Reservoir RocksReservoir Rocks
Log Interpretation
Interpretation is defined as the action of explaining the meaning of something.
Log Interpretation is the explanation of logs b, GR, Resistivity, etc. in terms of well and reservoir parameters, zones, porosity, oil saturation, etc.
Log interpretation can provide answers to questions on:
In a well evaluation the questions asked are simple, where is the oil and how much is there. Effectively the question is where will we perforate and how much will come out, will it produce.
These answers are available (usually) from log evaluation.
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Why Run Logs
Uses of logs has advanced over the 60+ years since the technique was pioneered. Simple correlation and hydrocarbon indication has advanced to geochemistry and resistivity profiling. Logs are employed to give information about the reservoir, from formation tops and marker beds to porosity and permeability of layers, to porosity and fluids and their types.
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Reservoir Rocks
The Reservoir
The elements of gas oil and water are not always present at the same time. Any combination is possible.
To have a reservoir all the elements are needed:
A reservoir rock
A source rock (but it may be far away from the actual reservoir).
The cap rock has to be on top.
The structure must be there.
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- source of organic material (terrestrial or marine)
- a suitable combination of heat, pressure and time
- an oxygen free environment
- a suitable basin
Organic material is needed as the source of the hydrocarbons. This material is “cooked” at high temperatures and pressures to give the liquid hydrocarbons. The process takes a very long time. The oxygen-free environment is needed otherwise the organic material cannot become hydrocarbons.
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Reservoirs are represented differently by geologists with rocks and reservoir engineers with the fluids.
The reservoir is pictured in two forms
The cross section
The Cross section shows the structure and the fluids
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Igneous:
(e.g. Marble).
The rocks forming the earth’s crust are broken down into three major classes reflecting their origins.
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Reservoir Rocks
Rock Cycle
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Comprise 95% of the Earth's crust.
Originated from the solidification of molten material from deep inside the Earth.
There are two types:
Plutonic - slow-cooling, crystalline rocks.
Volcanic rocks are those seen immediately after a volcanic eruption. They cool quickly resulting in an amorphous structure. They have no texture.
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Fractured granites form reservoirs in some parts of the world.
Volcanic tuffs are mixed with sand in some reservoirs.
Example: Granite Wash - Elk City, Okla., Northern Alberta,CA
A granite has no porosity or permeability of its own, however tectonic forces may fracture the rock. Into these fractures hydrocarbons can flow to create a reservoir.
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2) Metamorphic rocks
formed by the action of temperature and/or pressure on sedimentary or igneous rocks.
Examples are
Gneiss - similar to granite but formed by metamorphosis
Field Example: 1. Point Arguello - Monterey Formation is actually layers of fractured Chert and Shale. Oil is in the fractures
2. Long Beach, Calif. - Many SS producers on an Anticline above fractured Metamorphic basement rock
3. Austin, TX eastward - Lava flows of Basalt (Serpentine) from Volcanoes in ancient Gulf of Mexico
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Reservoir Rocks
Sedimentary Rocks
The third category is Sedimentary rocks. These are the most important for the oil industry as it contains most of the source rocks and cap rocks and virtually all reservoirs.
Sedimentary rocks come from the debris of older rocks and are split into two categories
Clastic and Non-clastic.
Clastic rocks - formed from the materials of older rocks by the actions of erosion, transportation and deposition.
Non-clastic rocks -
from chemical or biological origin and then deposition.
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Shallow or deep water.
This environment determines many of the reservoir characteristics
Frigg Gas Field -
North Sea
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Continental deposits are usually dunes.
A shallow marine environment has a lot of turbulence hence varied grain sizes. It can also have carbonate and evaporite formation.
A deep marine environment produces fine sediments.
The classical continental deposition of sand dunes produces an excellent reservoir quality reservoir rock. To create a reservoir the dune has to be buried with a source rock and cap rock providing the rest of the elements of the reservoir.
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Depositional Environments 3
The depositional characteristics of the rocks lead to some of their properties and that of the reservoir itself.
The reservoir rock type clastic or non-clastic.
The type of porosity (especially in carbonates) is determined by the environment plus subsequent events.
