Fundamental Drilling Fluids School: Basic Geology Notes

Basic Geology

Hydrocarbon Generation to Production

  • Process:
    • Generation (Source)
    • Migration
    • Accumulation
    • Exploration
    • Production

Petroleum System

  • Requires timely convergence of:
    • Mature source rock
    • Migration pathway
    • Reservoir rock
    • Seal or cap rock

Hydrocarbon Sources

  • Inorganic:
    • Chemical reactions form petroleum and coal naturally.
  • Organic:
    • Fossil fuels from plant and animal remains.
    • Transformation via:
      • Bacterial action
      • Heat and pressure
      • Catalytic reactions
      • Radioactive bombardment

Source of Hydrocarbons - Influences

  • Bacterial Action
  • Heat
  • Pressure
  • Time
  • Radioactive Bombardment
  • Burial
  • Chemical Reactions

Migration and Accumulation

  • Migration: Natural movement from source to reservoir rock.
  • Influenced by:
    • Buoyancy
    • Hydrodynamics
    • Capillary action

Buoyancy

  • Mechanism in hydrocarbon migration.
  • Petroleum is lighter than water and naturally rises above it; gas rises above oil.

Hydrodynamics

  • Forces by which one fluid flows past another, carrying the second fluid.

Accumulation

  • Buoyancy, hydrodynamics, and capillary action cause migration from source to reservoir rock.
  • Reservoir rock needs a cap rock or confining bed to seal the reservoir.

Origin of Petroleum

  • Geological requirements for a conventional hydrocarbon reservoir:
    • Source Rock
    • Migration Path
    • Cap Rock
    • Reservoir Rock
    • Trap

Source Rocks

  • Hydrocarbons originate from minute organisms in seas and lakes.
  • Organic-rich "muds" form in fine sediments after death.
  • Reducing environment strips oxygen, leaving hydrogen and carbon.
  • Compaction forms organic-rich rocks with low permeability.
  • Hydrocarbons migrate slowly to porous rocks, displacing water.

Hydrocarbon Migration

  • Two stages:
    • Primary migration:
      • From source rock to a porous rock with greater permeability.
    • Secondary migration:
      • Movement within the carrier bed to the trap.

Reservoirs Rocks

  • Porous and permeable lithological unit holding hydrocarbon reserves.

Cap Rocks

  • Low permeability unit impeding hydrocarbon escape from the reservoir.

Occurrence

  • Petroleum occurs in pores between sand grains; porous sandstones are excellent reservoirs.
  • In an anticlinal structure, petroleum is trapped by impermeable cap rock, with gas rising above oil.

Geology - Definition

  • The science which treats:
    • (a) Of the structure and mineral constitution of the globe; structural geology.
    • (b) Of its history as regards rocks, minerals, rivers, valleys, mountains, climates, life, etc.; historical geology.
    • (c) Of the causes and methods by which its structure, features, changes, and conditions have been produced; dynamical geology.

The Rock Cycle

  • Transportation
  • Weathering
  • Deposition
  • Uplift and exposure
    SEDIMENTS
  • Lithification
    • Compaction and cementation
  • Sedimentary Rocks
  • Metamorphism
  • Metamorphic Rocks IGNEOUS ROCKS
    • EXTRUSIVE
      • Consolidation
    • INTRUSIVE
      • Crystallization
  • Igneous Rocks
  • Melting
  • MAGMA

Types of Rocks

  • Igneous
    • Source: Molten materials in deep crust and upper mantle
    • Process: Crystallization
  • Sedimentary
    • Source: Weathering and erosion of rocks exposed at surface
    • Process: Sedimentation, burial and lithification
  • Metamorphic
    • Source: Rocks under high temperatures and pressures in deep crust
    • Process: Recrystallization due to heat, pressure, or chemically active fluids

Igneous Rock

  • Comprises 95\% of the Earth's crust.
  • Originates from molten material inside the Earth.
  • Two types:
    • Volcanic - glassy, fast cooling ⧫
    • Plutonic - slow-cooling, crystalline.
  • Igneous rocks were once hot molten liquid known as magma.
  • Magma cools and solidifies.
    • Intrusive (cool in the ground) : Granite
    • Extrusive (cool outside the ground) : Pumice / Basalt

Igneous Rock Varieties

  • GRANITE-speckled white & black
  • GABBRO-dark colored, evident crystals
  • BASALT-dark colored, fine grained
  • RHYOLITE-light colored, possible banding
  • OBSIDIAN-black, glassy
  • SCORIA-dark colored, numerous holes

Igneous Rock and Reservoirs

  • Igneous rocks can be part of reservoirs.
  • Fractured granites form reservoirs in some parts of the world.
  • Volcanic tuffs are mixed with sand in some reservoirs.

