Equivalent Circulation Density (ECD)
Equivalent Circulation Density (ECD) is the dynamic density of circulating mud in a borehole and is used to calculate the pressure exerted on the borehole wall when the mud is being circulated. Any fluid column exerts a pressure equivalent to its hydrostatic head on the walls of its container. Similarly, the drilling fluid also exerts pressure on the borehole, which is equivalent to the hydrostatic pressure of the mud column and is a function of the mud density. However, the pressure effect changes in dynamic conditions and is reflected as Equivalent Circulation Density (ECD).
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Newtonian Fluid
The fluids which exhibit constant viscosity under constant temperature and pressure conditions are called Newtonian Fluids. Viscosity is the resistance to flow that any fluid exhibits and in the case of Newtonian fluids, it varies only with changes in pressure or temperature. For Newtonian fluids, viscosity is a ratio of shear stress and shear rate. Light oil, water, salt water, sugar solution, glycerin, air, and other gases are examples of Newtonian fluids. Drilling fluid is not a Newtonian fluid.
Mud Rheology
Rheology is the study of the deformation or flow of material under stress or forces. The properties which affect the flow of material are called rheological properties. Mud Rheology refers to the properties of drilling fluid, which help determine how the fluid will flow under different conditions.
Mud rheological properties like viscosity, gel strength, and yield point along with other fluid properties are continuously measured during drilling. Required chemical treatment is carried out to maintain mud properties at the prescribed level to ensure the proper flow of drilling fluid, mud cake build-up, and primary control of the well under downhole conditions.
Annular Velocity
Annular Velocity (AV) is the velocity of drilling fluid in the annulus. It is an important element for achieving effective hole cleaning. Flow rate and annular cross-sectional area are used for calculating average annular velocity. The average annular velocity is a theoretical calculation considering a gauge hole. If there are some over-gauge areas in the well due to hole instability or caving, the risk of cuttings getting lodged in those enlarged sections should be carefully evaluated.
Cutting accumulation is severe in high-angle wells as the cuttings tend to accumulate on the low side of the well. The combined effect of mud rheology, annular velocity, and pipe rotation helps clean the hole by removing drilled and suspended solids from the well.
In general, a minimum average annular velocity of 150 ft/min is recommended for achieving effective cutting cleaning in a high-angle well. However, higher annular velocity also increases Equivalent Circulation Density (ECD). ECD higher than a certain value could cause losses. Hence annular velocity is optimized based on well configuration, well profile, and limits of open formations.
Dog Leg Severity
Dog Leg Severity commonly referred to as DLS is a measure of change in direction of a wellbore. It is described in degrees per 100 feet or per 30 meters. The difference between the directional measurements between two survey points is used for computing the DLS value. In simple terms, the difference in inclination of two survey points is used, but in reality, the directional measurements include inclination (inc) and azimuth (Az) of the well.
The actual computation of DLS is based on the 'Radius of Curvature' method. The formula used for computing DLS is: Dogleg severity (DLS) = {cos-1 [(cos Inc1 x cos Inc2) + (sin Inc1 x sin Inc2) x cos (Az2 – Az1)]} x (100 ÷ MD).
Higher DLS values result in higher torque and drag in drilling & tripping operations. It also risks creating key seats, which increase string wear and the possibility of fatigue failure. Severe key seating can also cause sting stuck-ups.
Blow Out Preventer
A Blow Out Preventer commonly known as BOP is a large valve placed on top of a well. As the name suggests, BOP is used to prevent a blowout in a well.
The conventional process of drilling a well maintains the pressure (force per unit area) inside the wellbore higher than the formation pressure. The difference between the hydrostatic pressure in the well and the fluid pressure in the formation being drilled is called overbalance. Overbalance is considered the 'Primary Barrier' in a well.
If for any reason, this primary barrier fails and the hydrostatic pressure in the well falls below the formation pressure, the formation fluid will start entering the well. If this situation is allowed to continue, the heavier drilling fluid will be completely displaced by the formation fluid, which will be gushing out of the well with high force due to pressure from the formation. This situation is called a blowout.
Blow-Out Preventers are designed to close the well and contain the pressure in such scenarios. BOP makes it possible not only to avoid a blow-out but also to circulate the kick out of the well by displacing the well with kill weight fluid. Different types of BOPs are placed on top of the well in the form of a stack. BOPs have different pressure ratings depending on the type of well.
