Geostatic Pressure
Geostatic Pressure is also known as Overburden pressure, Lithostatic Pressure, Confining Pressure, Vertical Stress, Total Stress. It’s defined as the pressure exerted by weight of rock and fluid in the rock on the formation layer at the point of interest. The equivalent density of the combined weight is referred to as the bulk density (ρb). Geostatic Pressure is normally estimated as average 1 psi/ft but varies from basin to basin depending on matrix density of rocks.
Overburden Pressure
Overburden pressure is also known as Geostatic Pressure, Lithostatic Pressure, Confining Pressure, Vertical Stress, Total Stress. It’s defined as the pressure exerted by weight of rock and fluid in the rock on the formation layer at the point of interest. The equivalent density of the combined weight is referred to as the bulk density (ρb). Overburden pressure is normally estimated as average 1 psi/ft but varies from basin to basin depending on matrix density of rocks.
Fracture Pressure
The pressure at which the rock breaks down is known as Fracture Pressure. Fracture pressure is higher than pore pressure and less than overburden pressure. Pore pressure and Fracture pressures are very important for designing a well. The specific gravity of mud is computed in such a manner that the hydrostatic pressure created by the column of mud is more than pore pressure but less than Fracture Pressure at any point in the well. In a deep water environment, the window between pore pressure and fracture pressure becomes narrow.
Casing points are carefully selected to ensure that the open hole is cased off before the Hydrostatic Pressure of the mud exceeds the estimated fracture pressure. Since Fracture Pressure while planning a well is an estimated value based on offset well data, ‘Leak Off Test (LOT)’ or ‘Formation Integrity Test (FIT)’ is carried out below critical casing shoes to calibrate Fracture Pressure at that point in the well. Actual values help extrapolation for the remaining part of the well and provide baseline value for future wells in the area as well.
Stress Caging - An effective wellbore-strengthening approach
Hydrostatic Pressure
Also known as Hydrostatic Head, Hydrostatic Pressure is defined as vertical pressure exerted by the weight of fluid. While planning a well, specific gravity of the drilling fluid (mud) is selected to ensure that the hydrostatic pressure exerted by the column of mud is more than pore pressure at any point in the well. This acts as a primary barrier for well control by ensuring that the formation fluid is held back from flowing into the well at any point of the time.
Pore Pressure
Pore pressure is the pressure of the fluid inside the pore spaces of the rock. It is also known as Formation pressure. If the pore pressure is higher than 'Normal Hydrostatic Pressure', it is called Abnormal and if it is lower than the normal hydrostatic pressure at a particular depth, it is called Subnormal pressure. Normal hydrostatic pressure is the pressure exerted by a vertical column of water with salinity normal for the specific geographic area.
The specific gravity of drilling fluid is planned such that the hydrostatic pressure created by the column of mud is more than pore pressure at any point in the well. This difference is called over-balance. Every company has safety policies to maintain pre-defined minimum over-balance depending on the type of well and reservoir fluid expected in a well.
High Performance Water Based Mud
Commonly known as HPWBM, the High Performance Water Based Mud systems are reformulated polymer water based systems designed to replicate the performance benefits of Oil Based Mud systems. HPWBM are designed to be able to achieve OBM benefits like shale stability, clay inhibition, lubricity, high ROP, while minimizing bit balling without the downside of adverse environment impact. In many wells a regular Water Based Mud (WBM) is used in shallow sections but due to water sensitive formations and high friction, Oil Based Mud systems are required in deeper horizons. In operations where cutting collection and environmentally safe waste disposal costs are high, HPWBM can prove to be more cost effective solution.
Invert Emulsion Mud
Invert Emulsion Mud system has water-in-oil emulsion. It has oil as continuous phase and brine as internal phase. Typical oil/brine ratios range from 95/5 to 50/50 in invert emulsion systems. Invert emulsions are usually tightly emulsified, low fluid loss oil muds. Relaxed Invert Emulsion mud is the one with relaxed fluid loss control, which have proven to improve drilling rates.
SBM
SBM is an acronym of 'Synthetic Based Mud'. It's an invert emulsion mud with synthetic fluid as base fluid rather than oil. SBMs have lower toxicity than 'Oil Based Mud' and are also called Low Toxicity Oil Based Mud (LTOBM). SBMs commonly use linear alpha olefins (LAO), straight internal olefins (IO), poly alpha olefins (PAO), vegetable oils, esters and ethers as base fluid. SBMs are more expensive than OBM but are more environmentally acceptable in offshore operations. Since synthetic oil is more biodegradable, environmental regulations in many areas could approve disposing off drilled cuttings in water without the need of any pre-treatment or collection and shipment to onshore location for safe disposal.
OBM
OBM is the acronym of Oil Based Mud. Oil-based mud is a drilling fluid composed of oil as the continuous phase and water as the dispersed phase. Additional chemicals are added as needed for achieving desired rheological properties. The oil base can be diesel, kerosene, fuel oil, mineral oil or synthetic oil. Barite is used as weighting agent to increase fluid density and organophilic bentonite is used for providing viscosity. There are two types of Oil Based Mud systems, 'Invert Emulsion Mud' and 'Oil Mud'.
OBM is used to address drilling problems such as, clays in the formation that would react or swell when exposed to water-based drilling fluid. Oil Base Mud also provides higher lubricity and reduces torque & drag downhole. Oil muds have the ability to drill formations where bottom hole temperatures exceed water-based mud tolerances. OBM has been used at temperatures approaching 550°F. Environmentally sensitive location however have strict regulations for disposal of drilling fluid and drilled cuttings. In many offshore locations, drilled cuttings need to be collected and shipped to onshore locations to safely treat and dispose as per environmental regulations.