The structure of a reservoir can also be determined by deposition; a river, a delta, a reef and so on.
This can also lead to permeability and producibility. of these properties are often changed by further events.
An example of changing properties with deposition is water depth. As clastic rocks are deposited by, for example a river, into the sea, the first deposits are the coarser sediments as they “fall’ out of the river first. Finer grains are taken further out into the deep ocean.
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Folding and faulting change the structure.
Dissolution and fracturing can change the permeability.
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Reservoir Rocks
Clastic Rocks
Clastic rocks are sands, silts and shales. The difference is in the size of the grains.
Sands are a reservoir rock, while shales are a source rock and a cap rock. The shales are very fine grained and although the can contain fluids this can only leak out in geological time, very slowly.
Shales and silts also contain other minerals than Quartz. The sediments are buried to create the sedimentary rock, initially filled with water.
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As the sediments accumulate the temperature and pressure increase expelling water from the sediments.
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Calcareous muds become limestone.
Sands become sandstone.
Another effect involves both the grains in the matrix and the fluids reacting to create new minerals changing the matrix and porosity. Fluids can also change creating a new set of minerals.
This whole process is called Diagenesis.
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Sediments are transported to the basins by rivers.
A common depositional environment is the delta where the river empties into the sea.
A good example of this is the Mississippi.
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Some types of deposition occur in rivers and sand bars.
The river forms a channel where sands are deposited in layers. Rivers carry sediment down from the mountains which is then deposited in the river bed and on the flood plains at either side.
Changes in the environment can cause these sands to be overlain with a shale, trapping the reservoir rock.
Ancient river beds below the current level can add up to a considerable thickness, although the individual river channels are small, they are numerous leading to a commercial reservoir.
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They consist of:
Carbonates usually have an irregular structure.
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Reservoir Rocks
Carbonate types
Chalk is a special form of limestone and is formed from the skeletons of small creatures (cocoliths).
Dolomite is formed by the replacement of some of thecalcium by a lesser volume of magnesium in limestone by magnesium. Magnesium is smaller than calcium, hence the matrix becomes smaller and more porosity is created.
Limestone CaCO3
Dolomite CaMg(CO3)2
Evaporites such as Salt (NaCl) and Anhydrite (CaSO4) can also form in these environments.
Limestones and dolomites are usually reservoir rocks. A very dense, low porosity limestone can, occasionally, become Dolomite. Dolomitization is a very important mechanism as it not only creates porosity but permeability paths vital to some reservoirs.
Chalk reservoirs tend to have very high porosity and very low permeability.
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Carbonates are formed in shallow seas containing features such as:
Reefs.
Lagoons.
Shore-bars.
A reef is the simplest carbonate deposition, the skeletons of the reef animals.
In the shallow lagoons, Calcium Carbonate is deposited. Shells and so on are added to the mixture. Changes in sea level allow the deposition of salt or anhydrite as a seal.
Carbonate deposition is very complex as the rocks themselves have “ particle “ sizes ranging from whole shells to line mud. The basic deposition is in shallow seas from biological and chemical action. CaCo3 is soluble hence can be transported around as a solute and then reprecipitated elsewhere.
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Porosity - quantity of pore space
Permeability - ability of a formation to flow
Matrix - major constituent of the rock
These are the major petrophysical properties of the rocks, they determine how much oil can be contained and how well it will flow.
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Definition of Porosity
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Reservoir Rocks
Porosity Sandstones
The porosity of a sandstone depends on the packing arrangement of its grains.
The system can be examined using spheres.
In a Rhombohedral packing, the pore space accounts for 26% of the total volume.
With a Cubic packing arrangement, the pore space fills 47% of the total volume.
In practice, the theoretical value is rarely reached because:
a) the grains are not perfectly round, and
b) the grains are not of uniform size.
The two packing models shown represent some of the possibilities .Cubic packing , with a porosity in excess of 47% is the theoretical maximum which is rarely reached.
These pictures are valid in a lot of cases as the sand sediments deposited are often of uniform size and shape. The addition of smaller grains will reduce the porosity.
Chalk often exhibits cubic packing.
