Annular Pressure Losses Research Articles

Evaluation and optimization of water-salt based drilling fluids for slim-hole wells in one of Iranian central oil fields

Abstract Slim-hole drilling in oil and gas industry has been developed extensively in recent years. Drilling fluid, in addition to providing appropriate rheological properties should produce a low annular pressure loss (APL) gradient while drilling. During slim-hole drilling, drilling mud hydraulics is an important concern, because due to reduced annular clearance, pressure loss can occur in drill pipe and annulus considerably. In this research, water based drilling muds were selected for the experimental work due to its low price, simple preparation and easy access to the required water. The research includes development and testing of water-salt based fluids. In this study, sixty-five samples of fluids were analyzed for their rheological parameters, the were considered using three various case study gas field wellbore configurations 6 1/8, 5 7/8 and 4 1/8 inch for their calculate APL gradients. Each composition was evaluated by the Power-Law and Bingham Plastic models and results of both models were compared. Choose of optimum fluids is based on suitable rheological properties, minimum annular pressure losses and maximum fluid transport ratio. Also, the effect of high temperature (180°F) on frictional pressure losses was studied. For an optimal formulation at high temperature from a well with distinct configuration 6 1/8 inch had an APL gradient of 0.149 psi/ft compared to 0.176 psi/ft at atmospheric condition. It was found that effect of high temperature on drilling fluids behavior is affirmative. On the other hand, the observed effect of high temperature on Power-Law fluids greater than Bingham Plastic fluids. Fluids with xanthan, in spite of high cutting carrying capacity and due to high viscosity have exhibited high frictional pressure loss. However, for some fluids, the annular pressure loss increased at high temperature due to green starch fermentation.

Numerical simulation of non-Newtonian fluid flow in partially blocked eccentric annuli

A cuttings bed formed on the lower side of the wellbore is the most common drilling problem confronted when horizontal or highly deviated wells are drilled. The phenomenon is undesirable and has a detrimental effect on wellbore hydraulics and drilling performance. To achieve an efficient drilling operation, hydraulics optimization should be implemented by precisely predicting annular pressure loss and other hydraulic parameters.The accumulation of cuttings at the lower side of horizontal or highly inclined wellbores, combined with wellbore eccentricity and fluid rheological properties, create a complex behavior of pressure loss. In this study, a numerical approach is used to investigate the influence of the aforementioned factors. A numerical solution is performed for laminar flow of non-Newtonian fluids. A non-orthogonal, boundary-fitted curvilinear coordinate is developed, which is based on elliptic partial differential equations (PDE) to accommodate annular flow geometry with arbitrary cross-sections. Finite difference numerical schemes adapted to the coordinates is then used to analyze the problem. The numerical model determines the effect of hydraulic parameters of fluid flow thorough an irregularly shaped annulus and considers the following factors: flow rate, fluid rheological parameters, annular geometry and bed appearance. Numerical predictions from the model are verified against existing models and experimental data in literature; and good agreements are obtained. The developed numerical model can be combined with a 1-D solids transport model for designing oil and gas well drilling operations.

Numerical Characterization of the Annular Flow Behavior and Pressure Loss in Deepwater Drilling Riser

In drilling a deepwater well, the mud density window is narrow, which needs a precise pressure control to drill the well to its designed depth. Therefore, an accurate characterization of annular flow between the drilling riser and drilling string is critical in well control and drilling safety. Many other factors influencing the change of drilling pressure that should be but have not been studied suffi- ciently. We used numerical method to simulate the process of drill string rotation and vibration in the riser to show that the rotation and transverse vibration of drill string can increase the axial velocity in the annulus, which results in the improvement of the flow field in the annulus, and the effect on pressure loss and its fluctuation amplitude. In addition, there are also multiple secondary flow vortices in the riser annulus under certain eccentricity conditions, which is different from the phenomenon in an ordinary wellbore. The findings of this research are critical in safely controlling well drilling operation in the deepwater environment.