Primary Barrier
The conventional process of drilling a well maintains the pressure (force per unit area) inside the wellbore higher than the formation pressure. The difference between the hydrostatic pressure in the well and the fluid pressure in the formation being drilled is called overbalance. Overbalance is considered the 'Primary Barrier' in a well.
The primary barrier keeps the well under control by ensuring that the specific gravity of the drilling fluid is more than the formation pressure and is aimed at disallowing the flow of formation fluid into the wellbore. Different companies have different policies for a minimum level of required overbalance as a primary barrier depending on the type of well and expected reservoir fluid.
Killing
If for any reason, the overbalance in a well is lost and the hydrostatic pressure in the well drops below the formation pressure, the formation fluid called influx can enter the well. Well Killing is the process of placing the heavier fluid back in the well by displacing out the influx for regaining primary control of the well.
Kick
The conventional way of drilling is to maintain an overbalance where the pressure (force per unit area) in the wellbore while drilling is maintained above the formation pressure. The difference between the hydrostatic pressure in the well and the fluid pressure in the formation being drilled is called 'Overbalance'. Overbalance is considered the primary barrier to well control and is maintained to disallow the flow of formation fluid into the wellbore. If for any reason, the overbalance is lost and the hydrostatic pressure drops below the formation pressure, the formation fluid can enter the well. This flow of fluid from the formation into the wellbore is called kick or influx.
Influx
The conventional way of drilling is to maintain an overbalance where the pressure (force per unit area) in the wellbore while drilling is maintained above the formation pressure. The difference between the hydrostatic pressure in the well and the fluid pressure in the formation being drilled is called 'Overbalance'. Overbalance is considered the primary barrier to well control and is maintained to disallow the flow of formation fluid into the wellbore. If for any reason, the overbalance is lost and the hydrostatic pressure drops below the formation pressure, the formation fluid can enter the well. This flow of fluid from the formation into the wellbore is called influx or kick.
Well Dominator
'Well Dominator is Schlumberger's test tool for carrying out positive pressure test on liner top. After cementing the liner, the conventional sequence of operations requires multiple trips for cleaning out cement inside the liner, scrapping, and carrying out a positive and negative pressure test to ensure the integrity of the liner top. Well Dominator is run in the string in a combination of another Schlumberger tool Well Commissioner to complete all operations in a single run thus saving rig time.
It is a ball drop tool that remains hydraulically inactive while cleaning-out the casing. Dropping a ball activates pads of Well Dominator for carrying out a positive pressure test of liner top. Once the pressure test is successfully completed, dropping another ball deactivates it. A negative pressure test can be performed by Well Commissioner, which is part of the same string.
Well Commissioner
'Well Commissioner' is Schlumberger's liner top test tool. After cementing the liner, multiple trips are required for cleaning out cement inside the liner, scrapping, and then running a retrievable packer assembly for carrying out a negative pressure test to ensure the integrity of the liner top. Well Commissioner is designed to save rig time by making it possible to clean out to cased hole TD and perform a negative pressure test at liner top in a single trip.
Inflow Test
An inflow test, also called a negative pressure test, is aimed at testing the barriers placed in the well for ensuring well integrity. The barriers in a well are installed to avoid fluid flowing past the barrier from the formation into the well or vice versa. To ensure well integrity, it is important to test whether the barrier fulfills this requirement.
An Inflow test is essentially the inverse of a positive pressure test performed in the well. In the inflow test, a differential pressure is created in the direction from formation to the well. It is achieved by reducing the hydrostatic pressure above a barrier to a level below the formation pressure. The test is designed to check if any formation fluid will leak past the barrier into the well.
Leak Off Test (LOT)
A leak-off test (LOT) is a pressure integrity test that is carried out to determine the strength of the formation or fracture pressure. The leak-off test provides important information required for safely drilling the next hole section by establishing the maximum mud weight, kick tolerance, maximum allowable surface pressure for shutting in a well, etc.
LOT is carried out immediately below the casing shoe. A small portion of the new formation is drilled below the casing shoe, the well is shut in and pressure is increased gradually in small increments by slowly pumping fluid in the well. The surface pressure is closely monitored with every increase. At some point, the fluid will leak into the formation either due to the opening of a fracture or a permeable flow path in the formation. This provides information on the strength of the formation below the shoe.