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Porosity and Grain Size
A rock can be made up of small grains or large grains but have the same porosity.
Porosity depends on grain packing, not the grain size.
In a clastic rock the grain size ( same size grains ) does not affect the porosity. Thus a sand, a silt and a shale can have the same porosity .The differences come in permeability where the grain size has a direct effect, large grains meaning higher permeability. This is the reason that a universal porosity - permeability transform does not work; two rocks with the same porosity but different grain sizes will not have the same permeability. The saturation can occur even in the same “ sandstone “ layer in a reservoir in a sequence where the grain size has changed during deposition e.g.. a firing up sequence.
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Diagenesis
The environment can also involve subsequent alterations of the rock such as:
Chemical changes.
Diagenesis is the chemical alteration of a rock after burial. An example is the replacement of some of the calcium atoms in limestone by magnesium to form dolomite.
Mechanical changes - fracturing in a tectonically-active region.
Sedimentary rocks are subject to changes over time. If water of a different chemical composition flows through the rock, reactions can occur changing the rock type or dissolving some of it.
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Carbonate Porosity Types 1
Carbonate porosity is very heterogeneous. It is classified into a number of types:
Interparticle porosity:
Each grain is separated, giving a similar pore space arrangement as sandstone.
Intergranular porosity:
Pore space is created inside the individual grains which are interconnected.
Intercrystalline porosity:
Mouldic porosity:
This set of porosity types is classified as fabric selective.
The porosity created due to changes in the composition.
Intercrystalline porosity can be due to dolomitisation.
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Pore spacing created by the cracking of the rock fabric.
Channel porosity:
Vuggy porosity:
Created by the dissolution of fragments, but unconnected.
This set of porosities are not fabric selective, i.e.. they happen to the entire rock. Fractures crack through any of the types of mineral or “ shell “ in the rock.
These two sets are not the end of the classification of carbonate porosity types, many more schemes exist.
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Porosity created after deposition is called "secondary porosity".
The latter is in two forms:
Fractures
Vugs.
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Fractures
Fractures are caused when a rigid rock is strained beyond its elastic limit - it cracks.
The forces causing it to break are in a constant direction, hence all the fractures are also aligned.
Fractures are an important source of permeability in low porosity carbonate reservoirs.
Fractures are classed as either being vertical or horizontal. Although they can appear at almost any angle, the majority are vertical to sub vertical. They can penetrate from an oil column down into the water, and, as they have very high permeability, can cause production problems.
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They do not contribute to the producible fluid total.
Vugs are caused by the dissolution of soluble material such as shell fragments after the rock has been formed.
They usually have irregular shapes.
The full definition of vugs is more complicated. They are irregular holes in the rock. They have been caused by dissolution of shell (etc) fragments and also some of the matrix surrounding them. They can vary widely in size from a few microns to metres. In this context they are regarded as being a centimetres at most. In most cases the vugs are not connected to each other in any producible manner and hence do not contribute to the formations productivity.
Carbonate rocks will frequently contain both vugs and fractures.
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Reservoir Rocks
Permeability Definition
The rate of flow of a liquid through a formation depends on:
The pressure drop.
The permeability.
The viscosity is a fluid property.
The permeability is a measure of the ease at which a fluid can flow through a formation.
Relationships exist between permeability and porosity for given formations, although they are not universal.
A rock must have porosity to have any permeability.
The unit of measurement is the Darcy.
Reservoir permeability is usually quoted in millidarcies, (md).
The major difference in the two properties porosity or permeability is that the former is a static rock property while the latter is a dynamic rock and fluid property.
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Reservoir Rocks
Darcy Experiment
The flow of fluid of viscosity m through a porous medium was first investigated in 1856 by Henri Darcy.
He related the flow of water through a unit volume of sand to the pressure gradient across it.
In the experiment the flow rate can be changed by altering the parameters as follows:
The flow rate increases with increasing pressure drop; it decreases with increasing length ; it increases with increasing surface area; it decreases with increasing viscosity. Putting this altogether gives an equation with the unknown as the permeability, K.
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Q = flow rate in centimetres3 / sec.
P1, P2 = pressures in bars.
