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Introduction to drilling fluid knowledge

2022-07-20

Section 1 Function, Type and Composition of Drilling Fluid

 

Drilling fluid testing refers to the testing of various circulating fluids that meet the needs of drilling work with their various functions during oil and gas drilling. Drilling fluid is also called drilling mud (Drilling Muds), or simply mud (Muds). The circulation of drilling fluid is maintained by a mud pump. The high-pressure drilling fluid discharged from the mud pump passes through the surface high-pressure manifold, riser, hose, tap, kelly, drill pipe, drill collar to the drill bit, and is ejected from the drill bit nozzle to clean the bottom of the well and carry cuttings. Then it flows upward along the annular space formed by the drill string and the well wall (or casing), after reaching the ground, it flows into the mud pool through the discharge pipeline, and then returns to the upper pool after being processed by various solid control equipment, and finally enters the mud pump circulation reuse. Various pipe fittings and equipment through which drilling fluid flows constitute a complete set of drilling fluid circulation system.

 

Drilling fluid technology is an important part of oil and gas drilling engineering. With the increasing difficulty of drilling, this technology plays an increasingly important role in ensuring safe, high-quality, and fast drilling.

 

1. The basic function of drilling fluid

 

(1) Carrying and suspending cuttings

 

The most important and basic function of the drilling fluid is to carry the cuttings broken by the drill bit at the bottom of the well to the surface through its own circulation, so as to keep the wellbore clean, make the tripping unobstructed, and ensure that the drill bit is always at the bottom of the well. Contact and break up new formations without causing repeated cuts to keep drilling safe and fast. When a single root is connected, tripped or the circulation is stopped for some reason, the drilling fluid will suspend the drilling cuttings in the well, so that the drilling cuttings will not sink quickly and prevent the occurrence of sand settling and sticking.

 

(2) Stabilize borehole wall and balance formation pressure

 

Wellbore stability and wellbore rules are the basic conditions for achieving safe, high-quality, and fast drilling. A drilling fluid with good performance should be able to form a thin and tough mud cake on the well wall by means of the filtration of the liquid phase to stabilize the drilled formation and prevent the liquid phase from invading the formation, reducing the hydration expansion and The degree of dispersion z. At the same time, during the drilling process, it is necessary to continuously adjust the density of the drilling fluid so that the liquid column pressure can balance the formation pressure, thereby preventing the occurrence of complex downhole situations such as well collapse and blowout.

 

(3) Cooling and lubricating drill bits and drilling tools

 

During drilling, the drill bit has been rotating at high temperature and breaking the rock formation, which generates a lot of heat. At the same time, the drilling tool is constantly rubbing against the well wall to generate heat. It is through the continuous circulation of the drilling fluid that the heat is absorbed in time, and then brought to the ground and released into the atmosphere, thereby cooling the drill bit and the drilling tool and prolonging its service life. Due to the existence of the drilling fluid, the drill bit and the drilling tool are rotated in the fluid, thus reducing the frictional resistance to a large extent and playing a good lubricating effect.

 

(4) Transmission of water power

 

The drilling fluid impinges on the bottom hole at extremely high flow rate at the bit nozzle, thereby increasing drilling speed and rock breaking efficiency. High-pressure jet drilling utilizes this principle, that is, using high pump pressure to drill, so that the high-speed jet formed by the drilling fluid has a strong impact on the bottom of the hole, thereby significantly increasing the drilling rate. When drilling with a turbo drilling tool, the drilling fluid flows through the turbine blades at a high flow rate from the drill pipe, so that the turbine rotates and drives the drill bit to break the rock.

 

However, drilling practice shows that as a high-quality drilling fluid, it is not enough to only do the above points. In order to prevent and minimize the damage to the oil and gas layers, modern drilling technology also requires that the drilling fluid must be compatible with the oil and gas layers that are drilled to meet the requirements of protecting the oil and gas layers; in order to meet the geological requirements, the drilling fluid used must be It is beneficial to formation testing and does not affect the evaluation of formations; in addition, the drilling fluid should not cause harm and pollution to drilling personnel and the environment, and should not corrode downhole tools and surface equipment or reduce corrosion as much as possible.

