drilling fluid sweeps

Copyright 2002, Society of Petroleum Engineers Inc. This paper was prepared for presentation at the SPE Annual Technical Conference and Exhibition held in San Antonio, Texas, 29 September2 October 2002. This paper was selected for presentation by an SPE Program Committee following review of information contained in an abstract submitted by the author(s). Contents of the paper, as presented, have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material, as presented, does not necessarily reflect any position of the Society of Petroleum Engineers, its officers, or members. Papers presented at SPE meetings are subject to publication review by Editorial Committees of the Society of Petroleum Engineers. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes without the written consent of the Society of Petroleum Engineers is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of where and by whom the paper was presented. Write Librarian, SPE, P.O. Box 833836, Richardson, TX 75083-3836, U.S.A., fax 01-972-952-9435.

Abstract Drilling fluid sweeps are commonly run in the field to help clean the borehole of cuttings that have not been removed with normal fluid circulation. Typically, these sweeps are used in vertical and deviated drilling operations. In high angle or extended reach wells, their use can be helpful in removing cuttings that have accumulated in sections where hole cleaning is not optimized. Sweep types usually fall into the following categories:

High-viscosity High-density Low-viscosity Combinations of any 2 of the above Tandem (one type followed by another)

Their effectiveness in the field is often quite spotty, often due to the application of a certain sweep type in the wrong drilling situation. Heretofore, there has been lacking a rigorous method of evaluating the effectiveness of sweeps at the rig site. Usually sweeps have been evaluated in the field based on observed or perceived quantities of cuttings coming over the shakers, a methodology that can be very subjective.

In this paper, a rigorous method of evaluating drilling fluids sweep efficiency is proposed. Here, information from pressure-while-drilling (PWD) tools and measured drilling fluid and sweep fluid properties is used to determine a mathematical prediction of cuttings out. In short, the method relies on the difference between mass in and mass out. Examples from field applications are given to illustrate the usefulness of this methodology.

With a more rigorous evaluation of drilling fluid sweep efficiency in hand, users can objectively determine the type

and frequency of application of drilling fluid sweeps and rig time spent circulating out ineffective sweeps can be minimized. Introduction Drilling fluid sweeps are usually applied in wells to augment hole cleaning, especially in high-angle or extended-reach wells where efficient hole cleaning is more difficult to maintain than in vertical or near-vertical wells. In the deviated wells, the drilled cuttings can accumulate on the lower side of the hole at angles greater than 35 - 40 from vertical. If left unattended, this accumulation can become severe enough to lead to hole pack-offs, stuck pipe, and other unwanted incidents of non-productive time.

Drilling fluid sweeps are formulated to achieve certain properties that provide additional lift to cuttings in vertical and near-vertical wellbores. In high-angle wells, drilling fluid sweeps can be used to scour the top layers of drilled cuttings accumulation and to displace lighter-density areas of the annulus where drilled cuttings accumulation is thought to occur.

There is not much information pertaining to use of drilling fluid sweeps that is available in the literature, a sign that this particular area of investigation has not received much attention among drilling fluids researchers. In a recent paper1, the authors described various types of sweeps and provided field evidence to support improved cleaning with changes in downhole measurements of annular pressure (PWD). In addition, the authors made extensive use of computer modeling to gauge drilling fluid sweep performance. A common problem associated with this approach involves the inherent assumptions implicit in the modeling process. It would be much better to use actual drilling and drilling fluid parameters to gauge hole cleaning improvement with use of drilling fluid sweeps.

The use of drilling fluid sweeps having a density higher than the drilling fluid system itself to improve cuttings transport is the subject of a U.S. Patent2. In this patent, small volumes of drilling fluid commonly weighted up with specially-sized barite particles were used in deviated wellbores, and the changes in ECD and hole cleaning efficiency were described. Again, any advantages using these sweeps in removing cuttings from the wellbore were only qualitatively described and / or modeled.