A = surface area, in cm2.
µ = viscocity in centipoise.
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Permeability and Rocks
In formations with large grains, the permeability is high and the flow rate larger.
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Permeability and Rocks 2
In a rock with small grains the permeability is less and the flow lower.
Grain size has no bearing on porosity, but has a large effect on permeability.
The flow rate through the small grained rocks is low hence the permeability is low. The formation contrasts with the one in the previous slide; with the same porosity the permeabilities can differ dramatically. The ultimate contrast is between a very fine grained shale with zero permeability and a coarse sandstone with a high permeability.
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Pore spaces able to retain hydrocarbon.
Permeability which allows the fluid to move.
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Reservoir Rocks
Clastic Reservoirs
Sandstone usually has regular grains; and is referred to as a grainstone.
Porosity
Permeability
Determined mainly by grain size and packing, connectivity and shale content.
Fractures may be present.
Sandstone reservoirs account for the majority of the worlds fields. There will always be bedding variations leading to differences in the quality of the reservoirs. The porosity and permeability are relatively simple to evaluate from core samples.
Fractures may be important in low porosity reservoirs.
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Porosity:
Determined by the type of shells, etc. and by depositional and post-depositional events
(fracturing, leaching, etc.).
Determined by deposition and post- deposition events, fractures.
Fractures can be very important in carbonate reservoirs.
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Impermeable cap rock keeps the fluids trapped in the reservoir.
It must have zero permeability.
Some examples are:
Zero-porosity carbonates.
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Reservoir Rocks
Source Rocks
Hydrocarbon originates from minute organisms in seas and lakes. When they die, they sink to the bottom where they form organic-rich "muds" in fine sediments.
These "muds" are in a reducing environment or "kitchen", which strips oxygen from the sediments leaving hydrogen and carbon.
The sediments are compacted to form organic-rich rocks with very low permeability.
The hydrocarbon can migrate very slowly to nearby porous rocks, displacing the original formation water.
The source rock is recognised on logs by a high GR (due to the high uranium content of organic material) and high resistivity. This is different from the low resistivity of most shales.
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Primary migration - from the source rock to a porous rock.
This is a complex process and not fully understood.
It is probably limited to a few hundred metres.
Secondary migration - along the porous rock to the trap.
This occurs by buoyancy, capillary pressure and hydrodynamics through a continuous water-filled pore system.
It can take place over large distances.
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Limestone CaCO3
Dolomite CaMg(CO3)2
Salt NaCl
Anhydrite CaSO4
Gypsum CaSO4.2H2O
Coal Carbon
Clastic rocks are classified initially by their grain size. There are many more complex classifications for this type of rock but this is the simplest. In this list Conglomerates and Sandstones are reservoir rocks, Siltstones and Shales are source rocks and shales are also cap rocks.
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The criteria for a structure is that it must have:
Closure, i.e. the fluids are unable to escape.
Be large enough to be economical.
The exact form of the reservoir depends on the depositional environment and post depositional events such as foldings and faulting.
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Reservoir Rocks
Traps General
Ghawar Oilfield - Saudi Arabia- Ls - 145 mi x 13 mi wide x260 ft
produces 11,000 b/d total 82B bbls
Gasharan Oilfield - Iran - Ls - 6000ft. Net pay total 8.5 B bbls
The key concepts are those of Net and Gross pay.
Gross pay is always > Net pay. This can also be described by the Net -to - Gross ratio which is always less than or equal to one.
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The simplest form of trap is a dome.
This is created by upward movement or folding of underlying sediments.
An anticline is another form of simple trap. This is formed by the folding of layers of sedimentary rock.
Structural traps describe all the large features and includes domes, anticlines and faults. These large scale reservoirs include most of the Middle East giants.
The difference between these two forms is in the manner of their creation, i.e. the forces applied to them.
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Reservoir Rocks
Fault Traps
Faults occur when the rock shears due to stresses. Reservoirs often form in these fault zones.
A porous and permeable layer may trap fluids due to its location alongside an impermeable fault or its juxtaposition alongside an impermeable bed.
Faults are found in conjunction with other structures such as anticlines, domes and salt domes.
