Cement Slurry Flow Properties and Mud Displacement

Learning Objectives After completing this topic " Cement job | Cement Slurry Flow Properties and Mud Displacement", you will be able to: Summarize the role of fluid rheology in ensuring good mud and cement displacement.Describe the difference between laminar and turbulent flow and the relevance to cementing procedures.Describe the role of proper mud conditioning in ensuring good cement job quality.Describe the differences among washes, preflushes and spacers pumped during a cementing job. Introduction https://youtu.be/ner86pR0118 The objective of cementing is to place a homogeneous mixture of cement and additives between a casing string and wellbore (or between concentric casing strings) in a manner that: Ensures strong bonds between the pipe and the cement, and between the cement and formationSupports the development of a uniform, competent sheath of cement around the casing over the entire cemented interval To achieve this objective, it is essential to remove any mud, drill cuttings and mud filter cake that could prevent cement from properly bonding with the pipe and formation, or even lead to uncemented voids behind the casing. One way to aid this removal process is to include hardware such as centralizers, wipers, and scratchers as part of the casing string; another is to reciprocate and/or rotate the casing during cement displacement. A more important consideration however, involves rheology - the study of flow and deformation of materials under applied forces. Of particular interest is the manner in which the displaced mud and the displacing cement behave under certain conditions of pressure, temperature and flow rate, and how this behavior affects mud, mud cake and cuttings removal. This course discusses the rheological character of cement slurry, how slurry behavior affects mud removal, and provide general guidelines for optimizing mud displacement. Application of Rheological Models Having a rheological model that accurately explains and predicts fluid behavior is essential to the successful planning and design of a cement job. An appropriately applied fluid model enables engineers to: Determine the rate of increase in frictional pressure losses.Predict the annular pressure after slurry placement.Optimize mud and mud cake removal through slurry turbulence.Evaluate slurry pumpability through different downhole restrictions.Evaluate the slurry’s ability to transport lost circulation materials. Before any cement is pumped, engineers can use such models to characterize the cementing process under various anticipated surface and downhole conditions. The models are incorporated into computer simulations using the following information: Data about the wellbore and casing geometryDrilling mud characteristicsNumber and placement of centralizersVolume, pump rate, and characteristics of the various fluids being pumped down the well These simulations predict various aspects of the cementing process, such as the pressures along the path of the cement as it is pumped down the well. Engineers routinely use cement simulations to model the complex process of mud displacement from the annular space. Predicting mud displacement is important for at least two reasons: First, if the cement flow does not displace the fluids from the annular space, the uncemented void filled with those fluids may create a flow path for hydrocarbons behind the casing. Figure 1 illustrates a cross-section of wellbore and casing showing how decentralized casing can allow cement to bypass drilling mud in restricted areas and leave an uncemented void space.Secondly, poor mud displacement increases the potential for reservoir fluids, particularly natural gas, to flow into and up the cement column as it sets. This gas flow can itself cause channeling and further compromise zonal isolation. Figure 1: Decentralized casing and void space Laminar and Turbulent Flow When a single-phase fluid flows below a certain rate through a pipe (or through the annulus between two pipes), the fluid particles flow in a single direction, across a line perpendicular to the pipe axis. The particles in the center of the pipe tend to move faster than those near the pipe wall, and there is no fluid motion transverse to the bulk flow direction. This condition is known as laminar flow. Plug flow describes a low-velocity, sub-laminar condition where a fluid moves uniformly as a homogeneous, relatively undisturbed body. Under certain conditions, the fluid particles swirl around in all directions within the annular space. The speed of flow increases rapidly with the distance from the pipe wall and remains fairly constant throughout the main body of the moving fluid. This is known as turbulent flow. Figure 1: Laminar, plug, and turbulent flow The distinction between laminar and turbulent flow is described by a dimensionless term known as the Reynolds Number (NRe), which relates fluid density, velocity, viscosity and the diameter of the flow conduit. For flow inside pipes, the Reynolds Number is expressed as [latex]N_{Re} = \dfrac{\rho \nu D}{\mu }[/latex] (1) For annular flow, the Reynolds Number is [latex]N_{Re} = 0.816\ \dfrac{\rho \nu \left (D_{2}-D_{1} \right )}{\mu }[/latex] (2) Where [latex]\rho[/latex] is the fluid density [latex]\nu[/latex] is the fluid velocity D is the inside diameter of the pipe D1 is the outside diameter of the pipe D2 is the hole diameter or the inside diameter of the outer casing μ is the fluid viscosity In field units ([latex]\rho[/latex] in [latex]\tfrac{lbm}{gal}[/latex], [latex]\nu[/latex] in [latex]\tfrac{ft}{s}[/latex], D in inches and μ in cp), equations 1 and 2 become [latex]N_{Re} = 928\ \dfrac{\rho \nu D}{\mu }[/latex] (3) [latex]N_{Re} = 757\ \dfrac{\rho \nu \left (D_{2}-D_{1} \right )}{\mu }[/latex] (4) In general, Reynolds numbers of less than 2100 indicate laminar flow, while Reynolds numbers greater than 4000 indicate turbulent flow. Between these values, flow is considered transitional. At low flow rates, fluids always flow in a laminar fashion and their flow behavior can be characterized by their viscosity. As flow rate increases, fluids transition to a turbulent flow regime, where the flow is less dependent on viscosity and more dependent on inertial forces. Viscosity, Shear Stress, and Shear Rate An examination of the Reynolds Number tells us that two of the most important properties in determining a fluid’s flow behavior are density [latex]\left (\rho\right )[/latex] and viscosity (μ), where density is the ratio of mass to volume and viscosity is the ratio of shear stress [latex]\left ( \tau \right )[/latex] to shear rate [latex]\left ( \gamma \right )[/latex]: [latex]\mu = \dfrac{\tau}{\gamma}[/latex] (1) This relationship between shear stress and shear rate applies to all fluid flow. The concepts of shear rate and shear stress can be represented in terms of two fluid layers (A and B) moving past each other when a force (F) has been applied (Figure 1). Figure 1: Shear rate and shear stress The shear stress is the frictional force that resists flow of the moving fluid layers past each other, while the shear rate is the rate at which one layer is moving past the other. The shear rate is therefore a velocity gradient equal to: [latex]\gamma = \dfrac{V_{A}-V_{B}}{d}[/latex] (2) Fluid Models Fluids can be broadly classified as Newtonian or non-Newtonian. Water and light oil (such as diesel oil and mineral oil) are examples of Newtonian fluids, while whole drilling mud and cement slurry are examples of Non-Newtonian fluids. Newtonian Fluids A Newtonian fluid is one in which the shear stress is directly proportional to the shear rate. The slope of the line depicting that relationship is the viscosity of the fluid, and is a constant that does not depend on flow conditions, but only on temperature and pressure. Such behavior is termed "Newtonian" as it follows a relatively simple model of fluid behavior first derived by Isaac Newton (Figure 1). Figure 1: Newtonian fluid: shear stress versus shear rate A plot of friction pressure gradient versus flow rate for a Newtonian fluid (Figure 2) shows that such fluids begin to flow immediately when a pressure gradient is applied (i.e., the line goes through the origin). Provided that the flow is laminar, the relationship is linear, like the shear stress/shear rate relationship. However, as the flow rate increases and the flow regime transitions to turbulent, the friction pressure increases faster than in laminar flow and the relationship is not linear - there is a slight upward curve to the line. When turbulent flow is achieved the linear relationship returns, but with a much faster increase in friction pressure with increased flow rate. Figure 2: Newtonian fluid: friction pressure gradient versus flow rate Non-Newtonian Fluids Drilling mud and cement slurry do not behave according to the Newtonian model, and so are termed non-Newtonian fluids. Non-Newtonian fluids have two characteristics that distinguish them from the simpler Newtonian model: The curve on the shear stress-shear rate relationship does not pass through the origin. This means that a certain amount of stress (a yield stress) must be applied before the fluid starts to flow.The slope of the line changes with increasing shear rate. This means that the viscosity decreases with increased shear ratein what is called shear-thinning behavior. Two models that have often been used to describe non-Newtonian behavior are the Bingham Plastic model and the Power Law model. One example of a non-Newtonian model, which combines aspects of the Bingham Plastic and Power Law models, is the Herschel-Bulkley or Yield Power Law model (Figure 3) described by the following relationship: [latex]\tau = \tau _{\gamma} + K\gamma^{m}[/latex] Where [latex]\tau _{\gamma}[/latex] is