 

Under normal circumstances, the cost of drilling fluid only accounts for 7-10% of the total cost of drilling. However, advanced drilling fluid technology can often save drilling time exponentially, thereby greatly reducing drilling cost and bringing considerable economic benefits.

 

2. Types of drilling fluids

 

With the continuous development of drilling fluid technology, there are more and more types of drilling fluids. At present, there are various classification methods for drilling fluids at home and abroad. Among them, the simpler classification methods are as follows:

 

According to its density, it can be divided into non-weighted drilling fluid and weighted drilling fluid.

 

According to the strength of hydration with clay, it can be divided into non-inhibitory drilling fluid and inhibitory drilling fluid.

 

According to the difference of solid phase content, the low solid phase drilling fluid is called low-solid phase drilling fluid, and the solid phase-free drilling fluid is called solid phase-free drilling fluid.

 

However, the general classification method is classified according to the composition characteristics of the fluid medium and the system in the drilling fluid. According to the different fluid medium, it is generally divided into three types: water-based drilling fluid, oil-based drilling fluid and gas-based drilling fluid. Recently, a type of synthetic-based drilling fluid has appeared. More specifically, it can be divided into seven types as shown in Figure 1-1.

 

Introduction to drilling fluid knowledge

 

Since water-based drilling fluids have always been dominant in practical applications, they are divided into several types according to the different compositions of the systems. The following are various types of drilling fluids that have been recognized in China on the basis of reference to foreign drilling fluid classification standards

 

(1) Dispersed Drilling Fluids

 

Dispersed drilling fluid refers to a water-based drilling fluid prepared with fresh water, bentonite and various treatment agents (referred to as dispersants) that disperse clay and drill cuttings. It is a common drilling fluid with a long history of use, simple preparation method and low preparation cost. Its main features are:

 

1. It can accommodate more solid phases, and is more suitable for preparing high-density drilling fluids.

 

2. It is easy to form a denser mud cake on the well wall, so its filtration loss is generally low.

 

3. Some dispersed drilling fluids, such as trisulfuric drilling fluids with sulfonated tannin extract, sulfonated lignite and sulfonated phenolic resin as the main treatment agents, have strong temperature resistance and are suitable for use in deep and ultra-deep wells. But it also has some disadvantages compared to other drilling fluid types. In addition to the poor inhibition and anti-pollution ability, the high solid content in the system has adverse effects on improving the drilling rate and protecting the oil and gas layers.

 

(2) Calcium-Treated Drilling Fluids

 

The compositional feature of calcium-treated drilling fluid is that the system contains both a certain concentration (mass concentration) of Ca2+ and a dispersant. Through ion exchange of Ca2+ with sodium bentonite with strong hydration, a part of sodium bentonite is converted into calcium bentonite, thereby weakening the degree of hydration. The function of dispersant is to prevent excessive flocculation of clay particles in the system caused by Ca2+, so as to keep it in a state of moderate flocculation, so as to ensure that the drilling fluid has good and stable performance. The characteristics of this type of drilling fluid are that it has strong resistance to salt and calcium pollution; and it has the effect of inhibiting the hydration and dispersion of the clay in the drilled formation, so it can control shale collapse and well diameter expansion to a certain extent. At the same time, it can reduce the damage to the oil and gas formation.

 

(3) Saltwater Drilling Fluids

 

Brine drilling fluids are formulated with brine (or seawater). It is of this type over the entire range from 1% (Cl- mass concentration of 6000 mg/l) up to saturation (Cl- mass concentration of 189000 mg/l). Brine drilling fluid is also a type of drilling fluid that has a strong inhibitory effect on clay hydration.

