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