Effects of drill string eccentricity on frictional pressure losses in annuli

It has been shown that drill pipe eccentricity can significantly reduce the annular pressure losses and must be taken in to account for accurate prediction of equivalent circulating density (ECD). This study investigates effects of axial flow of Yield-Power-Law (YPL) drilling fluid on frictional pressure losses in eccentric annuli. A numerical model is developed to simulate the laminar flow of yield-power-law fluids in eccentric annular geometries. The governing equations are solved numerically using an implicit finite difference method and employing a non-uniform grid distribution system. The numerical solution yields velocity profile in the annular space, which is integrated to obtain the volumetric flow rate. The frictional pressure losses are calculated using an iterative scheme that is based on convergence of the calculated volumetric flow rate to the given flow rate. Comparing the frictional pressure loss predictions of the model with three previous numerical studies indicates an excellent agreement for the case of non-Newtonian fluids. The numerical model is validated through extensive comparisons between the predicted frictional pressure losses and experimental data derived from the literature. Our analysis indicates excellent agreement between the model prediction and the experimental data. The numerical results conclude that the reduction in frictional pressure losses due to eccentricity is more prominent at high radii ratios. Results of extensive simulation scenarios for yield-power-law fluids are fitted using non-linear regression analysis, which provides a new correlation for prediction of frictional pressure losses of yield-power-law fluids in eccentric annuli. Comparing the predictions of the developed correlation with the published correlation in the literature reveals more accurate estimation of frictional pressure losses for the proposed correlation and hence safer controlled drilling operations.

Rheology and hydraulics of polymer-based foams at elevated temperatures

Foam has been successfully used for different operations like well stimulation, drilling, enhanced oil recovery (EOR), cleanout, and acidizing operations in the oil and gas industry. Additionally, the low liquid content of foam provides a distinct advantage in terms of lesser material requirements. However, accurate prediction of rheology is necessary for the success of field operations, which requires a rheological model that incorporates the effect of temperature on foam properties as it circulates downhole.In the present investigation, polyanionic cellulose (PAC) based foam was generated using nitrogen as the gas phase and its rheology was determined using a recirculating flow loop that has three pipe viscometers (3.05, 6.22, and 12.7 mm ID) and a fully-eccentric annular section (9.53 mm OD × 12.57 mm ID). Experiments were conducted within the temperature range of 24 to 149 °C and at various foam qualities (gas phase volume fraction). The foam was circulated at different flow rates and the differential pressure across each pipe section was recorded. All tests were conducted at 6.89 MPa.The foams displayed power-law fluid behavior in the shear rate range tested (100–5000 s−1), which is often experienced in the wellbore. Like its base liquid, polymer foam exhibited thermal thinning and a significant rheology change with temperature. Only high-quality foam (75%) at ambient temperature (24 °C) showed yielding behavior, which was measured in a pipe viscometer under static condition. The disappearance of yield stress at elevated temperature could be attributed to thermal thinning of the liquid phase that weakens the strength of bubble structure. Experimental data is used to develop new correlations to predict power-law fluid parameters as a function of temperature, base fluid properties, and foam quality. Moreover, the measurements are compared with the predictions of existing models, and discrepancies are observed, which could be attributed to the variation in foam generation technique, the nature and concentration of polymer, and the concentration of surfactant used in the experiments in which data was obtained to develop the models.Furthermore, annular pressure loss measurements obtained at low temperatures (24 and 79 °C) show predominantly good agreement with predictions of a hydraulic model that uses the new correlations. Discrepancies increased with temperature as the foam becomes unstable due to thermal thinning of the liquid film and subsequent weakening of bubble structure.

