Petro Data Mgt II- Drilling and Production Petroleum Professor Collins Nwaneri

Drilling Geology Overview – geology as it relates to drilling operations and basic principles of hydrostatic pressure exerted by a fluid at depth as it is important for drilling operations. Types of Rocks: 1. Igneous rocks – Formed by molten rock cooling and solidifying. what are examples of Igneous rocks? 2

Continued Sedimentary rocks – Formed by digenesis (chemical changes) or cementation of sediments. Metamorphic rocks – Formed by the physical changes of existing rocks by high pressures and temperatures. which of this rock type is likely to be a better source rock and why? 3

Continued Plate Tectonics – solids plates found underneath the earth surface floating on top of liquid rock or molten rock. Movement of the plates leads to rocks moving up or down within the earth crust. It can also need to rock beds becoming folded, broken and turned over. Fluid pressure and stresses within the rock will vary. Two major types of rock movements are: 1. Thrust fault (rocks are compressed together) to move in what direction? 4

Continued Normal fault(rocks are stretch apart) to move in what direction? 5

Lithology Lithology- a description of rocks that is based physical characteristics such as mineral comp, color, grain size and texture. Lithology affects many drilling decisions when planning and drilling a well. Examples of some lithologies: 1. Shale- consists of layers of clay minerals. They form about 75% of sedimentary rocks. Shale cause about 90 % of geology related drilling problems. What will happen if water based drilling mud is used to drill reactive shale formations? 6

Continued 2. Sandstone – consists of particles of sand (quartz), and maybe with traces of other minerals, such as iron. - clay minerals can also be found within sandstone if found within sandstone, how will it cause problems? - 11% of sedimentary rocks are made up of sandstone. Sandstone or other rocks must have porosity and permeability in other to be a reservoir. 7

continued What is a reservoir and define porosity and permeability? How is the porosity and permeability for sandstone and shale? Yes or no…..can a rock have permeability but no porosity. 3. Carbonates – composed of fossilized skeletons and minerals grains of calcite (crystals of calcium carbonates). Example is limestone. They are crystalline limestone and fossiliferous limestone. what is the difference between them? Most limestone's are fossiliferous. 8

Continued - Carbonates are often fractured due to their brittle nature. Fractured carbonates make prolific reservoir rocks as oil and gas collect in the fractures -They have high permeability and can produce high rates of hydrocarbons if intersected in a drilled well. - can also loose drilling fluids into the formation due to the fractures. Carbonates make up 13% of sedimentary rocks 9

Continued 3. Evaporates (salts) – occurs as result of sea water evaporating, leaving behind soluble salts. Less soluble salts are deposited first out of solution. Very soluble salts come out when dehydration is almost complete. - Salts can cause drilling problems…True or false. How? - What can you pump to dissolve flowing salt around drill bits in a well that causes the bit to stop drilling? - salt dome are good trap for reservoirs hydrocarbons. 10

Rock Strengths and Stress Rock strength varies depending on the types of stress applied on the rock (compressive, tensile or shear and may also vary due to the direction the stress is applied. - Tensile stress is negative rock stress -Compressive stress is positive rock stress -Shear stress is up and down rock stress Overburden stress – vertical stress due to weight of surrounding rocks. 11

Continued Stress distribution is affected by tectonic activities. -No or little tectonic activity …less horizontal compressive stress and high vertical compressive stress -With tectonic forces or other forces, stress differs. Example basic stress distribution magnitude in disturbed horizontal and vertical wells? 12

Principal stress Three stress resolved in perpendicular direction to each other. Two horizontal and one vertical stress. All compressive or tensile ( no shear stress). - stress orientation is important in designing a well and procedure to drill it successfully. Fracture pressure – rock ability to withstand pressure, which can cause tensile failure if not controlled. 13

Hydrostatic pressure Fluid impose hydrostatic pressure in a well and if the downhole pressure is not kept under control, it can cause an uncontrolled release of hydrocarbons (Blowout). Pressure is force/Area When formation pressure is higher than is normal for a depth this is called Over pressured formation 14

Geological input to drilling wells You need to know the relationship between hydrostatic pressure, fracture pressure and pore pressure for a successful drilling operations. 15