 

(4) Saturated Saltwater Drilling Fluids

 

It refers to the brine drilling fluid system when the NaCl content in the drilling fluid reaches saturation. It can be prepared with saturated brine, or firstly prepared as drilling fluid and then salted to saturation. Saturated brine drilling fluid is mainly used for drilling large sections of rock salt layers and complex salt gypsum layers that are difficult to deal with by other water-based drilling fluids, and can also be used as completion fluid and workover fluid.

 

(5) Polymer Drilling Fluids

 

The polymer drilling fluid is a water-based drilling fluid with some high molecular weight polymers with flocculation and coating as the main treatment agent. Due to the presence of these polymers, various solid particles contained in the system can be kept in a relatively coarse particle size range, and at the same time, the drilled cuttings are also protected by coating in time and are not easily dispersed into fine particles. Its main advantages are:

 

1. The density and solid content of the drilling fluid are low, so the drilling speed can be significantly improved, and the damage to the oil and gas layers is also less.

 

2. Strong shear thinning properties. Under a certain pump displacement, the viscosity and shear force of the annular fluid are high, so it has a strong ability to carry cuttings; while at the high shear rate at the bit nozzle, the flow resistance of the fluid is small, which is beneficial to Increase drilling speed.

 

3. The polymer treatment agent has a strong coating and dispersion inhibition effect, so it is beneficial to maintain the stability of the well wall. Therefore, since the 1970s, this type of drilling fluid has been widely used at home and abroad, and its process technology has been continuously improved and developed.

 

(6) Potassium-Based Polymer Drilling Fluids

 

Potassium-based polymer drilling fluid is a kind of anti-slump drilling fluid with potassium (or ammonium, calcium) salt and KCl of various polymers as main treatment agents. Among various common inorganic salts, KCl has the best effect of inhibiting the hydration and dispersion of clay; and the presence of polymer treatment agents makes this type of drilling fluid have various excellent characteristics of polymer drilling fluids. Therefore, it can achieve ideal anti-slump effect when drilling mud shale formations.

 

(7) Oil-Based Drilling Fluids

 

Drilling fluids that use oil (usually diesel or mineral oil) as the continuous phase are called oil-based drilling fluids. At present, ordinary oil-based drilling fluids with water content below 5% are rarely used, and water-in-oil emulsion drilling fluids with oil-water ratio in the range of (50-80): (50-20) are mainly used. Compared with water-based drilling fluids, the main features of oil-based drilling fluids are high temperature resistance, strong inhibition and resistance to salt and calcium pollution, good lubricity, and can effectively reduce damage to oil and gas formations, etc. . Therefore, the use of this type of drilling fluid has become one of the important means for drilling deep wells, ultra-deep wells, extended reach wells, horizontal wells and various complex formations. But on the other hand, its application is limited to a certain extent due to its high preparation cost and certain environmental pollution when used.

 

(8) Synthetic Drilling Fluids

 

Synthetic-based drilling fluid is a new type of drilling fluid with synthetic organic compounds as the continuous phase, brine as the dispersed phase, and containing emulsifiers, fluid loss reducers, and flow modifiers. Because the non-toxic and biodegradable non-water-soluble organics are used to replace the diesel oil commonly used in oil-based drilling fluids, this kind of drilling fluids not only maintains various good characteristics of oil-based drilling fluids, but also greatly reduces drilling fluids. The adverse effects on the environment during discharge, especially for offshore drilling.

 

(9) Gas-typed Drilling Fluids

 

Gas-based drilling fluids are mainly suitable for drilling low-pressure oil and gas formations, formations that are easy to leak and some heavy oil formations. It is characterized by low density and fast drilling speed, which can effectively protect oil and gas layers, and can effectively prevent the occurrence of complex situations such as lost circulation. Gas-based drilling fluids are usually divided into the following four types:

 

1. Air / Natural Gas Drilling Fluids

 

That is, dry air or natural gas is used as the circulating fluid in drilling. The key to its technology lies in that the injection pressure must be large enough to ensure that the annular flow velocity can be achieved to carry all the cuttings from the bottom of the well to the surface.

