STEP COMPANY Primary Development DelineationDiscoveryAbandonmentTertiarySecondaryExplorationKARAKTERISASI FORMASI

KARAKTERISASI FORMASIB. LUAS .TEBAL . VOLUME / ISIOOIP/OGIP = HxAx (1-Sw)x SEISMIK LOG CORE

Crucial Formation Evaluation QuestionsWhat is the storage capacity (e and t ) in a Complex Lithology Environment ? Are there hydrocarbons, what types of hydrocarbons and, how are they distributed?What is the permeability (deliverability)?Will the formation produce water free? (what is irreducible saturation (BVI))D.Marschall

Reservoir RocksReservoir rocks need two properties to be successful:Pore spaces able to retain hydrocarbon.Permeability which allows the fluid to move.

Rock PropertiesRocks are described by three properties:

Porosity - quantity of pore space

Permeability - ability of a formation to flow

Matrix -major constituent of the rock

Definition of Porosity

Porosity SandstonesThe 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, andb) the grains are not of uniform size.

Porosity and Grain SizeA 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.

QUARZT CEMENT FILL IN POROSITY

Carbonate PorosityIntergranular porosity is called "primary porosity".Porosity created after deposition is called "secondary porosity".The latter is in two forms:Fractures Vugs.

Carbonate Porosity Types 1Interparticle 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:Produced by spaces between carbonate crystals.Mouldic porosity:Pores created by the dissolution of shells, etc.Carbonate porosity is very heterogeneous. It is classified into a number of types:

Carbonate Porosity Types 2Fracture porosity:Pore spacing created by the cracking of the rock fabric.Channel porosity:Similar to fracture porosity but larger.

Vuggy porosity:Created by the dissolution of fragments, but unconnected.

FracturesFractures 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 lowporosity carbonate reservoirs.

VugsVugs are defined as non-connected pore space.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.

DiagenesisThe 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.

FRACTURED POROSITY

Porosity Frekuensi Distribution

High PorosityAverage 13 %St Dev 2%Low PorosityAverage 7%St Dev 1%Porosity Population Probability Distribution

STANDAR NORMAL DISTRIBUTIONMEAN =0STANDAR DEVIASI=1

POROSITAS VERSUS KEDALAMAN

ISOPOROSITY MAP

Rock PropertiesRocks are described by three properties:

Porosity - quantity of pore space

Permeability - ability of a formation to flow

Matrix -major constituent of the rock

Permeability Definition The rate of flow of a liquid through a formation depends on:The pressure drop.The viscosity of the fluid.The permeability.The pressure drop is a reservoir property.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).

POROSITAS DAN PERMEABILITAS

Darcy ExperimentThe flow of fluid of viscosity 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:

Darcy LawK = permeability, in Darcies.L = length of the section of rock, in centimetres.Q = flow rate in centimetres / sec.P1, P2 = pressures in bars.A = surface area, in cm2. = viscocity in centipoise.

Permeability and RocksIn formations with large grains, the permeability is high and the flow rate larger.

Permeability and Rocks 2In 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.

PERMEABILITAS PADA BATUAN

Relative PermeabilityTake a core 100% water-saturated. (A)Force oil into the core until irreducible water saturation is attained (Swirr). (A-> C -> D)Reverse the process: force water into the core until the residual saturation is attained. (B)During the process, measure the relative permeabilities to water and oil.

Relative Permeability Experiment

FRACTURE PERMEABILITY VS FRACTURE POROSITY

PERMEABILITAS VERSUS PORE THROAT

LITOFACIES PETROPHYSICS

CROSS BEDDED QUARZT RICH CARBONATE GRAIN STONEARGILACEOUS GRAIN STONE

SKELETAL WACKESTONEPELOIDAL GRAINSTONE

OOID SKELETAL GRAINSTONECEMENTED OOID SKELETAL GRAINSTONE

***As noted a rock can have porosity but no permeability. If it has zero porosity it will have zero permeability. In practical terms low porosity reservoirs ( < 10% ) exist.

*These are the major petrophysical properties of the rocks, they determine how much oil can be contained and how well it will flow.There are many other ways to describe a rock from a geological perspective. The minor constituents often determine how a rock behaves as a reservoir, hence they are included in the description. For example the shale content of a sandstone and the type of shale will be used. *The amount of porosity gives the volume of the reservoir containing fluids. As it is a fraction it can be described as a number e.g. 0.25 or commonly as a percentage, 25%. Porosity can range from zero to over 50%. In normal reservoirs the range of 20% - 39%.*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.*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.This implies that the silts and shales have porosity containing fluid. The fluid is water as the pore size is so small that capilliary forces prevent hydrocarbon from entering.

*Sandstones can also contain fractures and vugs, however this is rarer than in the carbonates. In the case of vugs the latter are soluble while sandstone is not. *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. Mouldic porosity is where specific shells have been leached from the rock.

*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.From a petrophysical point of view the porosity can be classed by the two main types.

*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.The amount of porosity in a fracture is very small as they are very fine, a few millimetres at most. Their most important property is the permeability.

*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.*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.Tectonic forces are always present. They crack the rock creating fractures.

*These are the major petrophysical properties of the rocks, they determine how much oil can be contained and how well it will flow.There are many other ways to describe a rock from a geological perspective. The minor constituents often determine how a rock behaves as a reservoir, hence they are included in the description. For example the shale content of a sandstone and the type of shale will be used. *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.The relationships between porosity and permeability are used in reservoirs to investigate changing permeabilities through the formation. They are often used as a lithofacies classification/description.*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.The key term in this equation is k/, called the mobility. This determines how a given reservoir will behave depending on the fluid in there. *Permeability is a metric ( but not SI ) unit.Note that the permeability depends directly on the flowrate, Q.

*The flow rate through the large pore spaces is high hence the permeability is high.*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.Due to bedding the permeability can change vertically to a clastic sequence . The vertical permeability kv is determined by the lowest permeability layer. The horizontal permeability kh does not have this problem. The anisotropy , Kv/Kh describes the difference between the two. This ratio is always less than or equal to 1.*The four stages are 100% water, oil and water mixture, residual oil and irreducible water.The first stage represents a water zone only. The last represents an oil zone. The residual oil stage is a reservoir that has been completely produced.The other stage is an intermediate stage, either a production stage or somewhere in the transition zone.*Initially, the core permeability will be the absolute permeability as there is only one fluid at 100% saturation.The relative permeability of water will drop to zero when Swirr is reached because no more water will move.The relative permeability to oil will rise but never reach the absolute permeability because there is still water in the pores.When water is forced in, the relative permeability of water will rise but not reach the absolute value for the same reason.