Oil and Gas Reservoir Formation Overview – The processes involved in petroleum generation, migration and accumulation into an exploitable reservoir will be described. Important rock properties for reservoir creation and fluid properties within the reservoir will be described. Hydrocarbons move upwards within the earth until they meet a barrier, and then may accumulate as an oil and gas reservoir. If there is no barrier, then they migrate to the surface as oil seep. 16

Continued They are seven main factors that all need to be present, in the right order, for a hydrocarbon accumulation to occur. These are the following: 1.Source rock 2.Hydrocarbon generation 3.Primary migration to a suitable structure (Reservoir) 4.Structural trap 5.Reservoir rock 6.Seal Rock 7.Secondary migration within the reservoir Which of this factors can be best attributed to what is called a dry wellbore? Mention at least two man made factors that can result in a dry wellbore? 17

Source Rock and Hydrocarbon Generation Buried remains of plant and animals accumulated over millions of years ago in slow moving or stationary water environments such as swaps, lakes, coastal regions and shallow seas. These organic materials plus other non-organic particles (clay minerals, fine sands, and silts) sank to the bottom and resulted in a build up of sediments over a long period. Increasing Temperature and Pressure with depth of this organic-laden sediments results to sedimentary rocks when they under go digenesis. 18

Continued About 99 % of all hydrocarbons deposit are found in sedimentary rocks. Chemical transformation also occur in the organic matter present in the pore spaces of the rock. Oil is generated from organic remains under certain conditions, with temperature as the most important factor. Oil generation starts at 50 deg. C and conversion to oil peaks 90 deg C and stops at 175 deg C. This temperature range between 50 deg C and 175 deg C is known as the oil window 19

Continued The decay of organic remains that generate gas occurs at below and above the oil window. Biogenic gas (generated by microbes) or swamp gas is generated at a temperature below 50 deg C. Thermal gas is generated at above 175 deg C. Heavy oil is generated at temperatures at the lower end of the oil window At Increasing temperature, lighter (more valuable) hydrocarbons are generated. At a high rock temperature of above 260 deg C, the organic mineral, which generates oil is destroyed. 20

Continued Petroleum is made up of: (83%) Carbon and (13%) Hydrogen and sometimes with small amounts of sulfur (up to 2%), nitrogen (0.5%) 90 % of most crudes oils contain hydrocarbons that are composed of carbon and hydrogen. Source rocks are rocks that produce hydrocarbons from organic matter that are buried within the rock pore space. Shale is the most common source rock for most oil and gas sedimentary rocks. 21

Continued Types of shale are Black, Green or Gray Shale Black shale has about 1 – 3 % of organic matter. Green or Gray shale has about 0.5 % of organic matter. Only about 2 % of deposited organic material becomes hydrocarbon. And only about 0.5 % is found in commercially exploitable reservoir. Coal comes from woody plant remains deposit. Low quality coal generated at lower temperature and pressure High quality black coal is buried deeper under higher temperature. Methane (Hydrocarbon gas) is also generated as a bye product from woody plants remains when coal is formed. This can migrate upwards to form a Gas Reservoir. 22

Vital Rock Properties Hydrocarbon is formed in pore spaces of a source rock. Rocks can be: 1. Highly or well consolidated 2. Poorly consolidated or unconsolidated This is controlled by the mineral bonding holding the rocks together. Which of the two above will have a stronger bonding? Two Important rock properties are Porosity and Permeability. 1. Porosity – the extent of porosity is measured as the fraction of the total rock volume occupied by the pore spaces. Porosity is expresses in percentage - Rock porosity is very important because without porosity hydrocarbon cannot be generated, migrate or accumulate in a reservoir. 23

Continued If 30 % of the total volume of a rock has pore spaces, what is the rock porosity? 2. Permeability – Ability of a rock to allow fluid to flow through it. It is measured in darcies. - Needs connected pore spaces. - Rock permeability is very important because oil generated in the source rock cannot migrate into a reservoir. -Shale has very low porosity and low permeability. -Hydrocarbon and water produced in the shale source rock are squeezed out by pressure and it takes a longtime for the fluids to migrate out. 24