 

2. Mist Gas Drilling Fluids

 

That is, a mist-like fluid formed by a small amount of liquid dispersed in an air medium. It is a transition form between air drilling fluid and foam drilling fluid.

 

3. Foam Drilling Fluids

 

Foam used in drilling is a dispersion system formed by dispersing gas medium (usually air) in liquid, and adding appropriate amount of foaming agent and stabilizer.

 

4. Aerated Drilling Fluids

 

Sometimes in order to reduce the density of drilling fluid, gas (usually air) is evenly dispersed in the drilling fluid to form aerated drilling fluid. Obviously, the more gas mixed in, the lower the density of the drilling fluid.

 

(10) Drill-in Fluids Drilling Fluids for Formation Damage

 

Control)

 

This refers to a type of drilling fluid used when drilling in a reservoir. When a well reaches its target layer, the designed drilling fluid should not only meet the requirements of drilling engineering and geology, but also meet the needs of protecting oil and gas layers. For example, the drilling fluid density and rheological parameters should be adjusted to a reasonable range, the filtration loss should be as low as possible, the selected treatment agent should be compatible with the oil and gas layers, and a suitable temporary plugging agent should be selected.

 

3. Composition of drilling fluid

 

Water-based drilling fluid is a multiphase dispersion system composed of bentonite, water (or brine), various treatment agents, weighting materials and drill cuttings. Among them, the average density of bentonite and drill cuttings is 2.6 g/cm3, and they are usually called low-density solid phases; while weighted materials are often called high-density solid phases. The most commonly used weighting material is API barite, which has a density of 4.2 g/cm3. Because bentonite is the most commonly used slurry material in water-based drilling fluids, it mainly plays the role of sticking and shearing, filtration reduction and wall building, so it and barite and other weighting materials are called useful solid phases. On the other hand, the drill cuttings are called useless solid phase. In the drilling fluid, the content of drill cuttings should be minimized through various solid control measures. The amount of bentonite should also be enough, and it should not be too large, otherwise it will cause the drilling fluid to stick too high, and it will also seriously affect the ROP and adversely affect the protection of oil and gas layers.

 

Oil-based drilling fluid is formed by adding water droplets as the dispersed phase, oil as the continuous phase, and adding appropriate amount of emulsifier, wetting agent, lipophilic solid treatment agent (organic soil, oxidized asphalt, etc.), lime and weighting materials. liquid system. Figures 1-2 and 1-3 show typical compositions of water-based and oil-based drilling fluids, respectively.

 

Introduction to drilling fluid knowledge

Figure 1-1-2 Typical composition of water-based drilling fluid Figure 1-1-3 Typical composition of oil-based drilling fluid.

 

Influence of drilling fluid on drilling speed in drilling engineering

 

Abstract: Combined with the actual situation, the influence of drilling fluid on drilling speed in drilling engineering construction is analyzed. First, the effect of drilling fluid on drilling fluid, and secondly, the influence of drilling fluid on drilling fluid performance during drilling construction process is analyzed. I hope that after the discussion, you can give relevant work staff to help.

 

Key words: drilling engineering; drilling fluid; drilling speed; influence

 

0 Preface

 

In drilling engineering, under different terrain conditions and different well types, the types of drilling fluids used are also different, but they must meet the requirements of safety and environmental protection, and will not cause any pollution problems to the formation. Therefore, it is necessary to strengthen the research and development of drilling fluids. It is necessary to add a certain amount of components to promote the smooth progress of drilling on the basis of crude oil properties. At the same time, it is necessary to meet the requirements of green environmental protection, energy saving and consumption reduction. This paper mainly studies the main functions of drilling fluids, and deeply analyzes drilling The performance of drilling fluids lays a solid foundation for the comprehensive development of drilling fluids.