SPE 77448

Drilling Fluid Sweeps: Their Evaluation, Timing, and Applications Terry Hemphill, SPE, Halliburton and Juan Carlos Rojas, SPE, BP Exploration

2 T. HEMPHILL AND J.C. ROJAS SPE 77448

Data Sources In this study, the input parameters and the results shown herein were obtained on a shallow TVD high-angle project drilled with 3.125-in OD Advanced Composite Coiled Tubing (ACCT). The use of ACCT has been recently reported in the literature3. No tubing rotation or drill string rotation was used during any of the time intervals covered in this paper. The 4.75-in open hole had a maximum horizontal displacement of 3487 ft. A hole pack-off incident led to the initial open hole being sidetracked and a second hole was drilled. A 5.5-in string of casing was set at 77 degrees from vertical. Hole angles on the initial open hole segment and the sidetrack ranged between 70 and 85 degrees from vertical in the tangent sections.

In this coiled tubing application, a synthetic-based invert emulsion drilling fluid (SBM) was used. Mud densities of the circulating system ranged between 9.4 and 10 lbm/US gal. Fluid rheological properties were held in a range deemed sufficient for cleaning in ERD-type well designs.

With the lack of any drill pipe rotation in this interval, hole cleaning in the tangent section was recognized to be poor. Hole cleaning modeling of the drilling conditions indicated that the top of the cuttings bed reached to the top of the coiled tubing. Accordingly, in the high-angle sections, modeling results showed approximately 50% of the cross-sectional annular area only was subjected to fluid flow. Sweep Dynamics In the study of drilling fluid sweep efficiency, the step-by-step dynamics of a sweeps movement through the wellbore should be recognized. In Fig.1, the measurements of PWD vs. time depict the various steps in this discussion.

In this sweep, a high-density sweep weighing 12.3 lbm/US gal was used. Its reported volume was 10 bbl. Before the sweep was pumped down the coiled tubing, a 9.6 lbm/US gal SBM was being circulated at a rate of 90 US gal/min. Baseline Equivalent Circulating Density (ECD) of the system was measured at 10.7 10.75 lbm/US gal. The blue line shows the measured mud weight in the coiled tubing, and indicates the maximum density in the tubing reached a maximum of 12.28 lbm/US gal and quickly tapered off as the high density sweep flowed into the annulus. Approximately 18 minutes after the high-density sweep entered the coiled tubing its leading edge began to show on annular pressure measurements. A maximum annular density of 11.93 lbm/US gal was recorded. This density includes the sweep density, annular pressures converted to ECD, and the weight of cuttings picked up by the sweep while passing through the wellbore. In all, the transit time of the high-density sweep in the annulus was 27 minutes. Once the sweep had passed out of the annulus and over the shakers, the baseline circulating fluid ECD was measured at 10.68 10.71 lbm/US gal. Because the high-density sweep was not isolated from the active system, it became incorporated in the circulating system and circulating mud weights going in the hole rose to 9.75 lbm/gal. As is usually seen with effective sweeps, the measured system ECD following the sweep is lower than that before the sweep was

pumped; in this particular case, a drop in ECD of 0.05 lbm/US gal was measured. Modeling of Sweep Efficiency Reviewing increases in PWD measurements solely to evaluate the effectiveness of drilling fluids sweeps is not recommended, especially so when high-viscosity sweeps are used. Often the increases in ECD due to the elevated viscosity of the sweep are enough to add several points to measured ECD readings. The situation becomes even more complicated when high-viscosity/high-density sweeps are used. Indeed, a rigorous method of modeling drilling fluid sweep efficiency is needed.

In this paper, such a method is proposed. In this protocol, drilling fluid sweep efficiency is evaluated on a mass flow out and mass flow in basis. A schematic of the various components is given in Fig.2. In this methodology the following parameters are utilized:

Baseline ECD of the drilling fluid system before the

sweep is applied (ECD1) Additional ECD resulting from higher density

sweep (ECD2) Additional ECD resulting from high viscosity of

sweep (ECD3) Additional ECD resulting from density of cuttings

removed from the hole by the sweep (ECD4) Total ECD of the sweep as measured by PWD

tools (ECDtotal). Time (T = 0 initially and T=T at end of evaluation period)

Accordingly, the ECD effects are given by: ECDtotal =

ECD1 + ECD2 + ECD3 + ECD4, and the ECD resulting from the density of the cuttings carried out by the sweep is ECD4 = ECDtotal ECD1 ECD2 ECD3.

To calculate the ECD effects in terms of mass flow out, the ECD profiles are integrated over time. Time intervals can be selected to give sufficient accuracy and maintain speed of calculations.