Numerical simulation of transport behaviour of small cuttings in extended reach wells

The purpose of this study is to examine the effect of drillpipe rotation and cuttings size on hole cleaning efficiency in a horizontal annulus flowing with a non-Newtonian fluid. It adopts computational fluid dynamics (CFD) simulation to predict annular pressure losses, cuttings concentration profiles, and cuttings velocity profiles. A combination of Wen-Yu drag force model and k-epsilon turbulence model closely predicted experimental pressure loss data from literature. The results indicated that drillpipe rotation improves the transport efficiency of small cuttings of size 0.5 mm better than larger cuttings size of 5 mm. Annular pressure losses increased with increasing drillpipe rotation but decreased with increasing cuttings size. This study is very unique as it reveals various flow patterns that may occur during drilling operation with different cuttings size and drill pipe rotation speeds. It also serves as a guide to optimize drilling hydraulics for efficient hole cleaning in extended reach wells. [Received: October 10, 2016; Accepted: May 9, 2017]

Annular Pressure Losses Prediction Using Power-Law Model with Polymer Water-Base Mud

Bottomhole Pressure (BHP) determination is quite crucial in the success of the drilling operations, especially on tight-drilling window environment. The calculation becomes much more critical when drilling with near-balance bottomhole pressure on the unconventional drilling techniques like Managed Pressure Drilling (MPD). Although Pressure-While-Drilling (PWD) technology can provide measured values of Bottomhole Pressure, it is so expensive and needs circulation to convey readings. Also, in case of tool failure, it consumes a lot of time to trip out of hole in order to change the tool, especially on deep-well drilling. On this paper, Bingham, Power-Law, Herschel-Bulkley and API RP13D rheological models have been utilized to calculate the Annular Pressure Loss (APL) on a deep well with polymer Water-Base Mud (WBM). A comparative study between model-calculated and PWD-measured values of APL for 15 points along the wellbore was conducted on the calculation of Annular Pressure Loss. Power-Law model was found the most optimum rheological model for polymer Water-Base Mud, due to exhibiting no Yield Stress. Power-law model gave 35 psi average error which can be tolerated most of the time during drilling operations.

An experimental investigation on flow pattern map and drift-flux model for co-current upward liquid-gas two-phase flow in narrow annuli

This study explored flow patterns and void fraction prediction models for co-current upward liquid-gas two-phase flow in narrow annular spaces. Experiments on two-phase flow in both narrow and wide annular conduits were performed in a 4-inch (0.102 m) outside-diameter test section, with 3.5-inch (0.089 m) and 2-inch (0.051 m) inner pipes as representations of different annular geometries. Flow patterns and liquid holdup were recorded while flowing air and water upward through the test sections. The experimental results showed that the two-phase flow pattern maps in narrow annulus and wide annulus are with different major regime transition lines. A new drift-flux model for water-air two-phase flow in narrow annuli was developed using experimental data collected in this study. The new model was compared against previous models that were developed for wide annuli, and the results showed a better match in void fraction predictions for two-phase flow in narrow annuli with experimental data. The understanding of two-phase flow in narrow annuli is vital for real-time prediction of void fraction and annular pressure loss in wells with gas influx during casing/liner/slimhole drilling operations. An enhanced prediction can ensure delivery of safer wells with lower non-productive time and associated trouble encountered during well control operations.

Research on Annular Frictional Pressure Loss of Hydraulic-Fracturing in Buckling Coiled Tubing