the yield stress, and K and m are empirically derived quantities known, respectively, as the Consistency Index the Power Law Index. Figure 3: Herschel-Bulkley or Yield Power Law As the flow rate increases and flow transitions from laminar to turbulent, the relationship changes and friction pressures increase much faster than predicted by the laminar flow relationship (Figure 4). All parts of the curve are nonlinear. Figure 4: Non-Newtonian fluids: friction pressure gradient versus flow rate Effect of Pump Rate and Equivalent Circulating Density on Displacement Higher cement pumping rates create turbulent flow and tend to increase the efficiency with which cement displaces mud from the annular space. Cement must be pumped fast enough for it to scour mud from the side of the wellbore instead of merely flowing past the mudcake. However, increased pump rates mean increased pressure and care must be taken to understand what the impact of these higher rates will be on downhole pressures. Equivalent circulating density (ECD) is the effective density of the circulating fluid (in this case cement) in the wellbore resulting from the sum of the pressure exerted by the hydrostatic head of the static column of cement (based on its density) and the friction pressure losses in the annulus when the cement is being pumped. This is commonly expressed in pounds per gallon [latex]\left( \tfrac{lb}{gal} \right )[/latex]. A modeling program can determine the dynamic annular pressures and ECD under various pump rates for various casing/wellbore combinations for a slurry with the following characteristics: 14.8 [latex]\left( \tfrac{lb}{gal} \right )[/latex] Class C slurry with fluid loss and dispersant additives 30.5 cp plastic viscosity 0.1 [latex]\left( \tfrac{lb}{100} \right )[/latex] sq ft yield point Figure 1: ECD under various pump rates Figure 1 illustrates that increasing the pump rate from 2 [latex]\left( \tfrac{bbl}{min} \right )[/latex] to 5 [latex]\left( \tfrac{bbl}{min} \right )[/latex] in 1000 feet of annular space between a 5.5 inch OD casing string and a 7 inch wellbore will result in a 9% increase in ECD compared with the static column (16.1 [latex]\left( \tfrac{lb}{gal} \right )[/latex] versus 14.8 [latex]\left( \tfrac{lb}{gal} \right )[/latex]). The same increase in pump rate will result in a 70% pressure increase in a 4.0 inch casing in 4.89 inch open hole. Rates above 2 [latex]\left( \tfrac{bbl}{min} \right )[/latex] may more than double the annular pressures in smaller annular gaps. At the same time, for 5.5 inch OD casing run in a 9 inch hole, where the annular space is 1.75 inches, the added friction pressure is insignificant compared to the hydrostatic head. This information is important in choosing a pump rate that will be fast enough to scour the annulus but not so fast that it will increase the effective circulating density, and thus the bottomhole pressure at the casing shoe, to a level that will fracture the formation and lead to lost circulation. Mud Conditioning The first step in an optimal cement job is drilling a good hole: Smooth and straight (few doglegs or washouts)In gauge (consistent diameter)Stable (free of spalling or shale swelling and encroachment)Clean (free of drill cuttings and excessively thick mud cake) Before the casing string is lowered into the hole, the drillstring will likely have been out of the hole for 12 to 24 hours while logging and caliper surveys are run to determine the exact dimensions of the hole for cement volume calculations. So it is important to have the hole and mud in good condition before removing the drill string. The mud should be conditioned to reduce gel strength, yield point, and plastic viscosity to minimum values before the drillstring is removed, taking care not to impair the mud's ability to suspend weighting agents needed to maintain proper hydrostatic head. After the casing has been run, and before pumping the slurry, the mud should be circulated to clean the hole and remove remaining cuttings and mud cake. At the same time, the casing can be worked up and down or rotated during mud circulation. In areas where mudcake removal is expected to be difficult, scratchers or wipers should be placed on the casing to facilitate this action. Properly placed centralizers also facilitate the circulation of mud around the casing in areas where off-centered casing could be a problem. At least two annular volumes of mud should be circulated at the highest rate possible without creating friction pressures that will cause lost returns. Here is where turbulent flow is needed to help with the cuttings and mud cake removal process. Tracers such as dye, grain, and colored beads can be used to monitor the volume of mud being circulated and make certain that at least 85% of the borehole volume is circulated before the cement slurry is pumped. By timing the travel time of these markers from the mud pump