Metamorphic Rocks

  • Formed by temperature, pressure, and/or chemical reactions on sedimentary or igneous rocks.
  • Examples:
    • Marble - from limestone
    • Carbon can change to graphite and graphite can change to diamond
    • Hornfels - from shale or tuff
    • Gneiss - similar to granite but formed by metamorphosis

Metamorphic Rock Formation

  • (Graphite) + Heat + Pressure -> (Diamond)
  • Limestone -> Marble

Sedimentary Rocks

  • Most important for the oil industry, containing source, cap, and reservoir rocks.
  • Come from debris of older rocks, split into two categories (are composed of particles which are broken from preexisting rocks. These particles are then transported by wind, water, or ice and deposited. After deposition, compaction (lithification) and cementation occur, forming sedimentary rocks).
  • Composed of:
    • Fragments Eroded From Older Rocks.
    • Minerals Produced by Chemical or Biological Processes.

Sedimentary Rocks - Classification

  • Classification of size, shape and fabric of sediment particles.
    • A. Grain size
      • Gravel >2mm
      • Pebbles 4-64 mm
      • Granules 2-4 mm
      • Coarse sand 0.5-2 mm
      • Medium sand 0.25-0.5 mm
      • Fine sand 0.06-0.25 mm
      • Silt 0.004-0.06 mm
      • Clay < 0.004 mm
    • B. Rounding
      • Angular
      • Sub-rounded
      • Well-rounded
    • C. Sorting
      • Poorly sorted
      • Well-sorted
    • D. Grains and matrix
      • Grain
      • Matrix

Sedimentary Rocks example

  • Parent Rocks
  • Erosion
  • Wind, Water, etc.
  • Transportation of Fine Rock Particles
  • Compaction
  • Hardening
    • Lithification
  • Sedimentary Rock
    • Sandstone
    • Shale
    • Limestone - Fossiliferous
      • The most common sedimentary rock in an drilling setting

Classification of Sedimentary Rocks

  • Three categories:
    • Organic
    • Chemical (Non-Clastic)
    • Clastic
  • Note: Commercial hydrocarbons are almost always located in the latter two types.

Organic Sediments

  • Such as coal accumulate from the remains of organisms such as plant remains.

Clastic and Non-Clastic Rocks

  • Clastic rocks: Formed from the materials of older rocks by the actions of erosion, transportation and deposition.
    • Rock type - Particle diameter
      • Conglomerate - Pebbles - 2 – 64 mm
      • Sandstone - Sand - 0.06 – 2 mm
      • Siltstone - Silt - 0.004 - 0.06 mm or 4 to 65 microns
      • Shale - Clay - < 0.004 mm or 4 microns
    • Clastic rocks are sands, silts and shales. The difference is in the size of the grains.

Rock Classification

  • Non-clastic rocks - from chemical or biological origin and then deposition.

    • Rock type - Composition
      1. Carbonates CaCO_3
      2. Dolomite CaMg(CO3)2
      3. Gypsum CaSO4•2H2O
      4. Anhydrite CaSO_4
      5. Halite NaCl
      6. Carnallite KMgCl3•6H2O
      7. Potash KCl
      8. Polyhalite K2Ca2Mg(SO4)4•2H_2O

Sedimentary Rock Types - Relative Abundance

  • Siltstone, mud and shale ~75\%
  • Sandstone and conglomerate ~11\%
  • Limestone and dolomite ~13\%

Types of Sedimentary Rock

  • Examples:
    • Sandstone
    • Shale
    • Breccia
    • Conglomerate
    • Limestone
    • Carbonate
    • Dolomite
    • Chalk
    • Marl
    • Reef
    • Chert
  • Grains settle to the bottom of the sedimentary basin
  • Increased pressure and temperature causes water to be expelled
  • Consolidation occurs to form the rock

Conditions for Hydrocarbon Producing Reservoir

  • To have a hydrocarbon producing reservoir,
    1. There must be a body of rock having sufficient porosity (Ø) to contain the reservoir fluids and permeability (K) to permit their movement
    2. The rocks must contain hydrocarbons in commercial quantities
    3. There must be some driving force within the reservoir, gas or water, to allow the fluids to move to the surface.