Compared with conventional hydraulic fracturing, coiled tubing (CT) annular delivery sand fracturing technology is a new method to enhance the recovery ratio of low permeability reservoir. Friction pressure loss through CT has been a concern in fracturing. The small diameter of CT limits the cross-sectional area open to flow, therefore, to meet large discharge capacity, annular delivery sand technology has been gradually developed in oilfield. Friction pressure is useful for determining the required pump horsepower and fracturing construction design programs. Coiled tubing can buckle when the axial compressive load acting on the tubing is greater than critical buckling load, then the geometry shape of annular will change. Annular friction pressure loss elevates dramatically with increasing of discharge capacity, especially eccentricity and CT buckling. Despite the frequency occurrence of CT buckling in oilfield operations, traditionally annular flow frictional pressure loss considered concentric and eccentric annuli, not discussing the effects of for discharge capacity and sand ratio varying degree of CT buckling. The measured data shows that the factors mentioned above cannot be ignored in the prediction of annular pressure loss. It is necessary to carry out analysis of annulus flow pressure drop loss in coiled tubing annular with the methods of theoretical analysis and numerical simulation. Coiled tubing buckling has great influence on pressure loss of fracturing fluid. Therefore, the correlations have been developed for turbulent flow of Newtonian fluids and Two-phase flow (sand-liquid), and that improve the friction pressure loss estimation in coiled tubing operations involving a considerable level of buckling. Quartz sand evidently increases pressure loss in buckling annular, rising as high as 40%-60% more than fresh water. Meanwhile, annulus flow wetted perimeter increases with decreasing helical buckling pitch of coiled tubing, therefore, the annulus flow frictional pressure loss rapidly increases with decreasing helical buckling pitch. The research achievement provides theoretical guidance for coiled tubing annular delivery sand fracturing operation and design.

Numerical Simulation of Pressure Loss in Largesize Annulus of Deepwater Drilling Riser

Large difference exists between largesize annular hydraulics and conventional well's. Using numerical simulation, this article analyze annular pressure loss of deepwater drilling riser with large size, describing how various parameters such as annular mud velocity, drill pipe rotation speed, mud property and annular clearance affect the pressure losses in a well, compared with that of a conventional wellbore as well. Theoretical calculations have shown: As for power law fluid in the largesize annular space, when it comes to laminar flow, an increase in drill pipe rotation speed can result in a decrease of annular pressure losses; In the contrary,under turbulent conditions, this may lead to a rise in annular pressure losses. The pressure drop dramatically along with an increasing in the annular space, in other words, the size of annular clearance is a major factor that affect the pressure losses in the annular space.

Calculation of the Open-Hole Extended-Reach Limit for an Extended-Reach Well

In this paper, we present a model for calculating the open-hole extended-reach limit for an extendedreach well which takes into account such factors as annular pressure loss, equivalent density of formation fracture pressure, etc. Studies have shown that the proposed model is the most accurate model available. We also present results of an analysis of the effect of different drilling parameters on the extended-reach limit of the well. Calculations have shown that the extended-reach limit is proportional to the equivalent density of the formation fracture pressure and inversely proportional to the pump rate, the drilling fluid density, and the mechanical rate of penetration (ROP).

Consideration of Tool-Joint Effects for Annular Pressure Loss Calculation

Consideration of Tool-Joint Effects for Annular Pressure Loss Calculation

Frictional pressure loss of drilling fluids in a fully eccentric annulus

It is common practice when drilling oil and gas wells to assume that the drillstring is placed concentrically in the annular space with either the open hole or previous casing strings in order to predict annular frictional pressure losses. The assumption of such a concentric annulus is, however, a considerable simplification that may not properly reflect the majority of drilling applications in the field. In fact, with an increasing number of deviated/horizontal and extended reach wells being drilled, a fully eccentric annulus is actually present in a large section of the wellbore.In this study, we apply experimental, analytical, and numerical approaches to investigate the impact of drillpipe eccentricity on the annular pressure loss while circulating non-Newtonian drilling fluids. The length of the experimental section of a flow loop was 27.74 m (91′) and it consisted of 0.0245 m (1″) steel drillpipe and 0.0571 m (2.25″) acrylic casing with the inner diameter of 0.0508 m (2″). Drillpipe was placed at the bottom of the casing, thereby simulating a fully eccentric annulus. Four Yield Power Law (YPL) drilling fluids were tested in this flow loop. Annular pressure loss for a wide range of laminar flow rates was recorded for each fluid. A numerical model based on a finite difference approach was developed to estimate the annular pressure loss. Subsequently, the experimental data was compared with the proposed model and also with several other widely used analytical and numerical approaches previously reported in the literature. The obtained results show that in the laminar flow regime, the annular frictional pressure loss in a fully eccentric annulus is considerably less than a concentric annulus, on occasions by less than 50%. In general, all the applied models under-estimated the effect of eccentricity on pressure loss. However, the novel proposed model showed the least discrepancy with the experimental data. Furthermore, it was found that the difference between the estimated and experimental results increases with increasing fluid yield stress. This suggests that models and/or correlations that are developed to correct for the eccentricity effect for fluids with negligible yield stress (for instance Power Law fluids) are not suitable to estimate the pressure loss for YPL fluids with elevated yield stress.

