervals along a line. Because of the geometry of reflections (incidence angle=reflection angle) data from the same spot in the subsurface is gathered from each shot at a different geophone. This means that the data is very redundant. This redundancy can be used to enhance the signal-to-noise ratio by summing redundant records. This process is known as stacking. The use of stacking brought about the most dramatic improvement in the quality of seismic data.

Why does stacking iprove the signal to noise ratio?

 
This line shows an example of common problem with marine data. The shallow structure appears repeated several times. This repetitions are known as multiples and are due to seismic energy bouncing between the seabottom and the surface of the ocean several times. Multiples can be removed during stacking.
Is everything that appears on a seismic line real structure?
 
 Another common problem:
During the initial calculation of moveout it is assumed that the layers are flat. If this is not true some of the reflected energy is misplaced on the section. For example in the case of the small sycline on the left reflections from three different spots on the syncline are plotted right below the receiver. The syncline appears as a bow tie on the seismic line. Migration is a process that corrects for this effect and restores the energy where it belongs.(Do not confuse with migration of hydrocarbons)
 

 
 Unmigrated line (notice bow ties, and diffractions)

 Migrated line. The syncline appears correctly.
   Seismic lines are presented with distance in the horizontal axis and travel-time in the vertical axis. In order to make accurate depth conversions, and good correlations with well data one needs accurate interval velocities for all the layers. On way to get this data is from the sonic log. A better way is to run a well velocity survey where a geophone is lowered down the well and seismic shots are done at the surface. This way the travel time and velocities can be measured directly.
   This is an example of a vertical seismic profile (a more sophisticated version of a velocity survey) used to correlate a well to the seismic data. Another way to do this is to use the sonic and density logs to calculate the reflectivity series, and then produce a synthetic seismogram that can be used to correlate with the real seismic data.


Other Geophysical methods

Gravity Surveying

  The gravitational acceleration at the earth's surface is about 9.8 m/s^2. However there are small variations due to differences in the distribution of mass below the surface. Gravity surveying takes advantage of this. The sketch to the left shows the principle behind a gravimeter. An increase in the pull of gravity will cause the spring to stretch slightly.
 

 This example shows the shape of the gravitational anomaly (solid black line) above a buried body with density lower than the surrounding area. A sedimentary basin could be such a body. Quantitative modeling of the anomaly provides an estimate of the depth, and shape of the basin. Gravity are much cheaper than seismic surveys, they provide a first approximation to the subsurface geology of an area. They are specially useful for locating salt diapirs because the salt is less dense than the surrounding rock. However gravity modeling is inherently ambiguous because a shallow body with a small density contrast can produce the same anomaly as a deep body with a large density contrast.

What types of traps might be found by this method?


3. Well Drilling and Completion.

 
 A modern rotary drill rig is composed of four separate systems:
Engines-Power everything
Hoist syst.-Used to lift, lower and suspend the drill string in the well
Rotating syst.-
Mud System
   The rotating system consists of the kelly, rotary table, the drill string, the drill collars and the bit.
 
 The mud system is used to pump drilling mud down the drill string and back up to the surface. This system has multiple functions:
Control the subsurface pressure via
Mud weight
Blow-out preventers (valves)
Prevent the hole from collapsing
Cool the drill bit
Remove the drill cuttings
Drilling mud is a key element of the drilling process. If the mud weight is too high the reservoir may be damaged, if too low there may be a blow out if a high pressure zone is encountered.
 
 Steel casing (heavy gage pipe) is used to maintain the integrity of the hole and to isolate specific strata.
Surface casing is always set in order to attach the blow-out preventers to control pressure. If the well is successful, production casing is lowered to the reservoir, cemented to the walls and perforated in front of the reservoir unit in order to be able to test and produce that interval.
 

 In some cases it is necessary to set an intermediate casing in order to isolate an over pressured (or under pressured) layer. Otherwise it would be necessary to maintain excessively high mud weight that would invade the reservoir damaging it.

Know how over pressured zones can be handdled.

 

 Wells are not always vertical. If the beds are tilted the well will tend to "walk" up-dip causing the well to deviate. In other cases the well is deviated on purpose, such as in order to drill several wells from a single surface location, or if it necessary to "side-track" the hole to avoid an obstruction. Directional drilling is done with a bit that is powered by a down hole motor (or turbine) instead of powered by turning the entire drill string from the surface.

How are horizontal wells drilled?


