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A Complete Solution to the Performance of Petroleum Drilling Fluid
source:admin time:2018-06-11
Density and Pressure Balance
Drilling fluid density MW (mud weight) or SG (specific gravity)
Through the mud liquid column to produce pressure on the well wall and bottom, to balance the formation of oil, gas and water pressure and rock side pressure, to prevent blowout, formation fluid invasion and protect the well wall. In addition, mud density has buoyancy on cuttings. Increasing mud density can improve the ability of mud carrying cuttings. The normal error of density scale is 0.01g/cm3
ECD and annular density
ECD: additional pressure loss due to mud flow is considered;
Annulus density: Based on the equivalent circulation density, the additional pressure loss caused by cuttings in the wellbore is considered;
Effect of drilling fluid density
·Balance formation pressure and prevent blowout, lost circulation and drilling fluid from being polluted by formation fluid;
·Balance formation pressure, keep well wall stable and prevent well collapse;
·Realize near balance drilling technology, reduce pressure holding effect and improve ROP;
·The density of drilling fluid should be selected reasonably to reduce the damage of drilling fluid to the producing formation.
The effect of density on drilling rate
The higher the mud density, the greater the liquid column pressure, the greater the bottom hole pressure difference, and the smaller the ROP.

Density control of Drilling Fluid
·High density: increasing the positive pressure difference will aggravate the reservoir pollution;
·The increase of liquid column pressure increases the drillable strength of bottom hole rock and causes repeated cutting of bottom hole rock
·Too low density: unstable sidewall and uncontrollable reservoir pressure
Principle of mud density determination
·According to the geological data, low density should be used as much as possible under normal conditions;
·When drilling the oil and gas reservoir, it is necessary to balance drilling as much as possible, protect the oil and gas reservoir and prevent blowout, so as to achieve "pressure without death, live without blowout";
·In near balanced drilling, the possible "suction effect" should be considered to increase the additional density;
·In order to prevent salt water pollution when drilling through high-pressure salt water layer, the density of mud should be properly increased for the formation that is easy to shrink and peel off;
Possible factors of increasing drilling fluid density
·Add weighting materials;
·Cuttings accumulation;
·The density of drilling fluid in the well will increase if the pump capacity fails to keep up with the rapid drilling;
·Increasing the yield value of drilling fluid will increase the ECD;
·Increasing the pump displacement or pressure will increase the ECD;
·Adding more electrolytes (salts);
·Adding high density brine into oil-based drilling fluid;
·Adding new pulp with higher density
Possible factors of drilling fluid density decrease
·Add clean water with lower density than drilling fluid;
·Downhole oil and gas invasion;
·Add new pulp or glue with lower density;
·Strengthening solid phase removal;
·Using centrifuge to remove (or recover) high density solid phase;
·Reducing the yield value of drilling fluid or pump discharge and pump pressure can reduce the downhole ECD;
• inflatable compounding into an aerated drilling fluid or using foam drilling fluid;
·It is possible to reduce the density of drilling fluid in the well by increasing the displacement of pump when the drilling speed is low.


Related Concepts
Hydrostatic pressure
Hydrostatic pressure is the pressure caused by the weight of the liquid column. Its size is related to the liquid density and vertical height, but not to the transverse size and shape of the liquid column.
Generally, the pressure value of unit height (or depth) increase is called pressure gradient. Hydrostatic pressure gradient is affected by liquid density, salt concentration, gas concentration and temperature gradient.
Overburden pressure
Overburden pressure refers to the pressure caused by the total weight of the formation matrix (rock) and the fluid (oil, gas and water) in the pore. The density of rock is related to porosity and burial depth.
Formation pressure
Formation pressure refers to the pressure acting on the fluid (oil, gas and water) in the rock pore, also known as formation pore pressure. The normal formation pressure is equal to the hydrostatic pressure from the surface to the underground. Its value is related to sedimentary environment.
Formation fracture pressure
At a certain depth in the well, the ability of bearing pressure is limited. When the pressure reaches a certain value, the formation will break. This pressure is called the formation fracture pressure PF. The size of formation fracture pressure depends on many factors, such as overburden pressure, formation pressure, lithology, formation age, burial depth and stress state of the rock.
Safe density window
Drilling fluid density range between formation fracture pressure and formation pressure.