Predicting pressure behavior during dynamic kill drilling with a two-phase flow

Dynamic kill drilling technology can effectively handle well kick, loss of circulation and wellbore collapse during surface drilling in deepwater. To ensure a safe and successful dynamic kill drilling operation, accurately predicting wellbore pressure is highly important. Based on dynamic kill drilling characteristics in deepwater, this paper develops a wellbore pressure prediction model with variable flow rate proportions. The complexity of the gas–liquid two-phase flow inside the annulus is included in the model. An analysis of the liquid holdup, annular density, annular pressure, annular pressure loss and bottom hole pressure during dynamic kill drilling is performed for a deepwater well using the wellbore pressure prediction model described in this paper. The results suggest that the key issue for dynamic kill drilling technology is controlling the mixture density by regulating the flow rates of the seawater and weighted drilling fluid. To demonstrate the model's validity, it was used to predict the wellbore pressure of two deepwater wells that had been constructed using dynamic kill drilling in the Gulf of Mexico (GOM). The wellbore pressure prediction model presented in this paper is valuable for guiding field operations involving dynamic kill drilling.

Modified Yield Power‐Law fluid flow in narrow annuli with inner rotating pipe

AbstractA proposed viscosity model for Yield Power‐Law fluid by Souza Mendes and Dutra (SMD), devoid of discontinuities at vanishing shear rate, is adopted in a 3D CFD simulation study. Radial distributions of axial, tangential, and coupled components of velocities are measured along concentric and eccentric narrow annuli. The flow is assumed to be fully‐developed, laminar, and steady state. Several parameters such as diameter ratios, eccentricities, and inner pipe angular speeds were varied to analyze their influence on the flow distributions and annular pressure losses. The simulated results of radial distributions of axial and tangential velocities show good agreement with experimental data and analytic solutions from previous studies. Annular flow instabilities in the form of Taylor vortices in the flow fields are briefly discussed. This study shows how reliable CFD simulations can replicate the actual, yet complex, oil and gas drilling operations.

CFD Method for Predicting Annular Pressure Losses and Cuttings Concentration in Eccentric Horizontal Wells

In oil and gas drilling operations, predictions of pressure losses and cuttings concentration in the annulus are very complex due to the combination of interacting drilling parameters. Past studies have proposed many empirical correlations to estimate pressure losses and cuttings concentration. However, these developed correlations are limited to their experimental data range and setup, and hence, they cannot be applicable to all cases. CFD methods have the advantages of handling complex multiphase flow problems, as well as, an unlimited number of physical and operational conditions. The present study employs the inhomogeneous (Eulerian-Eulerian) model to simulate a two-phase solid-fluid flow and predict pressure losses and cuttings concentration in eccentric horizontal annuli as a function of varying drilling parameters: fluid velocity, diameter ratio (ratio of inner pipe diameter to outer pipe diameter), inner pipe rotation speed, and fluid type. Experimental data for pressure losses and cuttings concentration from previous literature compared very well with simulation data, confirming the validity of the current model. The study shows how reliable CFD methods can replicate the actual, yet complex oil and gas drilling operations.