Oil Reservoir

  • An oil and gas reservoir is a formation of rock in which oil and natural gas has accumulated.
  • The oil and gas collected in small, connected pore spaces of rock and are trapped within the reservoir by adjacent and overlying, impermeable layers of rock

Clay, Claystone and Shale

  • Over 1/2 of all sedimentary rocks.
  • Particles too small to identify w/ naked eye.
  • Deposited in quiet (slow moving) water. ✓ deep ocean & continental slope- ✓ lakes- floodplains of rivers
  • Clays often posses porosity but very little permeability. Microfissures may be present in some clays. These can be plugged with polymers or gilsonite/lignite type materials.
  • Shale formations are often highly stressed tectonically or overpressured. Evidence of this will be seen by the size and shape of the cavings.
    • Rounded particles indicate particles that have been in the hole a long time.
    • Moon shaped/sliver particles indicate overpressure or tectonic pressures
    • Sticky particles indicate hydratable shales

Carbonates

  • These are a group of sedimentary rocks containing the anion CO_3 -- as the fundamental unit in their structure. eg. CHALK, LIMESTONE and DOLOMITE.
  • Carbonates (limestone and dolomite) normally have a very irregular structure.
  • Fractures can be very important in carbonate reservoirs.

Carbonate Types

  • Chalk is a special form of limestone (CaCO_3) and is formed from the skeletons of small creatures (cocoliths).
  • Dolomite (CaMg(CO3)2) is formed by the replacement of some of the calcium 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.

Dolomitization

  • Limestone fragments when in contact with Magnesium salts are transformed into dolomite.
  • There is a net loss in volume because the dolomite structure is very tightly packed.
  • Dolomitisation is a progressive phenomenon. Increasing Limestone Dolomitisation Dolomite
    • Limestone
    • Calcareous
    • Dolomite
  • The net loss in volume gives rise to vugs. Thus increasing the porosity.
  • 2CaCO3 + 2Mg^{2+} 2CaMg(CO3)_2

Sand, Sandstone and Siltstone

  • These are rocks made up of quartz grains. Sand has no cement holding the grains together.
  • Sandstone and siltstone have some or all of the grains cemented together either by precipitation of chemicals (carbonates), or by deposition of small solid fragments.
  • The porosity and permeability depends on the degree of cementation.
  • Drilling problems arise due to the porosity and permeability.
  • Also the formations may be poorly cemented in which case the well bore can collapse (especially in top hole or deviated wells). The mud weight may need to be increased in this case.

Evaporites

  • Sediments arising from the evaporation of saline waters.

  • The minerals are formed in the reverse order of their solubilities.

    • Increasing
      • Potash KCl or Magnesium Salts
      • Halites. NaCl or mixed salt.
      • Gypsum CaSO4 . 2H2O
      • Anhydrite CaSO_4
      • Dolomite (primary) CaMg (CO3)2

  • Note gypsum normally converts to Anhydride under heat and pressure.

  • Gypsum evaporites are non-porous so fluid loss control is not necessary.

Rocks and Loose Sediments

Loose SedimentsSedimentary rocks
gravelconglomerate
sandsandstone
siltsiltstone
clayshale

Properties of the Rock Fluid

  • Simultaneous existence of 2 or more fluids in porous rock requires to define following terms:
    • Porosity
    • Permeability
    • Sorting

Porosity

  • Porosity is defined as the relative amount of open space existing within a rock (or Porosity is the percentage of the formation that is occupied by pores or voids). It is expressed as a percentage of overall rock volume.
  • Porosity is the ratio of the void space in a rock to the bulk volume or size of the rock
  • Pore spaces in reservoir rock are container for accumulation of oil and gas
    • It is also referred to as storage capacity
  • These water, oil and gas are called pore fluids.

Porosity Classification

  • Primary porosity refers to spaces between the grains of sedimentary materials that existed during deposition.
  • Secondary porosity refers to spaces which develop within a formation as a result of events subsequent to the deposition. e.g.. dissolution by ground water, volume changes due to dolomitzation, tectonic fractures……

Porosity Factors

  • Porosity varies with:
    • (a) Grain size and shape.
    • (b) Grain distribution (Arrangement)
  • Normally porosity varies between 10\% to 30\%.
    • Cubic porosity provides the maximum possible porosity = 47\%
    • When grain are stacked in a rhombohedral fashion, porosity = 26\%
    • If formation consists of varying siwed grains in a closely packed arrangement porosity = 10\%
    • If grains are bricklike and loosely stacked porosity = 1\%

Type of Porosity

  • Effective porosity is the percentage of interconnected pore space or is the measure of the void space that is filled by recoverable oil and gas; or the amount of pore space that is sufficiently interconnected to yield its oil & gas for recovery.