 

1 Analysis of the effect of drilling fluid

 

During drilling construction, the main functions of drilling fluid are as follows:

 

In order to comprehensively improve the efficiency of drilling construction, it is necessary to clean up the bottom hole cuttings. One of the important functions of the drilling fluid is to clean up the cuttings, which is the effect of the drilling fluid circulation process. First, in the drilling construction stage, the drill bit It will continuously rotate at high speed, the hardness of the drill bit is greater than that of the rock, and the rock at the bottom of the hole will be directly cut into a large number of fine debris, and because the debris will have a relatively large frictional force, which will reduce the drilling speed, so the drilling process needs to be timely. To clean up the debris, the drilling fluid will directly enter the drilling tool and the well wall with the bit pressure, return the debris directly to the surface, and then circulate in the entire drilling environment to ensure that the drilling can be completed with high quality.

 

The speed of the drill bit in the drilling construction process is very high, and it will generate relatively large resistance after it collides with the rock. At this time, the temperature of the drill bit will be relatively high, which will have a relatively large negative impact on the drilling construction. In severe cases, it will directly lead to The drill bit is scrapped, and effective cooling measures cannot be taken in the drilling process, while the drilling fluid can reduce the temperature of the drill bit. First, the drilling fluid is mainly in liquid form in the well, which can increase the cooling area of ??the drill bit and achieve continuous cooling; secondly, the drilling fluid It will be continuously circulated, and the heat will be taken out at this time, which can ensure that the temperature of the drill bit will not be too high [2].

 

During the drilling construction process, the large-diameter rock will form fine debris under the high-speed grinding operation of the drill bit. At this time, due to the centrifugal force of the debris, it will directly adhere to the drill bit, resulting in relatively large friction force of the drill bit. , and the drill bit cannot be cleaned in the well, which will lead to problems such as sticking or a decrease in drilling speed under long-term influence, while the drilling fluid can directly bring out the debris and achieve the effect of lubricant.

 

Drilling Fluid Challenges

 

All drilling challenges are closely related to wellbore integrity throughout the drilling process. High-quality drilling engineering can complete all aspects of construction operations in the wellbore with the shortest non-productive time for drilling, logging, cased hole, cementing and completion.

 

The design of the drilling fluid system is key to achieving this goal. Today, let's talk about lost circulation and stuck pipe during drilling.

 

l Loss of circulation.

 

Loss of circulation is the influx of fluids from the formation into the wellbore due to the density of the drilling fluid being less than the pore pressure of the formation. Conditions that cause lost circulation include: natural and secondary fractures, high permeability and/or high porosity formations, etc. The cost of a lost-off event far exceeds the price of the drilling fluid used to deal with the lost-off event, because a lost circulation always results in a production downtime, including rig time and all service charges for drilling operations, and once the mud enters the reservoir or gas Reservoir can greatly reduce or even eliminate reservoir productivity and reduce crude oil production.

 

Loss of drilling fluid circulation in depleted formations can result in a drop in wellbore hydrostatic pressure. The influx of gas or water into the formation can exacerbate accidents when the hydrostatic pressure drops too low to stop the inflow of formation fluids. In this case, the operator should increase the drilling fluid density to further suppress the kick, while avoiding exacerbating the problem of drilling fluid loss. In addition, the pressure differential generated in the lost formation may cause the drill string to become embedded in the wall cake, a condition known as differential pressure sticking, in which case the drill string should be released mechanically or chemically quickly because it is stuck The longer the time, the lower the probability of release. Failure to release the drilling tools requires expensive salvage operations that cannot be carried out until the well is brought under control.

 

The Dropout Test (LOT) is the basis for preventing lost circulation. The method of operation is to seal the wellbore and pressurize the open hole immediately under the last string of casing before entering the next section. The strength of the wellbore at the casing shoe is tested at the point of pressure drop, which is generally considered to be one of the weakest points in the interval, the point of minimum pressure. However, a large number of formation fractures can seriously reduce the safe density window of the drilling fluid. Therefore, it is best to stop the test as early as possible when the test pressure point starts to decrease.