Annular Frictional Pressure Losses During Drilling—Predicting the Effect of Drillstring Rotation

Controlling the annular frictional pressure losses is important in order to drill safely with overpressure without fracturing the formation. To predict these pressure losses, however, is not straightforward. First of all, the pressure losses depend on the annulus eccentricity. Moving the drillstring to the wall generates a wider flow channel in part of the annulus which reduces the frictional pressure losses significantly. The drillstring motion itself also affects the pressure loss significantly. The drillstring rotation, even for fairly small rotation rates, creates unstable flow and sometimes turbulence in the annulus even without axial flow. Transversal motion of the drillstring creates vortices that destabilize the flow. Consequently, the annular frictional pressure loss is increased even though the drilling fluid becomes thinner because of added shear rate. Naturally, the rheological properties of the drilling fluid play an important role. These rheological properties include more properties than the viscosity as measured by API procedures. It is impossible to use the same frictional pressure loss model for water based and oil based drilling fluids even if their viscosity profile is equal because of the different ways these fluids build viscosity. Water based drilling fluids are normally constructed as a polymer solution while the oil based are combinations of emulsions and dispersions. Furthermore, within both water based and oil based drilling fluids there are functional differences. These differences may be sufficiently large to require different models for two water based drilling fluids built with different types of polymers. In addition to these phenomena washouts and tool joints will create localised pressure losses. These localised pressure losses will again be coupled with the rheological properties of the drilling fluids. In this paper, all the above mentioned phenomena and their consequences for annular pressure losses will be discussed in detail. North Sea field data is used as an example. It is not straightforward to build general annular pressure loss models. This argument is based on flow stability analysis and the consequences of using drilling fluids with different rheological properties. These different rheological properties include shear dependent viscosity, elongational viscosity and other viscoelastic properties.

Annular-Pressure Losses While Casing Drilling

This article, written by Dennis Denney, contains highlights of paper SPE 166103, ’Evaluation of Annular-Pressure Losses While Casing Drilling,’ by Vahid Dokhani and Mojtaba P. Shahri, SPE, University of Tulsa; Moji Karimi, SPE, Weatherford; and Saeed Salehi, SPE, University of Louisiana at Lafayette, prepared for the 2013 SPE Annual Technical Conference and Exhibition, New Orleans, 30 September-2 October. The paper has not been peer reviewed. Casing drilling is a method by which the well is drilled and cased simultaneously. The small annulus from casing drilling can create a controllable dynamic equivalent circulating density (ECD). Casing-drilling technology enables obtaining the same ECD as with conventional drilling but with a lower (optimized) flow rate and lower rheological properties and mud weight. Frictional pressure loss during casing drilling was evaluated with computational fluid dynamics (CFD). Having accurate models for ECD, including the effects of pipe rotation and eccentricity in the annulus, is essential for success in these challenging jobs. Introduction Casing drilling builds on experience gained from drilling liners to bottom in troublesome holes. The technique was implemented for drilling a formation sequence of highly pressured shale followed by a depleted reservoir. The major problem when drilling depleted reservoirs is the narrow operational mud-weight window. With advances in top-drive systems, retrievable bottomhole assemblies, and polycrystalline-diamond-compact bits, the technology enables completing a well by use of casing as the drillstring. An often-reported benefit of casing drilling is significantly fewer lost-circulation problems. The wellbore-plastering effect that casing drilling offers can enable drilling depleted zones while causing less formation damage. Plastering also enhances pressure containment by smearing the smaller drill cuttings into the pore spaces. The aim of this study was to simulate the casing-drilling operation through CFD modeling to evaluate the combined effect of eccentricity and pipe rotation on the velocity profile of a non-Newtonian fluid. Approach Initially, the geometry of casing drilling was constructed for a given wellbore condition. Then, the domain was discretized such that the result would not be grid dependent. A series of cases was designed to compare the CFD model with the analytical solution and validate the discretization scheme. Then, the non-Newtonian-fluid (yield-power-law model) simulation was run. Thereafter, an effort was made to analyze the effect of eccentricity and pipe rotation on the yield-power-law fluid. Assumptions In drilling operations, continuous fluid circulation through the annulus results in steady-state flow. In the shallow top-hole section, the fluid can be treated as an incompressible fluid. The simulated laminar-flow regime verified the CFD results with the analytical solution. It was assumed that a single-phase fluid flows through the annulus and that the pipe geometry provides a uniform concentric annulus along the test section. For simplicity, the effects of drill cuttings were neglected in the simulation to be able to validate the CFD results with the analytical solution. Initially, the casing was treated as stationary (no pipe rotation) with a no-slip condition at the walls (both inner-pipe and wellbore). The pipe and the wellbore were assumed to be smooth. Also, the geometry was held uniform along the pipe (i.e., the effect of tool joints on pressure loss was neglected). The pipe section was considered to be 5 or 10 m long.

