Near-Bit Stabilizer
As the name suggests, if a stabilizer is installed in the bottom hole assembly just above the bit, it is called a ‘Near-bit stabilizer’. Since the drill bit has a pin connection, near near-bit stabilizer has box-to-box connections to connect between the drill bit's pin end and the drill collar's pin end. A near-bit stabilizer reduces the bit tilt and provides more stability while drilling, which helps reduce bit whirl.
Junk Slot Area
As part of the drilling process cuttings are generated, which need to be cleared as soon as possible from the bottom. The hydraulic energy is used to aid the drilling and also to circulate the drill cuttings out of the hole. However, if there isn't enough space for the cuttings to get past the drill bit, they may not escape quickly. If the cuttings are not cleared efficiently, they tend to clog the bit and stay at the bottom getting reground, adversely affecting the drilling efficiency. Hence the drill bits are designed with several flow paths, which serve as conduits for cuttings to get past the bit. These flutes are called junk slots.
The Junk Slot Area is measured in square inches and is the total cross-sectional area of these slots when viewing the bit face-on. Junk slots can vary in shape and size. While drilling a soft formation, the rate of penetration is high, which generates cuttings at a faster rate. Hence the bits for drilling softer formations need to be designed with larger junk slot areas to be able to clear the drilled cuttings faster. Junk slots also allow drilling fluid to flow freely and prevent surge and swab while tripping.
Stabilizer
Stabilizers are thick-walled hollow cylindrical components with stabilizing blades on their circumference. They are the components of the bottom hole assembly and are used to provide stability to the drill string. Stabilizers help avoid unintentional sidetracks, minimize vibrations, and enable weight and torque transfer to the bit efficiently by ensuring that the drill string rotates at or near the center of the borehole. Stabilizers have box and pin ends with API connections to be able to easily install at the desired position in the drilling assembly.
Overflush
The overflush is the fluid stage of the stimulation treatment cycle that is pumped after pumping the main treatment fluid. Normally a weak acid or brine is used as an overflush fluid. When the main treatment fluid, which is normally a mixture of Hydrofluoric and Hydrochloric or organic acids, comes in contact with the formation, it could produce some byproducts. The objective of overflush is to displace the main treatment acid and also to flush out any byproducts or non-reacted mud chemicals out of the wellbore after the culmination of the treatment cycle.
Preflush
Stimulation treatment contains multiple fluids that are pumped in sequence to achieve the best results. Preflush is the fluid stage that is pumped before the main treatment. Preflush is used for dissolving calcareous materials, rust, or displacing brine before the main treatment fluid comes in contact with the formation. The presence of these materials could cause the plugging of formation pores when coming in contact with the main treatment fluid. The composition is decided based on the type of reservoir rock and formation damage.
Reservoir geochemistry
Reservoir geochemistry is the study of the chemical composition of reservoir rocks and fluid in the reservoir. The reservoir fluid can be either water, oil, or gas. Their chemical analysis and study of their interactions with the reservoir rock and the surrounding formation provide valuable information. Reservoir geochemistry helps understand reservoir fluid's origin, migration, and accumulation. It also aids in evaluating reservoir quality, compartmentalization, and potential productivity of a reservoir.
Sidetracking
The process of drilling a deviated section initiated from the already drilled part of the original wellbore is called Sidetracking a well. Sidetracking can be planned, intentional, or maybe accidental as well. A sidetrack can be initiated from either a cased hole or an open hole depending on the reasons for a sidetrack.
Sidetracking a well requires a specialized set of equipment, exhaustive planning, and directional drilling expertise. A well can be sidetracked either from the vertical, deviated, or horizontal part of the original wellbore depending on the reasons for sidetracking.
Stress Caging
Stress Caging is the technique of strengthening the wellbore so that the hole can be drilled without inducing downhole losses. Stress caging not only helps avoid downhole complications by avoiding loss of circulation but could also reduce the number of casing strings required for drilling the well to the planned target depth.
The technique achieves the objective of formation strengthening by treating weak formations with drilling fluids containing engineered particulate lost circulation materials. The process of stress caging aims to increase hoop stresses in near wellbore regions and seal shallow fractures at the wellbore formation interface continuously while drilling.
Stress Caging - An effective wellbore-strengthening approach
Extended Leak Off Test (XLOT)
The Extended Leak-Off test (XLOT) is a technique used to measure the stress in oil and gas wells and is used to obtain horizontal stress data for predicting wellbore stability. Surface pressure is applied to the fluid column to determine the pressure at which the fracture will propagate into the exposed formation.
XLOT test procedures are similar to routine leak-off tests but with multiple leak-off cycles, longer data collection time, and better data acquisition and analysis.
Stress Caging - An effective wellbore strengthening approach
Formation Integrity Test (FIT)
The Formation Integrity Test (FIT) is essentially a test of formation strength to a predetermined value. FIT is carried out by gradually increasing the bottom hole pressure after drilling out the casing shoe and a few feet of formation. The intent of FIT is not to pressurize the wellbore until the formation fractures but to test the strength of the formation to a pre-established limit. Since it is carried out below the casing shoe, it also tests if any cement channeling can compromise the well's integrity.