  • 36% Porosity vs. 20% Porosity, effect of cement material.

    • Ineffective porosity is the ratio of the volume of isolated or completely disconnected pores to the total or bulk volume.
    • Ø_i = (Vol. of completed disconnected pores) / (Total or bulk vol. of reservoir rock)
    • non-porous and non-permeable
    • unconnected pore spaces: porous and non-permeable
    • connected pore spaces: porous and permeable
    • High Porosity VS. Low Porosity

Type of Porosity Grades

  • Total porosity or absolute porosity is the ratio of the entire void spaces in the reservoir rock to the bulk volume of the rock
  • Porosity % - Grade
    • 25~20 - Very good
    • 20~15 - good
    • 15~10 - moderate
    • 10~5 - poor
    • 5~0 - no value

Permeability

  • Permeability is a measure of interconnectivity of the open spaces within a rock.
  • It depends on porosity but not proportionately.
  • When porosity is zero, permeability is zero. When porosity is 20\%, for example, permeability may be zero, if the pores are not connected.

Typical Permeabilities

  • Ranges from Unfractured igneous & metamorphic rocks to Gravel.

Factors That Affect Porosity

  • Particle shape
  • Packing
  • Particle sizes
  • Cementing materials
  • Overburden stress
  • Vugs and fractures

Permeability Equation

  • General equation for permeability is
  • K = (Q * μ * L) / (A * ΔP)
  • Where,
    • K = permeability (Darcy)
    • Q = flow rate through the rock
    • μ = viscosity of the flowing fluid
    • L = length for which the differential pressure is considered
    • A = Flow cross sectional area

Permeability - Cement Effect

  • Effect of cement material
    • POOR PERMEABILITY: Cement blocks the pores, so the pores are not conected.
      • Porosity 36\%.
    • GOOD PERMEABILITY: The pores are conected.
      • Porosity 20\%.

Porosity & Permeability - Pore

  • Porosity: PORE
  • Permeability: PORE

Porosity & Permeability - Effect

  • Porosity is the void space in the rock, expressed as a percentage of the rock volume
    • It is the porosity that effects the volume of oil/gas in place.
  • Permeability is a measure of how easy it is for fluids to flow through the pore system
    • It is the permeability that affects well productivity
    • Pore throats are narrow restrictions between grains which connect larger voids

Sorting

  • Sorting is the range of grain sizes that occurs in sedimentary materials.
  • Well-sorted materials have grains of the same size, while poorly sorted materials have grains of many sizes.
  • Controls porosity & permeability
    • Large Pore Spaces Yield Good Porosity And High Permeability.
    • Poor sorting yeilds smaller pore spaces and lower permeability.

Sorting - Well-sorted sandstone

  • Well- sorted sandstone : Good porosity and permeability.
  • More permeable
  • Plenty of open spaces that connect to each other
  • Water flows easily from space to space

Sorting - Poor sorting

  • Poor sorting: Less permeable
  • Much lower porosity and permeability
  • Small particles fill up the spaces leaving little room for water to move
  • It's hard for water to find a path through the soil

Determination of Porosity

  • Several methods: involves only the determination of two out of 3 (Vp, Vm, & Vb)
  • Bulk volume by the following methods
    • Coated sample immersed in water, or
    • Water-saturated sample immersed in water, or
    • Dry sample immersed in Hg method (no more Hg in the labs)
  • Grain volume: by Melcher- Nutting method in which the sample is crushed and its volume measured with a pychnometer

Laboratory Measurement of Porosity

  • Under laboratory the following are measured;
    • Vp, pore volume directly measured indirectly
    • Vb, bulk volume directly measured indirectly
  • Porosity measurement
    • Vb, bulk volume directly measured
    • Direct measure, Vb
  • Common shaped sample (cylinder, or cubic) measured the dimensions and consider bulk volume A * L

Porosity - Measurement of Vb

  • Measurement of Vb, Irregular & regular sample shapes Two means explained here;
    • Volumes faulted (volumetrically)
    • Methodologies gravity (gravimetrically)
  • To use above method must prevent fluid penetration into the pore sample by:
    • Coating with wax
    • Saturating the core with same fluid
    • Using mercury

Porosity Measurement of Vb

  • Mercury volume displacement by the rock sample when completely immersed in the liquid
  • Mercury volume addition after core sample is included in mercury is bulk volume.