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Stress Caging - An effective wellbore-strengthening approach
Negative Pressure Test
A negative pressure test, also called the inflow test is aimed at testing the barriers placed in the well for ensuring well integrity. The barriers in a well are installed to avoid fluid flowing past the barrier from the formation into the well or vice versa. To ensure well integrity, it is important to test whether the barrier fulfills this requirement.
A negative pressure test is essentially the inverse of a positive pressure test performed in the well. In the inflow test or negative pressure test procedure, a differential pressure is created in the direction from the formation to the well. It is achieved by reducing the hydrostatic pressure above a barrier to a level below the formation pressure. The test is designed to check if any formation fluid will leak past the barrier into the well.
USIT
Ultra-Sonic Imaging (USI) Tool is an advancement on conventional sonic tools used for cement evaluation. It uses a single rotating transducer and receiver to emit and receive ultra-sonic waves and provide complete circumferential casing coverage. Another improvement of the USI Tool is the presentation of data in a color map, which allows the interpreter of the log to quickly identify intervals of good cement and the segments with problems in the cement sheath. Since its introduction, USI tools have proved to be a superior cement evaluation tool that overcomes most of the problems found with the limitations and interpretation constraints of other cement bond logging tools.
Deflocculation
Deflocculation is the process of breaking up the flocculent state. Defolocculants are thinning agents, which reduce the viscosity to disperse the particles flocking together in the mud system. It should be noted that deflocculated mud is not the same as dispersed mud. Traditionally, tannins, lignosulfonates, and lignites are used as deflocculants but many high-performance WBMs use short-chained polymers or surfactants instead.
A deflocculated mud usually shows much improved filter-cake qualities, lower yield point, and gel strengths. Filter-cake quality is improved because the platelets become detached from each other and can lie flat to form a thin, low-permeability filter cake. Lowering yield point and gel strength beyond a point may not be desirable and should be adjusted through appropriate chemical treatment. If yield points and gels are lowered too far, the suspension and cutting capacity of the mud could reduce.
Flocculation
Flocculation is the process where clays, polymers, or other charged particles become attached and form a fragile structure. After the clay is depressed in the mud system, the platelets arrange themselves according to their electrical surface charges. Positively charged edge to negatively charged face and floc together.
Flocculation is not desirable, since it results in a high yield point and high gel strength. Common causes of flocculation are high ionic strength, a high concentration of shale in the mud and high temperature, or contamination with an influx from the formations. Deflocculants are used for neutralizing the charges on the clay surfaces.
Cement Bond Log (CBL)
Cement Bond Logs (CBL) and Variable Density Logs (VDL) are conducted after the primary cementing operation for evaluating the quality of the cement bond behind a cemented casing or liner. It is a sonic tool that is run on wireline and uses a combination of transmitter and receivers to measure sonic wave arrival for assessment of cement bond quality. The principle used is that when no cement is bonded to the casing, the pipe is free to vibrate, whereas when the casing is bonded to hard cement, casing vibrations are attenuated proportionally to bonded surface.
A bit and scrapper run is recommended before the CBL-VDL log to remove any cement or scale from the casing wall. The tools should be properly centralized and the wellbore should be fluid-filled for accuracy of results since gas or air bubbles induce inaccuracies.
Radial tools were developed to overcome some limitations of conventional CBL-VDL tools. The Radial CBL-VDL tool permits a more accurate evaluation of cement distribution. It uses more sensitive transducers and evaluated the cement quality around the circumference providing precise location of partial bond and channeling.
Variable Density Log (VDL)
Variable Density Logs (VDL) and Cement Bond Logs (CBL) are conducted after the primary cementing operation for evaluating the quality of the cement bond behind a cemented casing or liner. It is a sonic tool that is run on wireline and uses a combination of transmitter and receivers to measure sonic wave arrival for assessment of cement bond quality. The principle used is that when no cement is bonded to the casing, the pipe is free to vibrate, whereas when the casing is bonded to hard cement, casing vibrations are attenuated proportionally to bonded surface.
A bit and scrapper run is recommended before the CBL-VDL log to remove any cement or scale from the casing wall. The tools should be properly centralized and the wellbore should be fluid-filled for accuracy of results since gas or air bubbles induce inaccuracies.
Radial tools were developed to overcome some limitations of conventional CBL-VDL tools. The Radial CBL-VDL tool permits a more accurate evaluation of cement distribution. It uses more sensitive transducers and evaluated the cement quality around the circumference providing precise location of partial bond and channeling.