Normal Faults - Nigeria,
Trends (Victoria, TX)
Drag Faults - Wyoming,
most Rocky Mountains
Faulting is an important mechanism in most reservoirs. It forms reservoirs in its own right and also breaks other reservoirs down into specific blocks. Well testing helps determine the fault parameters such as distance from a well, angle and so on.
Faulting of older blocks creating grabens also makes depositional environments for new reservoir formation.
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Salt Dome Trap
Salt Dome traps are caused when "plastic" salt is forced upwards.
The salt dome pierces through layers and compresses rocks above. This results in the formation of various traps:
In domes created by formations pushed up by the salt.
Along the flanks and below the overhang in porous rock abutting on the impermeable salt itself.
Example: Gulf of Mexico, Spindletop,TX, North Sea (Ekofisk
Dome with fractured Chalk as reservoir rock)
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Reservoir Rocks
Stratigraphic Traps
Point Bars - Powder River Basin, WY, Clinton SS in Western Ok,
Michigan - Belle River Mills
Devonian reefs (Barriers and Atolls) - Alberta CA. (Leduc & Redwater)
Midland Basin &Delaware Basin of West TX - Barrier Reefs
Stratigraphic traps describe the traps associated with the depositional environment. Reefs, channels and bars are from specific environments.
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Reservoir Rocks
Reservoir Mapping
Reservoir contours are usually measured to be below Mean Sea Level (MSL).
They can represent either the reservoir formation structure or fluid layers.
Most reservoir maps in the world use m.s.l. as the reference. Depths of the layer increases away from the crest of the structure.
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The formation to be measured is masked by the borehole.
The borehole contains fluids and is of an irregular shape.
The sensor has to be able to measure the formation property accurately and send the information to surface.
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Perfect shape no problems except if very large.
Ovalised hole; will give problems for some tools. Best to run two calipers.
Irregular borehole, gives problems for most tools.
The first problem for measurement is the borehole shape. This depends on the formation being drilled, regional stresses and the drilling practice used.
The best case is the perfectly circular hole. This will only cause problems if it is very large. Ovalised boreholes are often caused by local tectonic stress imbalance. A lot of tools will lie along the long axis and the caliper measuring a large hole size. This may cause too much correction to be applied hence two caliper measurements at 90Þ to each other is preferred as it gives an indication of the borehole shape.
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Tool Positioning - 1
Some tools are run centralised in the borehole in order to measure properly.
These include laterolog and sonic devices.
Special centralisers are put on the tool.
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Some tools are run eccentred, pushed, against the borehole wall.
In some cases this is done with an eccentraliser.
In other cases a caliper arm does this job.
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Tool Positioning - 3
Some tools are run with “stand-offs” to position them at a fixed distance from the wall.
The induction family are usually run in this manner.
Stand offs are physical devices placed on the tool to keep it a fixed distance from the wall. Their use is to keep the tool away from the wall but still in a known position. In some cases (the induction family) this is done to optimise the tools functioning.
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Borehole fluids 2
Oil based mud will not allow current to pass so electrical logs will not work.
Foam and air muds will not transmit sonics signals. Neutron tools are also affected.
Mud salinity affects electrical and induction tools in different manners.
Additives such as barite affect density, gamma ray and photoelectric effect measurements.
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Reservoir Rocks
Borehole - Temperature
Increasing temperature affects the measurements in some tools. The most affected is the thermal neutron devices.
High temperature also affect the performance of the electronics in the tools.
Temperature affects the mud resistivity (it decreases with increasing temperature).
Temperature is measured during each logging run.
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Volume of Investigation
The tool shown here measures all around the borehole. It is omni-directional.
An example of this type of tool is the Gamma Ray.
Some of the “signal” is in the borehole. Most comes from the invaded zone.
Tools are constructed to measure a certain volume of formation. This volume depends on the physics of the measurement being made and the type of sensor.
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Volume of Investigation 3
This type of measurement has the sensor facing in one direction only.
Examples of this are the neutron porosity and bulk density measurements.