Primary Migration The first two conditions necessary for the birth of a reservoir are : 1. Organic rich source rock. 2. Temperature and time for hydrocarbon generation. Finally, the source rock should be next to a permeable rock or channel that allows for hydrocarbon migration. -Conduit of hydrocarbon can be provided by permeable sandstones, fractures in the rock or ancient reefs. 25

Structural Traps A structural trap has the ability to trap the hydrocarbon as a result of it’s primary migration from a source rock. Maybe formed in a deformed rock as a result of movements within the earth crust. Examples of structural traps are: 1.Anticline 2.Salt Dome 3.Angular Unconformity 26

Reservoir Rock Hydrocarbon (gas or oil) and water are found in pore spaces or fractures within the rock matrix. Most reservoirs are sandstones which have good porosity to hold hydrocarbon and high enough permeability for production. Carbonates (limestone) with fractures and/or pit tend to be prolific reservoirs with high porosity and permeability. Reservoir rocks that have clay within the pore spaces tend to reduce permeability around the wellbore. Reservoirs may have different layers of varying characteristics that leads to what is known as directional permeability….difference in permeability due to direction of hydrocarbon flow. 27

Seal Rock Impermeable rock that is a seal above a reservoir rock to prevent upward migration of hydrocarbon. Clean shale, salt and unfractured limestone are examples of seals. Seal rocks are a source of a formation pressure transition from a normal pressure to overpressure. 28

Secondary Migration Formation of hydrocarbon pools due to movement of hydrocarbon droplets within the reservoir. Also another step may occur when the earth crust movement shifts the pools position within the reservoir rock. Accumulations can be affected by: 1.Buoyancy 2.Other Impermeable barriers 3.Hydrocarbon accumulations in carbonate reservoirs 4.Different layers of shale in large sandstone may distribute reserve with earth crust movement. 29

Continued 5. Faults 6. Uplift and erosion 7. Cap rock fracturing Single Phase Reservoirs – contain only single fluid (gas or oil). Oil is seldom found without some gas or some water. Most Reservoirs are multi-phase – they contain mixtures of gas, oil, and water Secondary migration separate’s this fluid out by gravity, and gas sits at the top (Gas Cap), and then oil under gas and water under oil (lightest fluid at the top of heaviest fluids at the bottom) 30

Reservoir Drives Energy that is used to move the hydrocarbons to the surface from the reservoir when first drilled. Most oil when first drilled have sufficient pressure (energy) in the reservoir to push the oil to the surface. Types of energy sources: 1. Gas Drive – partially or completely isolated from the pressure regime in the surrounding rock. Oil production causes the gas cap to expand and looses it energy. - The temperature and pressure in the reservoir will drop and there will not be enough energy left to drive the oil out of the reservoir. 31

Continued Gas drive is not an efficient long-term production producer. The following can be done if there is no sufficient pressure left to drive out oil in order to increase the reservoir pressure: 1.Inject more gas 2.Ignite oil underground by injecting air 3.Install a down hole pump to pump oil to the surface 32

Continued 4. Inject gas into the well (gas lift) 5. Inject water and chemicals in some part of the reservoir All of the above method are called secondary recovery method. 2.Water Drive – has a reservoir that is connected hydraulically to an area regime (i.e such as aquifer that is open to the atmosphere). - In addition, in a sedimentary rock sequence, there is a local water table(rock pore spaces with salt water, which exerts a pressure at depth. (Principles of hydrostatic pressure) 33

Continued - The water from the local water table pushes the oil to the oil well. - Water drive last longer than gas drive. - water is eventually produced as oil is driven out from the well and may cause what is known as water blocking in the well (Increase in the amount of water in the pore spaces blocks oil) 34

Problems Related to fluids in the reservoir Hydrogen sulfide can be produced from degradation by bacteria of the oil in a reservoir - it is extremely toxic and it is also called sour gas. Carbon dioxide in reservoirs can lead to corrosion of steel in a wet environment. - This can cause corrosion problems for well tubular. Hydrocarbons that have both H 2 S and CO 2 are normally treated before they are sold. 35