Rheology of Drilling Fluid
Rheological properties of drilling fluid (DF) are the characteristics of flow and deformation of drilling fluid under the action of external forces. For example, the plastic viscosity, dynamic shear, apparent viscosity, effective viscosity, static shear and thixotropy of drilling fluid are rheological parameters.
Influence of mud rheology on drilling
·It will affect the carrying of cuttings and ensure the bottom of the well is clean.
·Influence on suspended cuttings and barite
·Impact on ROP
·It affects borehole rules and downhole safety.
Shear stress and shear rate
Shear rate: the increment of velocity (DV / DX) in unit distance perpendicular to the flow direction. A kind of
Shear stress: internal friction resistance to flow per unit area in the process of liquid flow.
Basic flow pattern of fluid
Newtonian fluid: the shear stress is proportional to the shear rate. Plastic fluid: Bingham fluid, suitable for water-based drilling fluid system.
Pseudoplastic fluid: power-law fluid, suitable for polymer system expansion fluid.
Common rheological parameters
Definition: the total reflection of internal friction between solid particles, between solid particles and liquid, and between liquid molecules when drilling fluid flows. – basic factors affecting mud viscosity • clay content (high content, high viscosity) • dispersity of soil particles (increasing plastic viscosity) • coalescence stability or flocculation strength (structural viscosity) • properties, molecular weight and concentration of polymer treatment agent.
Funnel Viscosity
Definition: the time that a certain volume of fluid (700ml) measured by funnel viscometer flows out of 500ml. The unit is seconds. The funnel viscosity is related to the plastic viscosity and yield value of the mud, as well as the size and shape of the instrument.
Structural viscosity
Definition: the interaction between dispersed phase particles or the friction increased by space grid structure is closely related to the yield value (T0) of mud.
Shear dilution –
Definition: the phenomenon that the apparent viscosity decreases with the increase of shear rate - for Bingham fluid, the lower the h-plastic, the higher the T0, that is, the greater the t0 / h plastic ratio, the higher the shear dilution capacity.
In every part of the actual drilling well (such as drill pipe, bit water hole, annulus, etc.), the shear rate is different, resulting in different effective viscosity. The larger the t0 / h plastic ratio, the stronger the shear thinning ability, the better for high pressure jet drilling, and the higher the shear rate, the better for sand belt.
Static and dynamic shear
The shear force of drilling fluid is static shear force. The essence of colloidal chemistry is gel strength. The strength of gel depends on the number of structural links and the strength of single chain. Dynamic shear force of drilling fluid: it reflects the interaction between clay particles and polymer molecules (the ability to form space grid structure) during laminar flow.
Thixotropy –
Definition: the property that the mud becomes thinner (shear force decreases) after mixing and thicker (shear force increases) after standing. – expression method: thixotropy expression: 10s shear (initial cut), 10min final cut (final cut) - the drilling technology requires that the drilling fluid has good thixotropy. When the mud stops circulating, the shear force can quickly increase to a proper value, which is conducive to the suspension of drilling cuttings, and does not cause the pump pressure to be too high after standing.
Adjustment of rheological parameters
·Reduce PV: reduce the solid content as much as possible by reasonable use of solid control equipment, water dilution or chemical flocculation.
·Increase PV: add clay with low mud making rate and crude oil; increase polymer concentration to improve filtrate viscosity of drilling fluid.
·Reduce YP: add viscosity reducer suitable for the system to break up the grid structure formed in the drilling fluid. If it is due to the flocculation of Ca2 + and Mg2 +, the structure formed by Ca2 + and Mg2 + can be weakened.
·Increase YP: pre hydrated bentonite can be added or the amount of polymer can be increased. For calcium treatment or other brine drilling fluid, the concentration of Ca2 + and Na + can be increased appropriately.
·Reduce N value: increase the content of high molecular weight polymer and inorganic salt in drilling fluid, and add pre hydrated bentonite into brine drilling fluid system, can reduce N value.
·Decrease or increase K value: basically the same as adjusting PV and YP.