 

Formation Integrity Testing (FIT). To avoid damaging the formation, many operators match casing shoe locations to determine if the wellbore can withstand the expected maximum mud density while drilling the section. If the casing shoe maintains a pressure equivalent to the specified mud density, the test is considered successful and drilling resumes. When the operator selects a bulk operation or a tie-in operation, if the test fails, some remedial action (usually cementing) should be performed before resuming drilling to ensure the capacity of the wellbore.

 

Loss of circulation is such a common phenomenon that prevention is critical, but so is effective remediation. Because of the high cost of most weighted, treated drilling fluid systems, lost circulation materials are used in many operations where a lost zone may exist. Preventive pretreatment can be conveniently carried out using a lost circulation material that can be carried in the drilling fluid without significantly affecting the rheology or leakage characteristics of the drilling fluid. Pretreatment can mitigate wellbore breathing effects (swelling), seepage losses, and potential lost circulation in drilling depleted zones. As long as a sufficient density is maintained, the hydrostatic pressure can be reduced in a targeted manner to reduce the loss of drilling fluid when leakage occurs. Since the lost layer also poses the risk of differential sticking, rotary and reciprocating trips help reduce this risk. If the location of the lost zone is known, it is best to pull the drill string over the affected area.

 

It is common practice to use a mixture of various plugging materials and to combine treatments of several types and particle sizes. Less expensive traditional materials include calcium carbonate, paper, cottonseed husks, nut shells, mica and cellophane. Since leakage has always been one of the most expensive problems facing the oil and gas industry, the purpose of repairing the lost zone quickly and safely regardless of changes in annular pressure has enhanced the development of lost circulation materials that can be combined with Fracture widths remain relatively consistent, closing pores.

 

In some cases, this deformable, expanding lost circulation material has a high success rate in preventing and repairing severe lost circulation and lost circulation. For example, conventional operations cannot solve the serious lost circulation problem, which can be placed under certain pressure conditions through a locatable and hydratable plugging material. Under the downhole temperature conditions, the anti-seepage material of the plugging material expands rapidly, filling and bridging the fractures, allowing the drilling and cementing operations to resume quickly, sometimes in 4 hours or less. Alternatively, lost circulation material products can rapidly react with drilling fluids after the entire lost zone is discovered, forming dense, flexible plugs that fill fractures and adhere to the wellbore. In some cases, this type of plug has proven so effective that the formation's naturally fractured formation pressure gradient actually increases, allowing the operator to resume drilling and increasing the mud weight beyond the pre-treatment value. limit.

 

l stuck drill.

 

Complications related to sticking account for nearly half of the total cost of drilling, making it one of the most expensive problems in drilling operations. In highly deviated and horizontal wells, sticking and well control are major issues that contribute to the high cost of drilling operations.

 

When the annulus pressure exceeds the formation pressure, the drill string may be pulled toward the wellbore and embedded in the filter cake deposited on the wellbore as the drilling tool passes through the depleted layer (as shown). When the drill pipe contacts the filter cake, the internal filter cake pressure drops, causing the pipe to stick to the wall due to the pressure differential. During directional drilling operations, because sliding can better control the direction, the drilling tool needs a period of sliding time. During this time, the drill string does not rotate for a period of time, but continues drilling through the downhole motor, but the stop of the drilling tool will affect the drilling. Borehole cleaning. Good borehole cleaning is important in all wells, but is critical for highly deviated wells where cuttings can settle in the lower part of the wellbore, forming cuttings bed. If the cuttings cannot be removed in time and effectively, the drilling tool assembly will be stuck by the accumulated cuttings bed, which is another form of stuck pipe. At this time, you can try to increase the circulating drilling fluid discharge However, it may cause excessive pressure in the wellbore, which could result in fracture of the formation under the packer. Such drilling accidents often occur. Although drilling fluid alone cannot solve all problems, the professional mastery of drilling fluid and adjusting the formula of drilling fluid can effectively prevent the occurrence of wellbore instability, thereby helping drilling operations to achieve.

