Summary The occurrence of barite sag has been a well recognized but poorly understood phenomenon in the drilling industry resulting in problems such as lost circulation, well control and stuck pipe. The financial impact on drilling costs, usually resulting from rig-time lost while circulating and conditioning the drilling fluid system, is not trivial. Recurring barite sag problems reportedly have resulted in the loss of drilling projects. Originally thought to occur under static conditions, barite sag is recognized now to occur more readily under dynamic, low-shear-rate conditions. Industry experts have offered a variety of measuring parameters, based upon empirical data, that only partially correlate with the occurrence of barite sag. Prediction of barite sag in dynamic flow has created an engineering challenge. The effect of shear rate on dynamic barite sag, for invert-emulsion drilling fluids, has been studied and quantified using new and advanced technology. A new field viscometer capable of measuring viscosity at shear rates of 0.0017 sec–1 and an eccentric wellbore-hydraulics model were used to develop and understand this relationship. Changes in mud weight as a function of shear rate, hole angle, annular velocity (AV), and eccentricity correlate with ultralow-shear-rate viscosity. Based upon experimental results, field technology has been developed to predict the potential for barite sag of invert-emulsion drilling fluids and to provide remedial measures through ultralow-shear-rate-viscosity modification. The efficacy of using traditional rheological measurements as indicators of barite sag potential is addressed. Introduction Recent advances in drilling technology have resulted in greater numbers of directional wells being drilled as operators strive to offset ever-increasing operating costs. Deviated drilling allows operators to exploit reservoir potential by drilling multiple wells from a single site, and to increase production by penetrating the pay zone in a horizontal, rather than a vertical plane. With consideration to eliminating drilling problems such as torque and drag, stuck pipe, low rates-of-penetration and wellbore stability, these wells are being drilled increasingly with invert-emulsion drilling fluids. Despite their considerable technical merits and advantages, invert-emulsion drilling fluids are not always trouble-free. First, these fluids are generally more viscous at surface conditions than water-based drilling fluids, and efforts are made to reduce viscosity by minimizing additives used for suspending barite. Second, fluid flow in a deviated wellbore is skewed by the effects of drillpipe eccentricity, typically resulting in low shear rates under the eccentric pipe, creating conditions conducive to barite sag. As a result, the frequency of problems associated with barite sag when drilling highly deviated wells is higher with invert emulsions, compared with water-based systems. Prior Laboratory Studies In the field, barite sag is defined roughly as the change in mud weight observed when circulating bottoms-up. Several laboratory investigations of barite-sag mechanisms and potential have been undertaken over the past decade. Results from a laboratory study presented by Hanson et al.1 found that barite sag is most problematic under dynamic, not static, conditions. Results indicate that barite sedimentation and bed formation occur while drilling fluid is being circulated and that fluid-like beds can "slump" downward when circulation is stopped. An important conclusion from this work was that barite sag generally observed in the field is due primarily to barite deposition occurring under dynamic conditions. Bern et al.2 induced barite sag by circulating at low flow rates with an eccentric drillpipe. Drillpipe rotation tended to prevent bed formation and served to aid in removing beds formed on the lower side of the test section. The barite sag tendency of some fluids tested at low flow rates was so great that beds were observed "avalanching," slumping down the test section and being incorporated back within the system. The authors concluded that the combined effects of hole angle, low AV, and a stationary, eccentric drillpipe were conducive to inducing dynamic barite sag. There appear to be several "schools-of-thought" on the relationship between rheological properties and barite sag. Using laboratory devices to measure static barite sag, several researchers concluded that the API gel strength measurement is an unreliable indicator of static barite sag potential.3,4 Dynamic oscillatory techniques were used by Saasen et al.4 to measure the linear viscoelastic properties of near-static gel networks, and found to be reasonable predictors of static barite sag potential. Kenny and Hemphill5 showed that the Herschel-Bulkley yield-stress coefficient, t0, correlates with static barite-sag potential; however, they cautioned that t0 should not be the only parameter used for dynamic barite-sag predictions. The low-shear-rate-yield point (LSRYP), an extrapolated yield stress calculated from 6 and 3 rev/min readings, was deemed by Bern et al.6 to be a reasonable approximation of the true yield stress of a drilling fluid. They suggest that while the expertise exists to control static barite sag, the influence of rheological properties on dynamic barite sag is not well understood. A common theme in the published literature is that