The Relationship between Drilling Fluid Rheology and Drilling
1. Affect drilling speed
·Through the influence of hydraulic parameters on the drilling speed, viscosity on the transmission of water power, viscosity on the size of ECD (equivalent circulating density), pressure holding effect, reduce the drilling speed.
·Shear dilution characteristics affect the transfer of water power.
2. The relationship between the rheological property of drilling fluid and borehole purification
·The transport process of cuttings in wellbore laminar flow: disadvantages of peak laminar flow turbulence: favorable for carrying rocks
·The disadvantages of turbulent flow, large displacement, high pump pressure, low apparent viscosity, large sinking speed of cuttings, wall erosion, are not conducive to wall stability.
·Drill string rotation
3. The relationship between rheology of drilling fluid and wellbore stability
·Wall instability caused by mechanical factors: the possible arrangement of three principal stresses in the wall rock. When the difference between S1 and S3 (maximum and minimum principal stresses) is greater than the strength of the rock, shear failure occurs
·Wellbore instability caused by chemical factors: prevention of shale hydration and expansion
·The rheology and flow pattern of drilling fluid are related to wellbore stability.
4. The relationship between the rheology of drilling fluid and the pressure excitation in the well
In the process of tripping out and drilling, due to the up and down movement of the drill string and the start of the mud pump, the pressure of the fluid column in the well changes.
The factors affecting the exciting pressure are: the movement speed of the drill string, the degree of mud inclusion of the bit and the drill string, the gap in the annular space, well depth and mud performance (viscosity and shear force).

Filtration and Lubrication Performance
The filtration loss is a phenomenon that the free water in mud seeps into the porous formation under the action of pressure difference. Type: instantaneous loss, dynamic loss and static loss.
Parameters of drilling fluid filtration
1. API filtration (loss of water)
Volume of filtrate measured at 100 ± 5 psi (690 ± 35 kPa) for 30 minutes at room temperature
2. HTHP filtration (loss of water)
Simulate the actual temperature, the top pressure is 600 psi (4140 kPa), and the back pressure is 100 psi, twice the volume of filtrate measured in 30 minutes.