 

Introduction to drilling fluid knowledge

 

In highly deviated wells and horizontal wells, gravity increases the contact area between the drill string and the formation. Appropriate control of the lubricity of the drilling fluid and the quality of the filter cake in the permeable formation can help reduce the occurrence of stuck pipe accidents.

 

Mechanical causes of sticking include: isolation caused by poor wellbore cleaning, shale expansion, wellbore collapse, formation plastic flow (ie, plastic fluid in the wellbore). Preventing sticking requires close monitoring of early warning signs such as torque and drag increases, signs of excessive cuttings loading, stuck sticking encountered while running, and loss of circulation that occurs while drilling.

 

Depending on the cause of the sticking, the drilling fluid density needs to be increased (to stabilize the expanding shale) or decreased (to protect the depleted zone and avoid differential sticking). The coefficient of friction of the drilling fluid is an important factor in whether the drilling fluid can effectively prevent sticking and/or enable it to work freely. Oil-based and synthetic-based drilling fluids provide maximum lubricity; inhibited water-based drilling fluids can be treated with lubricants (usually 1 to 5 percent by volume) and formulated into a thin, impermeable filter cake, Added protection against sticking. High-performance polymer water-based drilling fluids designed as replacements for oil-based drilling fluids and synthetic-based drilling fluids have a high degree of natural lubricity and may not require the addition of lubricants.

 

Borehole cleaning

 

Effective drilling fluid selection and management is critical to the successful outcome of high-angle or horizontal extended-reach drilling. In addition to being able to effectively protect the reservoir, the most important challenges for horizontal extended-reach wells include the narrow pressure window between pore pressure and fracture pressure, ECD control, wellbore cleaning, torque and drag reduction, wellbore stability, recrystallization problems such as rock settlement and drilling fluid circulation loss. A large number of drilling history data show that 30-60 degrees of well inclination is the most difficult for wellbore cleaning, and the control of annular return velocity, drilling fluid rheology, drill string rotation speed and eccentricity of BHA can help drilling The fluid effectively transports the cuttings to the surface, reducing the impact of long-term and massive accumulation of cuttings on the wellbore stability.

 

Using hydraulic engineering calculation software can accurately predict drilling fluid performance under actual downhole conditions, including static and dynamic temperature field, drilling fluid rheology, hydrostatic column pressure, ECD, annular pressure loss, cuttings migration and transportation efficiency, The influence of the eccentricity of the drill string and its hydraulic model characteristics are established and corrected through real-time pressure while drilling (PWD) data.

 

Real-time feedback from calculations allows drilling fluid engineers to optimize wellbore cleaning in a number of ways, including:

 

Adjust surface mud properties to meet changes in downhole conditions;

 

Adjustment of construction machinery parameters, such as drilling speed, flow rate, control rotation speed, etc.;

 

Downhole cuttings cleaning operations that can be effectively designed and implemented.

 

Considering the pumping and fluid handling capabilities of the drilling platform, during the drilling design stage, casing design, bit selection and drilling fluid properties can be optimized upfront to achieve the best drilling conditions. Accurate fluid mechanics modeling should include Bingham plasticity, power law, and Heba rheological models. At the same time, the rheological properties of surface drilling fluids are measured with a six-speed rheometer, and the temperature and pressure of the wellbore profile can be predicted using hydraulic calculation software. Change conditions to determine the actual annular shear rate at any depth in the well, and for rock-carrying calculations, the hydraulics calculation software can also accurately predict the cuttings load in the annulus, cuttings bed height, drill string rotational speed and drill pipe eccentricity , and the maximum recommended ROP for the given conditions. These tools are suitable not only for most conventional drilling situations, but also for optimizing drilling performance in unconventional well operations such as deepwater operations, high pressure and high temperature wells and slim hole drilling operations.