4. Formation Evaluation.

 
 During drilling well side geologists monitor many parameters that help figure out the stratigraphy that is being drilled, as well as detect any hydrocarbons that may be present.
Log of the well cuttings
Log of gas and gas chromatography
Oil shows
Drilling rate
Mud weight
Any kicks (high pressure zones)

Drilling rate depends on lithology. Sandstones are fast to drill, shales are more difficult as well as carbonates.

 

 Cores can be used to sample any unit of interest. Sidewall cores are collected by lowering a tool that has hollow sampling bullets attached with a wire. Small cylindrical plugs are recovered when the tool is pulled back out.

Conventional cores are cut with a bit that cuts a cylinder of rock and traps it inside the drill string.

   Drill-stem-tests (DST): It is possible to test the fluids in an open hole by setting packers above and below the interval of interest. This way a unit is isolated and the formation fluids are allowed to flow into the drill string. This way the formation pressure, and permeability can be measured and the formation fluids sampled. DST's are often unreliable because it is difficult to completely isolate the reservoir unit. Also frequently some of the drilling mud has invaded the formation, so pristine fluids do not flow into the well.


Well Logs
 

 Well logs are the main tool for characterizing a well. The book has a reasonable summary of the different types of logs available and the principles behind them.

The table on the left summarizes the main types of logs and their uses. The principal uses of well logs are:
Lithologic determinations
Stratigraphic correlations
Evaluation of formation fluids
Porosity determination
Correlation with seismic data
Location of faults and fractures
Determination of the dip of strata
  This is an example of the use of the Gamma ray (GR), SP, Resistivity (Rsn, Ril), Neutron (CNL), and Density (FDC) logs to identify a gas-rich zone. The Gamma Ray and SP indicate the location of the reservoir bed, the high Resistivity at the top of the bed shows that it is saturated with hydrocarbons, the cross-over of the Neutron and Density logs shows that the hydrocarbon in question is gas.
   

   6. Field Development

Objectives:

1. Maximize Rate of rrturn of investiment (Recover invested dollars as fast as possible, plus some profit)

2. Maximize Ultimate Recovery of oil and gas. (Be able to sell the greatest amount of oil and gas possible over the life of the field)

These two objectives are somewhat in conflict because pumping the oil out too fast will damage the reservoir. So a good development paln is required

 

 Reservoir Simulations

In order to predict the production potential for each well location and for the field as a whole, a good reservoir model is required. This requires knowledge of: trap geometry , porosity distribution, permeability distribution (including fractures), water saturation, oil-gas ratios, and pressure regime. The model typically grids the field area, assigns values for all these parameters to each cell, runs the flow equations and outputs production rates for oil, gas and water and predicts the life of each well. In practice, the parameters of the field are poorly known to begin with, but become better constrained as wells are drilled and produced.

 

 Production Histories

Typically oil production ramps up rapidly and then declines as reservoir pressure goes down, as water enters the well, etc. Gas production climbs more slowly if it comes from gas exsolution (which also occurs due to the pressure drop).

 

 Reservoir Pressure regimes

Three common pressure regimes are : Gas exsolution drive, gas cap drive, and water drive. Gas exsolution drive occurs in reservoirs filled with liquid hydrocabons only. As the pressure drops, gas comes out of solution repalcing some of the fluid that has been removed thus helping sustain the pressure. If the reservoir contains both oil and free gas, as the oil is produced, the gas expands helping sustain pressure. In this case pressure does not drop as fast in the reservoir and there is higher ultimate recovery than under gas exsolution drive. Water drive occurs when there is an active acquifer. In this case there is very little pressure loss, but water tends to invade the reservoir more easily as it is driven by the hydrostatic head of the acquifer, instead of only by the pressure difference created by pumping the well. Reservoirs of this type may have high recovery factors, but also high water production.

 

 Well Spacing

One of the objectives of reservoir models is to determine what the most efficient well spacing should be in the field. Also to decide what the best distribution and position of injection wells should be. For fields in dipping strata a common strategy is to use 'line drive" where a row of injection wells are used to displace the hydrocarbons towards the updip production wells.

Deviated Wells

In order to minimize surface impact, or in order to reach reservoirs that are not direcrtly below the well location, deviated wells are used. Off shore, essentially all develoment wells are deviated from a central platform. Also, horizontal wells can maximaze the volume of reservoir rock in contact with the borehole thus giving access to larger reservers from a single well and they can be orriented to intersect facture systems at the ideal angle.

   

 

 Fluid Injection

An important issue to consider when planning an injection program is that the injected fluids will follow the highest permability paths within the reservoir. Fluids injected into high permeability zones may bi-pass large volume of hydrocarbons and invade adjacent wells.