Mud Cake Quality of Drilling Fluid
1. Quality control of mud cake
The quality of mud cake is determined by the type and distribution of solid phase in mud and the degree of hydration and dispersion of clay. The quality of mud cake can be improved by adding high-quality soil and mud cake improvement materials (pf-tex, pf-gra)
Evaluation of mud cake quality:
Appearance: thin, strong toughness, performance evaluation: use the method of measuring penetration loss of water, i.e. use the mud cake as API loss of water or HTHP loss of water to judge the sealing ability of mud cake. In any case, we hope to obtain better quality mud cake.
2. Harm of drilling fluid loss
Hazards of high water loss:
·After the formation has been soaked for a long time, it causes borehole shrinkage, mud and shale peeling and collapse
·The water infiltrates into the production layer, makes the clay of the oil layer expand, the permeability of the oil and gas layer decrease, and the production capacity decreases
·Hazards of poor mud cake quality:
·The mud cake is thick and loose, and the friction coefficient is high, which increases the contact area between the drilling tool and the well wall, and the risk of sticking mud cake is high
·Easy to mud the bit or block the bit water hole
·Increase of lifting force during tripping out
·Hindering casing running and cementing between cement slurry and well wall
·Electric logging is easy to encounter jamming, which affects well wall sampling
3. Water loss process of drilling fluid
Water in drilling fluid = chemical combined water + adsorbed water + free water instantaneous loss of water.
At the moment of new well formation, mud water will permeate into the formation without mud cake dynamic loss. In the case of mud circulation, the mud cake is built, thickened and even balanced, and the water loss rate is gradually reduced from the beginning to a constant value.
Static loss of water: when the circulation is stopped, there is no scouring force of mud flow on the mud cake. With the loss of water, the mud cake is gradually thickened and the loss of water is gradually reduced.
The larger the static water loss is, the thicker the mud cake is, the larger the ratio of the solid volume content CC in the mud cake to the solid volume content cm in the drilling fluid is, and the thickness of the mud cake will decrease instead. When the solid content in the drilling fluid increases to close to the solid content in the mud cake, the thickness of the mud cake will increase significantly, so the high-quality viscosity should be selected as the material for slurry preparation.
The permeability of mud cake is the key factor to reduce water loss. The main factors that affect the permeability of mud cake are the particle size and size distribution of solid particles in drilling fluid and the concentration of particles. The more fine particles in the drilling fluid, the smaller the average particle size, the lower the mud cake permeability; the wider the solid particle distribution, the smaller the mud cake permeability; the permeability in the mud cake also depends on the proportion and content of colloidal particles (d < 10-5 μ m) in the drilling fluid.
4. The relationship between water loss and wall building and drilling
·Excessive loss of mud water will cause collapse, shrinkage and damage of water sensitive mudstone and shale.
·Mud cake thickness will cause: increase of lifting force, and even mud cake sticking. It is easy to cause bit mud package and increase tripping pressure. It prevents the casing from running in, and is not conducive to the cementation of cement and well wall during cementing.
·Requirements for water loss and mud cake: minimize water loss and control the nature of free water when the cost is feasible. – mud cake: thin, dense and tough.
5. Regulation of water loss and wall building
The key to control the loss of water and wall building of mud is to control the permeability of mud cake, which depends on the size, shape and hydration degree of clay and other particles that make up mud cake.
concrete measure
·Bentonite is used to form dense mud cake and add proper amount of soda ash, caustic soda or organic dispersant to improve the dispersion of clay particles.
·CMC or other polymers are added to protect clay particles, and CMC plays a role of hole plugging.
·Add very fine colloidal particles.
6. Control principle of drilling fluid loss
The water loss of mud is not the smaller the better the limestone, dolomite and cemented tight sandstone are affected by the water loss very little. The drilling speed determines that the water loss of the upper stratum is not strictly controlled. Therefore, the reservoir section with the liberation of drilling speed must be controlled with the smaller water loss. The water loss should be strictly controlled for the stratum that is easy to absorb water, expand and collapse. When the well is shallow, it can be relaxed. When the well is deep, it should be strictly controlled for the open hole time The short time can be relaxed, and the long time of naked eye should be strict.
7. Overview of drilling fluid performance - sand content
Sand content of mud:
Percentage of the volume of sand in the mud that cannot pass the 200 mesh sieve (0.074mm).
Generally, the sand content of mud shall be controlled below 0.5%, otherwise, the following hazards may occur:
·Mud density increases and drilling speed decreases
·The friction coefficient of mud cake increases, which is easy to cause mud cake sticking and sticking
·With the increase of sand content in mud cake, the permeability of mud cake is enhanced, resulting in the increase of water loss
·The mud cake is thick and loose, which makes the electric logging stuck and the cementing quality is poor
·Bit, drilling tool and mechanical equipment are severely worn

PH Value of Drilling Fluid
PH control
·The clay particles in the mud are stable because of the large cation exchange capacity in the alkaline medium;
·Some organic treatment agents can play a better role after forming sodium salt in alkaline environment;
·The low pH value makes the organic treating agent easy to ferment at high temperature;
·When the pH value is too high (usually > 10), the adsorption of OH - on the clay surface will cause the hydration and expansion of the clay, which is not conducive to collapse prevention and reservoir protection;
·The pH of conventional water-based mud is usually controlled at 8 ~ 9;
·Cl content: it can estimate the salinity and K + content of the drilling fluid, and it is also the judgment parameter of salt invasion;
·K + content: measured when containing KCl or other potassium salt, as the basis for potassium salt supplement;
·Ca2 + content: it reflects one of the parameters of mud hardness, and is also the judgment parameter of cement invasion and gypsum invasion;
·PF: phenolphthalein alkalinity of filtrate, which can be analyzed more specifically than pH value;
·MF: the methyl orange alkalinity of the filtrate, which can be more specific than the pH value to analyze the acidity and alkalinity of the filtrate;
·PF / MF: it can help to judge the invasion of HCO3 - or CO32 -;
·MBT (methylene basket exchange capacity): it reflects the amount of dispersed clay in the mud. It can also be used as a reference for mud displacement before mud conversion.
·PM: phenolphthalein alkalinity of mud, which can be more specific than pH value to analyze the pH of mud.

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