 

Since drill pipe eccentricity affects the surrounding flow distribution, for highly deviated and horizontal wells, the use of high viscosity drilling fluids may be the best option for wellbore cuttings carryover. In order to induce flow, the stress exerted on the fluid must exceed the yield stress of the fluid. However, in the narrow annular space created by the eccentric drill pipe, little or no flow may occur, and the cuttings layer will remain in the deviated well, a problem that may be exacerbated by pumping high-viscosity drilling fluids.

 

Barite Settlement

 

Barite subsidence becomes a problem in highly deviated wells (maybe at 35° inclination, but is increasingly likely at ≥50°, then tapers off as distance approaches 75 - 90°) . The most severe barite sedimentation usually occurs in the 45 to 65 degree range. Barite subsidence will lead to a decrease in the density of the drilling fluid in the upper well section, especially when the density of the drilling fluid at the bottom of the well increases under long-term static conditions. The barite settles in the lower part of the wellbore and slides to the bottom, creating a heavy buildup of silt around the lower part of the drill string.

 

Barite settling can lead to well control problems and pipe sticking problems, and can exacerbate wellbore cleaning problems. A well-designed wellbore cleaning process can help prevent or reduce subsidence. Field results have demonstrated significant success in preventing subsidence with appropriate wellbore cleaning methods. When using clay-free emulsifiers, barite settling is rarely a problem based on data collected by downhole pressure measurement tools and drilling fluid density measurements recorded while circulating up the bottom hole.

 

Salt formations and formation fracture zones

 

The five major problems usually encountered when drilling into the salt layer are: the formation pressure increases due to the presence of interlayer shale in the salt layer; the wellbore is easily eroded when drilling the salt layer and the shale above and below the salt layer; the creep caused by the salt-gypsum layer Excessive torque and reduction problems occur; well control problems and leaks lead to mud loss.

 

Of these problems, the loss of large amounts of drilling fluid below the salt layer is the most challenging, making it impossible for drilling fluid engineers to use oil-based and synthetic-based drilling fluids to drill in the fractured zone of the formation. Based on the assessment of drilling risk control, it is decided whether to use synthetic-based drilling fluid or saturated brine drilling fluid. A high level of drilling fluid can help increase ROP without causing excessive washout, but prolonged use of undersaturated water-based drilling fluids can cause excessive wellbore enlargement where the drilling fluid is exposed the longest.

 

Using hydraulic calculation software and PWD data, it is possible to perform hydraulic analysis before drilling into the salt formation and reaching the fracture zone of the formation. If drilling through fractured formations cannot be continuously tracked by downhole measurement tools, the results of hydraulic analysis can help operators to drill with the minimum ECD in the fractured zone of the formation.

 

Mud shale instability

 

Shale makes up the bulk of the drilled formation and causes wellbore instabilities ranging from erosion to complete wellbore collapse. Shale is a fine-grained sedimentary rock composed of clay, silt, and in some cases fine sand. Shale types range from black shale rich in organic matter and finely dispersed to sandy shale, but all of them are permeable A common feature of extremely low sexuality and a high proportion of clay minerals.

 

The use of water-based drilling fluid for overbalanced drilling in shale formations can make the drilling fluid pressure penetrate the formation. Due to the saturation and low permeability of the formation, a small amount of mud filtrate entering the formation will cause the pore fluid pressure near the wellbore to increase significantly, and the pore fluid The increase in pressure can reduce the effective support of the drilling fluid, which can lead to the destabilization of the drilling fluid. Some polymeric water-based drilling fluids achieve the same effect of shale inhibition as oil-based drilling fluids and synthetic-based drilling fluids by using inhibitors and encapsulants to prevent shale hydration and dispersion.