low-shear-rate viscosity is a rheological parameter of importance in determining the capacity of a drilling fluid to minimize or prevent the occurrence of barite sag, particularly dynamic barite sag.1–8 Most authors refer to "low-shear rate" as that corresponding to the 3 rev/min dial reading (~5.1 sec–1), the lowest operating speed of the 6-speed viscometer. Dye et al.9 recently concluded that the magnitude of dynamic barite sag in an eccentric annulus, using invert-emulsion drilling fluid, is highest at annular shear rates below 3 to 5 sec–1. This study demonstrated that viscosity measurements taken at ultralow shear rates (<2 sec–1) correlate with the management of dynamic barite sag. Theoretical Foundation of Study Drawing on the work of previous researchers, we postulated that dynamic barite sag can occur when:the drillpipe (or inner cylinder) is in a fixed, eccentric position, thereby ensuring a wide distribution of point velocities in an eccentric annulus;drilling fluids are circulated at constant shear-rate over an extended period of time; andviscosity levels at these shear rates are insufficient to retard barite sedimentation.
It is important to predict the Equivalent Circulating Density (ECD) of fluids as closely as possible for safe drilling and completion of wells. This is especially true for those challenging wells where the margin between hole collapse and formation fracturing is very narrow. These characteristics of narrow safe drilling windows are commonly seen in extended-reach drilling (ERD) and deepwater wells, and pose significant drilling and cementing challenges. Over recent years, the changes in ECD caused by fluid compressibility, downhole fluid rheology, drillpipe eccentricity, and rate of penetration have been widely studied. In contrast, work on the modeling of drillpipe rotation effects on pressure drop and ECD have received relatively little attention. Early studies have looked at the relationship between circulating pressure drop, especially in slimhole drilling configurations, and at very high drillpipe rotational speeds. More recently the predicted effects of rotation on ECD in concentric and eccentric wellbore geometries have been studied and model improvements have been made. These technical approaches are summarized in this paper. The effect of drillpipe rotational speed on the predicted ECD for different drillstring combinations are presented and discussed. The calculated effects of drillpipe rotation on pressure drop and ECD are compared to actual field measurements using downhole annular pressure gauges. From the results discussed in this paper, the reader can gauge the accuracy of the current calculation methods. The changes in ECD with drillpipe rotation can now be better predicted, something especially important for those wells whose safe drilling window is narrow. Ultimately this reduces drilling and completions risk and helps assure safe and efficient well construction. Introduction Many studies on the effects of drillpipe rotation on annular drilling hydraulics have been performed over the years1,2,3. In these studies, the authors have looked at hydraulic effects in terms of changes in annular pressure drop (?P). They generally concluded that annular pressure initially drops with increasing drillpipe rotation speed, and later increases with increasing drillpipe rotation speed. In slim hole drilling studies4,5,6, it was found that a rotating drillpipe can produce large increases in annular pressure drop when annular clearances are small. Later, with the advent of downhole pressure drilling tools, researchers7,8,9 found that there was indeed a positive link between drillpipe rotation speed and Equivalent Circulating Density (ECD), a term that mathematically combines Equivalent Static Density (ESD) and annular pressure drop converted to density by the following equation: (1) ECD* = *ESD* + *?P/0 .052/TVD Figure 1 and Figure 2 show results from published work8,9 that clearly demonstrate a relationship between drillpipe rotation speed and increasing ECD. Figure 1 shows data for a 12.25-in section, while Figure 2 contains data for 8.75-in and 8.5-in sections. In presenting the data, the authors did not publish a model to account for the measured changes in ECD. Today, researchers now generally agree that there is a positive relationship between drillpipe rotation speed and annular pressure drop / ECD. Until recently there has not been published a comprehensive mathematical model that accurately predicts the effects of drillpipe rotation on annular pressure drop / ECD. Results using a preliminary model10 were published for concentric wellbores, and subsequently results for a more comprehensive model11 were published for eccentric wellbores. Studies of drillpipe rotation effects in circulating invert emulsion drilling fluids were made, and the combined effects on ECD were measured. In these "fingerprinting" exercises performed on a North Sea deviated well, key operational and drilling fluid parameters were recorded and the subsequent ECD measurements were measured and recorded. This body of work represents the most comprehensive set of data from which to study the effects of drillpipe rotation on annular pressure changes. In this paper, the data set from the North Sea experiments is used to validate the mathematical model constructed for drillpipe rotational effects in concentric and eccentric wellbores.