This pattern is in a single direction. hence the tool sees a volume of the formation just in front of its sensor. This type of tool is eccentered as any other borehole position would make it read too much of the borehole. These tools see a few inches into the formation, again measuring the invaded zone.
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Reservoir Rocks
Vertical Wells
In vertical wells, with homogeneous layers all types of tool are reading in the same formation.
In horizontal (or highly deviated) wells the deep reading resistivity tools may read a different layer to the shallow reading tools.
In addition the omni-directional tools (e.g. GR) may read different layers from the single direction devices.
The tools are built to read correctly in an infinite homogeneous formation. This situation applies reasonably well with vertical wells. In the horizontal case the focusing of the deep resistivity tools may make them read beyond the layer seen by the shallower tools. This causes confusion when trying to use combinations of both types of measurement.
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An exploration well targets a suspected reservoir.
An appraisal well evaluates a discovery.
A development well is used for production.
The three categories of well have different objectives and hence different logging problems. Exploration wells, in unknown conditions, pose the greatest questions while development wells are usually the simplest to log and evaluate.
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Structural information obtained from surface seismic data.
Rough geological information can be provided by nearby wells or outcrops.
Approximate depths estimated from surface seismic data.
The exploration well is often in very unknown territory. Th surface seismic will give structure, outcrops will give some idea of the geology. Depths, fluids porosity, saturation etc. are all unknowns. The logging suite has to cover all eventualities, a switch in mud type or higher than expected resistivities may require a change of resistivity tool.
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1927 - First electrical log recorded.
1930s - SP, Short Normal, Long Normal and Long Lateral combined, Core Sample Taker.
1940s - Gamma Ray and Neutron, 3-arm Dipmeter using SP, then electrical measurements, Induction tool.
1950s - Microlog tool, Laterolog tool, Sonic tool, Formation Tester.
1960s - Formation Density tool.
1980s - Resistivity Imaging tool, Advanced Sonic tools
1990s - Advanced testing tools, Induction imaging tools, Azimuthal Laterolog tools, Ultrasonic imaging tools, Epithermal porosity tools, Magnetic resonance tools
Tools and acquisition systems have continued to be developed since the first log was recorded in 1927 by the Schlumberger brothers Marcel and Conrad. Some development has improved existing measurements, the simple electrical log has become the Azimuthal Resistivity Imaging Tool. Other are new measurements added to the battery of existing techniques such as nuclear magnetic imaging.
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Early resistivity logs were used to find possible producing zones.
high resistivity = hydrocarbon
SP was used to define permeable beds, compute Rw and determine shaliness.
Resistivity was also used to determine "porosity".
Archie developed the relationship between resistivity, porosity and saturation.
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The simplest evaluation technique consists of recognising the hydrocarbon zone using the porosity and resistivity curves
Water
Water
Shale
Hydrocarbon
This is a fast quicklook technique to recognize hydrocarbon zones. In a water zone the porosity and resistivity will track each other, as the porosity decreases there is less water hence the resistivity increases and vice versa. In shale the resistivity usually reads low and the porosity reads high.
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Reservoir Rocks
Interpretation Procedure
This interpretation procedure follows some simple guidelines to arrive at a final answer. The input is the environmentally corrected and quality checked log data. This is an important step which cannot be avoided if a proper answer is required. Additional information such as core data may also be used. This information is zoned, broken into sections of interest (the reservoir) and other (such as shale and bad hole).
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Zoning
Zoning is the first step in any interpretation procedure. During zoning, the logs are split into intervals of:
1) Porous and non-porous rock.
2) Permeable and non-permeable rock.
3) Shaly and clean rock.
4) Good hole conditions and bad hole conditions.
5) Good logs and bad logs.
Zoning Tools:
Resistivity.
The objective of zoning is to eliminate (or put aside for later study) zones which are not of prime interest, i.e. non reservoir or poor data quality. The best tools to use are the simple ones, the SP and GR which react to simple phenomena. The caliper is good as it often shows shale as bad hole and clean zones as having mud cake, in addition to showing bad hole where the log response is poor.
The neutron-density-Pef are good but the first two also react to the fluid type and the Pef may be affected by barite.
The resistivity is the last tool to use as it is affected mainly by fluids.
Formation