Numerical Simulation of Large-Diameter Annular Pressure Loss in Riser Segment of Deep-Water Drilling

Large-diameter annulus and conventional annulus are very different in hydraulic property. Based on numerical simulation, a study on pressure loss of large-diameter annulus was carried out. The influence of annular velocity, pipes rotation speed, mud property and annular diameter of marine riser segment on pressure loss was also analyzed and compared with that of conventional segment. The results show that under the condition of large-diameter annulus, the pressure loss of power law fluid laminar flow decreases with the increase of the rotational speed of drill pipe and the contrary tendency occurred when it is turbulent flow. With the increase of the flow behavior index and consistency coefficient, annular pressure loss increases exponentially and linearly respectively. From an overall perspective, with the increase of annular size, the annular pressure loss sharply reduced, and it has great effect on pressure loss.

Hydraulic Effect of Tool Joints on Annular-Pressure Loss

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper SPE 142282, ’The Hydraulic Effect of Tool Joints on Annular-Pressure Loss,’ by Majed Enfis, SPE, and Ramadan Ahmed, SPE, University of Oklahoma, and Arild Saasen, SPE, Det Norske and the University of Stavanger, prepared for the 2011 SPE Production and Operations Symposium, Oklahoma City, Oklahoma, 27-29 March. The paper has not been peer reviewed. For a successful drilling operation, downhole pressure, or equivalent circulating-density (ECD), control is critical. Conventional drilling techniques require maintaining the bottomhole pressure (BHP) at a level between the pore and fracture pressures. In deepwater wells, the margin between these pressures is very narrow; therefore, the BHP and ECD must be predicted accurately and be maintained within that narrow margin to avoid kicks and circulation losses. The presence of a tool joint changes the annulus geometry between the drillpipe and the casing or hole, resulting in strong turbulence and fluid acceleration that generate additional viscous dissipation and pressure losses. Theoretical and experimental studies examined the hydraulic effects of rotating and nonrotating tool joints. Introduction While drilling oil and gas wells, drilling mud is circulated in the wellbore to transport cuttings to the surface and control the BHP. In conventional drilling operations, the mud is injected into the well through the drillstring (drillpipe) with a high-pressure pump. The mud flows through the drillpipe and exits the drill bit where it enters the annulus and flows up the annular space to the surface carrying rock cuttings. Significant pressure losses occur as the mud flows through the drillpipe and the annulus. The BHP, which is a critical parameter in any drilling operation, is a function of the hydrostatic head of the mud and annular-pressure loss. Hence, accurate prediction of the annular-pressure loss leads to a good estimation of the BHP and the ECD to prevent kicks and circulation losses. The ECD is a commonly used drilling term that represents the BHP in terms of equivalent fluid density. This means that it combines the density of the fluid and annular-pressure loss. Previous studies focused on the hydraulic effects of a nonrotating tool joint. The purpose of this study was to examine hydraulic effects of tool joints under both static and dynamic conditions, and to develop models for predicting the change in annular-pressure loss caused by the tool joints.