The drilling community generally believes that pipe rotation helps hole cleaning during both drilling and cementing. However, there has been a lack of reliable calculations to describe the effect and application in engineering and design and deployment. This paper discusses the effect of pipe rotation on velocity profile and pressure drop in concentric and eccentric annulus during the axial flow of non-Newtonian fluid. The calculations are applied to the design of hole cleaning during drilling and cementing. In the absence of reliable calculations one can only guess the impact of pipe rotation on hole cleaning. This work gives an engineer the tool to design for improved hole cleaning and hence save time and money during well construction and production. Pipe rotation is all the more important in an eccentric annulus to remove the drill cuttings and the gelled drilling fluid from the narrow annulus. This is because in an eccentric annulus, in the absence of pipe rotation, the fluid flows preferentially through the wider annulus. The pipe rotation speed (i.e., the RPM) required to move the gelled drilling fluid and the drill cuttings in various annulus configurations and eccentricities are presented. The results discussed in this paper can be applied for effective hole cleaning during drilling and cementing, which should save non- productive time (NPT) during drilling and cementing and also reduce remedial and intervention costs during well production. Introduction A number of studies on the hydraulic effects of drill pipe rotation have been performed over the years1,2,3. In these studies, the authors observed that drillstring rotation served to lower pressure drop and Equivalent Circulating Density (ECD). Others studied the effects of drillstring rotation in Newtonian fluids from a theoretical perspective and concluded rotation had a significant effect on pressure drop, especially in smaller diameter gaps through which fluid was moving in laminar flow4. Several studies were later conducted in support of slim hole drilling efforts. In these studies, it was found that the rotating drillstring can produce large increases in pressure drop and ECD5,6,7. Finally with the use of pressure-while-drilling (PWD) tools in hydraulic studies8,9,10, a positive link between pipe rotation speed and increasing annular pressure drop was clearly seen. To summarize, industry researchers now generally agree on the positive relationship between drillstring rotation speed and annular pressure drop. Given the link between annular pressure drop and velocity, the relationship between annular point velocity and drillstring rotation speed exists. In an earlier paper11a rigorous engineering method was discussed to determine pipe rotation effects on axial flow for concentric pipe geometry. The effects of pipe rotation on pressure drop and local velocities were presented. The work discussed in this paper expands the previous work to include the effect of drillstring eccentricity. The effects of pipe rotation on pressure drop at different eccentricities are discussed. The effects of rotation on local velocity profiles across the annular gap are shown using the maximum level of drillstring eccentricity used in this paper. The combined effects of rotation and eccentricity on hole cleaning also are discussed.