back to the shale shaker, one can compare the volume of mud pumped during that time to the calculated borehole volume. Washes, Preflushes, and Spacers Intermediate fluids are sometimes pumped as a buffer ahead of the cement to prevent it from contacting and mixing with the mud in the annulus, which can cause inefficient displacement. Drilling mud contamination of cement can also reduce its compressive strength. These intermediate fluids can be: Chemical washes or preflushes that contain no solids and have a density and viscosity comparable to that of water (such as a 5% hexametaphosphate solution to help disintegrate the mud cake).Spacer fluids that contain solids and are mixed at various densities and viscosities. One variation on a preflush is a cement scavenger slurry - a dilute mix of cement and water pumped ahead of the cement slurry in turbulent flow to help remove gelled mud and filter cake. Common washes contain surfactants and dispersants, often similar to those used in cement slurries. Spacers typically contain viscosifiers like polyacrylamides, guar gum, cellulose derivatives, or biopolymers. The density and rheological properties of the spacer should lie between those of the drilling mud and the slurry. The volume of spacer pumped should be enough to extend across at least 500 feet of annular distance. Any expected reduction in hydrostatic head due to a spacer being pumped must be carefully considered. If the overbalance is calculated to be less than the minimum required overbalance, at any stage of the cement job, a weighted preflush should be used instead of a water spacer. Just as the mixing of mud and slurry in the casing is prevented by the use of wiper plugs, similar plugs can be used to prevent the mixing of the spacer with both the mud (ahead) and the slurry (behind) in the casing. Surface flowlines should also be flushed thoroughly to ensure that any cross-contamination of mud, spacers, and cement slurry is minimized. Assignment Cement job | Cement Slurry Flow Properties and Mud Displacement Cement Job Design: Laminar and Turbulent Flow You are evaluating an intermediate casing string for the Assignment well, and you want to determine the type of flow in the annulus. The string is 9-5/8" in a 12-1/4" Open Hole, and you are using Class G neat cement (density 15.8 ppg). In this size string, [latex]1\ BPM = 0.298\tfrac{ft}{sec}[/latex]. Determine whether the flow is laminar, transitional, or turbulent at 1, 2, 3, 4 and 5 BPM. Solution [latex]D_2\ - D_1 = 12.25\ - 9.625\ = 2.625\ in[/latex] [latex]r = 15.8\ ppg[/latex] [latex]n\ = 1,\ 2,\ 3,\ 4,\ and\ 5\ \tfrac{bbl}{min} \times 0.298 \tfrac{ft}{sec}[/latex] [latex]m\ =\[/latex] taken from the provided graph So the [latex]Re = 757 \times 2.625 \times 15.8 \times 0.298 \times \tfrac{Flow\ Rate\ (BPM)}{Viscosity\ (cP)}[/latex] [latex]= 9356 \times \tfrac{Flow\ Rate\ (BPM)}{Viscosity\ (cP)}[/latex] Flow Rate (BPM)Viscocity (cP)EquationReFlow Regime1250Re= 9356×1 / 25037Laminar2100Re= 9356×2 / 100187Laminar340Re= 9356×3 / 40701Laminar412Re= 9356×4 / 123119Transitional56Re= 9356×5 / 67797Turbulent Assessment "Cement job | Cement Slurry Flow Properties and Mud Displacement" 1. What is viscosity? A .The ratio of pressure to depthB .The ratio of mass to volumeC .The ratio of stress to strainD .The ratio of shear stress to shear rate Correct 2. What describes a dilute mix of cement and water pumped ahead of the cement slurry in turbulent flow to help remove gelled mud and filter cake? A .Cement scavenger slurry CorrectB .ViscosifiersC .Spacer fluid 3. Which of these hole conditions would be considered favorable for obtaining an optimal cement job? (Select all that apply.) A .Free of drill cuttings CorrectB .Increasing hole diameter with depthC .Thick, protective mud cakeD .Consistent hole diameter CorrectE .Free of swelling shales CorrectF .Few doglegs or washouts Correct 4. Identify a characteristic of turbulent flow within the annular space. A .Fluid moves uniformly as a homogeneous, relatively undisturbed, body.B .There is no fluid motion transverse to the bulk flow direction.C .Fluid particles swirl around in all directions. Correct 5. Knowing the ECD when pumping a cement slurry up a casing/wellbore annulus is primarily important to preventing ________. A .lost circulation due to high wellbore pressure at the casing shoe CorrectB .premature setting up of the cementC .excessive waiting on cement time 6. Rheological models are used to predict _________. A .mud displacement CorrectB .compressive strengthC .setting timeD .thickening time 7. Identify examples of non-Newtonian flow models. (Select all that apply.) A .Power Law CorrectB .Bingham Plastic CorrectC .Equivalent circulating densityD .Herschel-Bulkley CorrectE .Reynolds Law Recommended for You Introduction to Well Cementing Chemistry and Classification of Oil well Cements Cement Additives in Drilling Cement Testing Procedures