Porosity Measurement of Vb

  • Gravity method
    • A Wtk = dry core weight
    • Wthg = mercury weight
    • B Wtb = mercury weight and core that forced inside mercury
    • Mercury weight faulted = (Wtb - (Wtk + Wthg))
    • Core bulk volume = Mercury volume faulted =

Porosity Measurement Vp

  • Fluid saturation inside rock H2O
    • Wtk = Dry sampleweight
    • Wtt = Saturated sample weightfluid
    • Wtf = Fluid weight in pore = Wtt -Wtk

Porosity Measurement Vp

  • Boyle’s Law : Porosimeter
    • V1 = Volume of cylinder helium reference in P1 & T1
    • Vl = Connector tube volume cylinder helium reference to cylinder sample
    • Vs = Volume of empty cylinder sample
    • Vg = Volume of core sample

Porosity Measurement Vp

  • Boyle’s Law : Porosimeter
  • Cylinder sample steel line sample
  • P3 V3 T1
  • P1 V1 = P3 V3
  • V3 = V1 + Vl + Vs -Vc - Vg
  • Helium tank
  • P1 V1 T1

Bulk Density

  • Bulk density is defined as the dry weight of soil per unit volume of soil.
  • Bulk density considers both the solids and the pore space; whereas, particle density considers only the mineral solids.

Formation structures

  • PRINCIPAL TYPES OF GEOLOGIC TRAPS:
    • FAULT
    • ANTICLINAL
    • STRATIGRAPHIC
    • DOMAL

Source/Reservoir/Cap rocks

  • Source Rock:
    1. Limestone
    2. Shale
  • Reservoir rocks
    • Major Reservoir Rocks:
      1. Sandstone
      2. Limestone
      3. Dolomite
    • Minor Reservoir Rocks:
      1. Shale
      2. Fractured metamorphic and igneous rocks
      3. Volcanic sediments
  • Cap rocks:
    • Shale
    • Clay stone
    • Tight limestone
    • Anhydrite
    • Gypsum

Source/Reservoir/Cap rocks - Trap

  • WHAT IS TRAP!!!!!
  • This barrier is generally referred to as a trap. Varying densities make the gas phase rise, while the water settles to the lowest point, and the oil remains in the middle.
  • Traps are categorized as structural or stratigraphic.
  • Accumulation and Traps
    • A trap is the place where oil and gas are barred from further movement ----Levorsen 1967-
    • A trap is produced by geological conditions that cause HC (oil and gas ) to be retained in a porous reservoir (or at least allowed to escape to a negligible rate).

The Geological HC Traps

  • Three types of traps
    • Structural trap
    • Stratigraphic trap
    • Combination trap

Structural Traps

  • A structural trap is formed by folding or faulting of the rock layer that contain HC.
  • Most common structural traps are anticline traps, fault traps, domes and plug traps.
  • A few examples of structural traps are :
    • Anticline
    • Syncline
    • Fold
    • Fault
    • Salt dome

Structural Traps - Anticline Trap vs Syncline Trap

  • Anticline
    • Cap rock
    • Reservoir rock
    • Gas
    • Oil
    • Water
  • Syncline

Structural Traps - Dome and Plug Trap

  • A roughly symmetrical upfold in which the beds dip away from the crest in all directions.
  • Most domes are elongated oval domes.
  • SALT PLUG

Structural Traps - Fault Trap

  • Normal Fault / Reverse Fault
  • 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.

Structural Traps - Fault Types

  • Thrust fault
  • Wrench fault

Structural Traps - Fold Trap

  • A fold is a bend or flex in a layer or layers of rock

Stratigraphic Traps

  • A stratigraphic trap is caused either by a nonporous formation sealing off the top edge of a reservoir bed or by a change of porosity and permeability within the reservoir itself.
  • Two general kinds of stratigraphic traps are:
    • The disconformity trap
    • The angular unconformity trap
    • Lenticular trap

Stratigraphic Traps - Angular Unconformity Trap

  • Erosional Surface
  • Seal
  • Reservoir Rock
  • Oil
  • Angular Unconformity

Combination Traps

  • Combination traps are formed by a combination of folding, faulting, changes in porosity and other conditions – some structural and some stratigraphic in origin.

Summary

  • To summarize, it is necessary to have three ingredients to create an oil field.
    • You must have a source rock to generate the oil.
    • You must have a Reservoir Rock to store the migrating oil and allow it to flow.
    • You must have a geologic trap to cause the migrating oil to stop it’s journey upward an accumulate in quantity.