Summary Weighting-material sag is a reoccurring problem with many oil-based drilling fluids. Attempts to correlate sag tendencies to various rheological properties commonly used to benchmark drilling fluids have had limited success in prevention and anticipation of sag problems in the field. This paper presents a new testing apparatus for dynamic and static settling-rate (sag) measurements, which has proved to provide a better understanding of the sag phenomena and a better means to characterize fluid performance. This apparatus greatly expands the precision of sag measurements over previous techniques and allows testing conditions similar to those experienced downhole. Good correlation has been found between settling-rate measurements and performance of drilling fluids in the field. Introduction Sag is a variation in density of a drilling fluid caused by settling of suspended particles or weighting material in a wellbore. Laboratory and field experience suggests that sag is often worse in dynamic situations caused by pumping, pipe rotation, and tripping. However, sag can occur in either static or dynamic conditions. In the presented apparatus, measurements are performed at prescribed shear rates, elevated temperatures to 177°C (350°F), and pressures to 690 bar (10,000 psi). Additionally, the apparatus requires only a 50-cm3 sample for complete analysis. The settling-rate measurements obtained are useful in planning and as a diagnostic tool for sag performance in active drilling-fluid systems. Preliminary Laboratory Studies A typical way to control the shear of a non-Newtonian drilling fluid is to use a concentric-cylinder configuration with the sample fluid occupying the annulus. If either the outer or inner cylinder is rotated relative to the other, the annular fluid is subjected to an approximately uniform shear field that can be modeled easily. The configuration is comparable to the common oilfield viscometer and is commonly referred to as "Searle geometry" if the inner cylinder rotates relative to a stationary outer cylinder or as "Couette geometry" if the outer cylinder rotates relative to a stationary inner cylinder. Cylinder rotation combined with axial flow of the annular fluid would more closely resemble the borehole configuration, but would greatly complicate the computational modeling and control. Flow loops usually expose the sample to a range of shear rates in contrast to the constant shear rates possible in the simpler system. A flow loop also would require a high-pressure pumping system, as well as added unnecessary bulk, sample volume, and system complexity. A preliminary study apparatus was assembled (Fig. 1), which consisted of a clear-plastic outer cylinder approximately 2 m (6 ft) long and 7.62 cm (3 in.) internal diameter (ID), with sealing caps closing the ends. Bushings in the caps supported a rotatable concentric inner stainless-steel tube of 3.81-cm (1.5-in.) outside diameter. This gave a diameter ratio of 0.50. In later studies, another clear tube was centered in the original outer tube with an internal diameter of 5.08 cm (2 in.), giving a diameter ratio of 0.75. The narrower annular gap more closely approximates ideal Searle flow. The entire apparatus was pivoted on a bench-mounted knife edge, near the center, and tilted at 45° from vertical. A pivoted strut from the top end of the tube rested on a electronic laboratory digital scale, setting the angle of tilt and allowing the measurement of the imbalance force. A gear motor mounted on the upper end of the outer tube was arranged to belt drive the inner cylinder. The motor speed was adjustable by an electronic drive. A temperature-controlled bath was connected to the inner rotating tube in a way that allowed the tube to rotate while fluid from the temperature controlled bath circulated through it. When the annulus of the tubes was filled with a sample of drilling fluid, changes in the center of gravity could be tracked by monitoring the scale readings. Sample taps at intervals along the bottom side of the sloped outer tube allowed measurements of the density of the fluid at that those points.
Summary Many laboratory studies evaluating the cuttings transport capabilities of water-based and oil-based drilling fluids have been published. Few attempts have been made to investigate both fluid types under identical, controlled conditions. Those that considered both fluid types measured cuttings accumulation in the annulus and not fluid velocity. In this comparative study, the efficiency of water-based and oil-based muds in cleaning the inclined annulus at varying fluid velocities was investigated. Introduction A major problem for drilling operations on high angle/horizontal wells around the world is inadequate cuttings transport. Increasing environmental concerns and economic restraints are forcing operators to consider application of extended-reach drilling techniques to even greater depths and lateral displacements, thereby increasing the magnitude and severity of the cuttings transport problem. Many researchers have identified fluid velocity as the key parameter affecting hole cleaning in high angle situations. Previously published studies that have considered water-based muds (WBM) and oil-based muds (OBM) together were principally concerned with cuttings accumulation in the annulus and not with fluid velocities. In this paper, cuttings transport capabilities of WBM and OBM are evaluated under conditions of critical and subcritical flow. The effect of fluid rheological properties on cuttings transport is also addressed. In addition to the Bingham Plastic and Power Law rheological models, this paper applies the more accurate Yield-Power Law [Herschel-Bulkley] model to better understand the problem of cuttings transport from a rigorous fluid flow perspective. Recent advances in the modeling of fluid flow in eccentric annuli are employed to document the effect of fluid rheological properties on flow velocities.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.