Dispersant
Drilling and completion fluids are prepared by adding various liquid and solid chemicals a liquid phase. Dispersants are aimed at breaking up solids and liquids as fine particles or droplets, thus improving the separation of particles to disperse well in the system and avoid any settling. Without dispersant, the particles may lump together and settle out of the liquid phase, changing rheological properties of the fluid system. Dispersants are also used in case of oil spills. They break up oil into very small particles, which get easily diluted or dispersed in water.
Oxygen Scavenger
Oxygen dissolved in water can cause corrosion in the drill string. Oxygen scavenger reacts and neutralizes the dissolved oxygen to reduces corrosive effects of the fluid on metal components of downhole equipment. When fluid returns to surface, it again comes in contact with the atmosphere and begins to pick up oxygen, thus reducing effective concentration of scavenger. Hence oxygen scavenger level is measured and continuously injected at the pump suction. Common oxygen scavengers have sulfite (SO3–2) or bisulfite (HSO3–) ions, which combine with dissolved oxygen in the fluid system to form sulfate (SO4–2). Nickel or cobalt based catalysts are used for this reaction.
Turbulent Flow
Turbulent flow is a flow regime characterized where the flow is irregular and chaotic. Turbulent flow has a random pattern with cross flows. It has rapid variation of pressure and flow velocity in space and time. Flows at Reynolds numbers larger than 4000 are typically turbulent in nature. Flows in the range of Reynolds numbers 2300 to 4000 are in transition zone between laminar and turbulent flow.
Laminar Flow
Laminar flow occurs when fluid flows in parallel layers with layers sliding past one another smoothly without any lateral mixing. There are no cross-currents perpendicular to the direction of flow. Laminar flow has a parabolic pattern with velocity of the layers the centre of a conduit being the maximum and gradually reducing towards the walls. A low Reynolds number below 2300 usually indicates laminar flow where as the flows with Raynolds number from 2300 to 4000 would indicated in transition between laminar and turbulent flow.
NACE
NACE is the abbreviation of 'National Association of Corrosion Engineers'. NACE is globally recognized as a premier authority for corrosion control solutions. NACE was established in 1943 with headquarter in Houston, Texas, USA and has centres all across the globe. NACE offers training & certifications and also sets industry standards for corrosion control. NACE also has journals, publications and holds conferences to assist industry in effectively addressing corrosion prevention and control in the industry.
Acid Brittleness
Acid brittleness is also known as SSC, Sulfide Stress Cracking or Hydrogen Embrittlement. It is a spontaneous brittle failure of high strength allow or steel in presence of humid hydrogen sulfide. When susceptible alloys or steel comes in contact with Hydrogen Sulfide, the chemical reaction makes metal sulfide and releases atomic hydrogen. Atomic hydrogen can combine to result in H2 which disperses into metal matrix taking up more space and weakening the bonds between the grains of metal. Formation of molecular hydrogen causes sudden failure due to cracking under tensile stress. This failure is generally limited to steels or alloys having a hardness of 22 or greater on the Rockwell C scale.
Hydrogen Embrittlement
Hydrogen Embrittlement is also known as SSC, Sulfide Stress Cracking or acid brittleness. It is a spontaneous brittle failure of high strength allow or steel in presence of humid hydrogen sulfide. When susceptible alloys or steel comes in contact with Hydrogen Sulfide, the chemical reaction makes metal sulfide and releases atomic hydrogen. Atomic hydrogen can combine to result in H2 which disperses into metal matrix taking up more space and weakening the bonds between the grains of metal. Formation of molecular hydrogen causes sudden failure due to cracking under tensile stress. This failure is generally limited to steels or alloys having a hardness of 22 or greater on the Rockwell C scale.
Sulfide Stress Cracking
Sulfide Stress Cracking is also known as SSC, Hydrogen Embrittlement or acid brittleness. It is a spontaneous brittle failure of high strength allow or steel in presence of humid hydrogen sulfide. When susceptible alloys or steel comes in contact with Hydrogen Sulfide, the chemical reaction makes metal sulfide and releases atomic hydrogen. Atomic hydrogen can combine to result in H2 which disperses into metal matrix taking up more space and weakening the bonds between the grains of metal. Formation of molecular hydrogen causes sudden failure due to cracking under tensile stress. This failure is generally limited to steels or alloys having a hardness of 22 or greater on the Rockwell C scale.
Corrosion Coupon
Corrosion coupons are metal strips that are used for measuring rate of corrosion in a system. Pre-weighed strips are installed in the coupon rack. The corrosion rate in a specific operating system is measured by comparing the initial weight with the weight of the strips after the test period of 60, 90 or 120 days as planned. Measurement of corrosion rate helps in planning maintenance, repair needs and also to assess material failure possibilities.
CRA
CRA is abbreviation of 'Corrosion Resistant Alloy'. CRA material should be considered whenever there is potential presence of water, particularly when combined with CO2, H2S or chlorides. CRA are metal alloys engineered to resist degradation of metal surface in corrosive environment. Stainless steel is commonly used CRA and provides resistance to corrosion. Stainless steel with varying percentages of Chromium, Nickel, and Molybdenum are used in oil & gas wells depending on corrosivity assessments in the application.