Since the maximum mud weight is limited by the fracture gradient, FIT is useful to know what maximum mud weight can be used for drilling. FIT is also used as a basis for casing depths and well control options in case of influx.
Stress Caging - An effective wellbore-strengthening approach
Wellbore
Wellbore refers to the hole drilled in the ground either on land or offshore to explore or produce oil, gas, or water. A wellbore is drilled using a drill bit through several formation layers and rock types. Drilling fluid is used and is designed to provide the required hydrostatic pressure for ensuring wellbore stability, avoiding the influx of formation fluid, removing drilled cuttings, and cooling downhole. Based on types of formations, pore pressure, and fracture pressure estimations, wellbores are cased with steel casing and cement at predetermined intervals. Part of a wellbore can be left uncased as well depending on the design, rock type, and objectives of the well.
Corrosion Cap
Corrosion Caps are also known as Temporary Abandonment Caps or TA Caps. They are used for suspending an offshore well at the seabed. Once an offshore well is drilled and needs to be temporarily suspended for moving the rig off location, leaving the casing or wellhead open at the seabed could allow botanical growth or corrosion of exposed threads. This could make it difficult to reconnect to the well for any future operations. Hence a Corrosion Cap is installed to protect the threads and seal areas.
Corrosion caps are installed and retrieved with a running tool. Corrosion caps can also offer pressure containment if a check valve is installed in the stem of the cap. Normally after disconnecting casing strings at MLS, the corrosion cap is installed for the topmost threads. However, caps can be installed for each casing string for isolating the string and its annular space if required.
Mud Line Suspension (MLS)
Mud Line Suspension (MLS) systems are installed at or near mudline (seabed) in offshore wells . MLS supports the weight of casing strings that are run in the well as per well configuration and makes it possible to use a surface BOP & wellhead stack while drilling an offshore well. It also facilitates safely suspending the well at the seabed, disconnecting and reconnecting on a later date for future operations.
Well Completion
Completion is the process of making the well ready for production or injection. Types of completion vary significantly depending on the objectives, reservoir, reservoir fluid properties, reservoir potentials etc.
Logging
A log is defined as a systematic record of data. In the oil field, logging refers to the process of recording formation information downhole for evaluating properties and reservoir characteristics. A well log is a plot of different formation properties versus depth. Interpretation of well logs can help identify lithologies, porosity, permeability, etc and provide valuable information in identifying a pay zone. Various different types of tools are run in the well on either wireline or drill pipe to measure different formation properties. Logging tools use electrical, electronic, sonic, and radioactive materials to be able to measure specific rock properties.
Particle Size
The size of solid particles in the mud system is measured in microns. A micron(µ) is 1/1000 of a millimeter or 1/25400 of an inch. API classification of particle sizes is as below:
Particle Size, microns(μ) Particle Classification Sieve Size
Greater than 2000 Coarse 10
2000 to 250 Intermediate 60
250 to 74 Medium 200
74 to 44 Fine 325
44 to 2 Ultra-Fine 400
Less than 2 Colloidal –
Low Gravity Solids
The solid particles in the mud system that have a density lower than the weighting materials like barite or hematite are called 'Low Gravity Solids' (LGS). Drilled solids and bentonite clay are considered LGS. Solids in the mud system are measured as lbm/bbl or Vol %. Solids percentage is continuously measured on the rig site to maintain their concentration within design limits. The density of LGS is assumed to be 2.60 gm/cc.
High Gravity Solids
High Gravity Solids (HGS) are dense materials that are used as weighting material to increase the specific gravity of drilling fluid. Barite and Hematite are commonly used HGS in drilling fluid. Specific gravity to Barite is 4.2 whereas, for Hematite, it is 5.505.
Swabbing
If a drill string, casing, or logging tool is pulled out of the hole too fast, the string creates a piston effect pulling some mud out of the hole with it. This tends to reduce hydrostatic pressure in the well. The fluid in the annulus travels downwards to fill the void. This downward movement of fluid creates friction with the wellbore wall that acts upwards. The resultant pressure reduction created by this situation is called 'Swab Pressure'. If the swabbing reduces the pressure too much and the net hydrostatic pressure falls below the formation pressure, it may invite a kick (wellbore influx) into the wellbore. This may require carrying out well control procedures to secure the well.
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Surge
While running in the drill string or casing in the well, the downward motion of the string forces the drilling fluid out of the flowline. This upward flow causes frictional force with the wellbore wall and pipe that acts downwards. At the same time, the fluids immediately adjacent to the string are dragged downwards. This creates a piston effect and adds additional pressure to the hydrostatic pressure. This additional pressure is called 'Surge Pressure'. Excessive surge pressures can increase the Bottom hole pressure to high levels and induce losses.
More Topics and Illustrations in Upstream Oil & Gas
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