To calculate the mass flow in of a drilling fluid sweep, certain properties of the sweep must be known: Density of sweep Volume of sweep Pump rate

From this information, the mass flowrate in of the sweep can be calculated and subtracted from the mass flowrate out of the sweep to finally arrive at a value for the mass of cuttings brought out of the hole by a drilling fluid sweep. In the event that the pump rate is not constant during the entire sweep process, then the pump rates should be integrated over small time intervals to arrive at a final flowrate.

Other Modeling Considerations Other factors that could be involved in affecting sweep results should be discussed:

SPE 77448 DRILLING FLUID SWEEPS: THEIR EVALUATION, TIMING, AND APPLICA TIONS 3

Negative mass cuttings out values Drill pipe rotation Turbulent or transitional flow Fairly clean wellbores

Sometimes in the calculation procedures, a negative value of mass cuttings out can result. A negative value is attributed to inexact or estimated input parameters and, when such an incident happens, the mass cuttings out value is set to zero.

It is known that other factors such as drill pipe rotation can contribute to ECD and/or bring cuttings out of the hole in drilling applications. In this methodology it is assumed that drill pipe rotation is either non-existent or held constant during the entire sweep evaluation process. Introducing variable drill pipe rotation in the evaluation process complicates PWD interpretation.

When increasing the density of a high-density sweep (e.g., with no viscosifiers added), small increases in viscosity are usually seen. Given that the sweep passes through the annulus in the laminar flow regime, these increases in ECD will usually be very small. This situation, of course, is quite different for sweeps in transitional or turbulent flow where the density component predominates in pressure loss calculations.

For sweeps having a higher-viscosity than the circulating system, the additional ECD caused by the high-viscosity (ECD3) can be either measured if the data is available or predicted using hydraulic models. If/when hydraulic modeling is applied, the volume of the viscous sweep must be included in the annular volume and the ECD effects must be evaluated in different annular sections to obtain an overall contribution to ECD by the viscous sweep.

Lastly, it should be mentioned that, to be effective, sweeps require the wellbores to have significant levels of drilled cuttings accumulation. The best-formulated sweep will have little effect if the wellbore is clean. In this coiled tubing project it can be safely assumed the hole experienced significant cuttings accumulation when hole angles reached 30 from vertical or greater. Sweep Efficiency Modeling Results In the coiled tubing drilling application, hole cleaning in the high angle zone was poor, as should be expected in non-rotating drilling situations. Accordingly, many drilling fluid sweeps were run while drilling to augment the hole cleaning efforts. Three principal types of sweeps were run:

High-viscosity High-density Combination high-viscosity / high-density

These sweeps were run at various places in the annulus, sometimes in the high angle section of the wellbore and at other times in the near-vertical section of the annulus. A total of 7 sweeps are discussed in this paper, and the pertinent fluid and pumping properties and the calculated sweep efficiency

results are found in Table 1. The ECD results for the 7 sweeps are also shown in Figs. 4-6 and are segregated by sweep type. In these figures, the Delta ECD is the measured increase in ECD over that of the baseline drilling fluid ECD.

Fig.3: 3 high-density sweeps (Sweeps 1, 2, 4) Fig.4: 3 high-viscosity sweeps (Sweeps 3, 6, 7) Fig.5: Sweep 5 Fig.6: Sweep 4

Large differences in sweep performance are seen: Sweep 4 (high density) and Sweep 5 (combination high density and high viscosity) produced the largest increases in ECD. The other 5 sweeps had little effect on increasing ECD and hence would not be expected to bring much in the way of drilled cuttings out of the wellbore. Of these 5 sweeps, 2 were high-density sweeps and the others were high-viscosity sweeps. When the viscous properties of the high-viscosity sweeps are backed out of the ECD increases seen at surface, the calculated amount of cuttings brought out by the high-viscosity sweeps is nil. Clearly, the sweep efficiency evaluation technique described in this paper can compare sweeps on a quantitative, not just a qualitative, basis.

From ECD results alone, Sweep 5 appears to be nearly as effective as Sweep 4. However, when the sweeps viscosity and density properties on ECD are evaluated, it is apparent that Sweep 5 brought little-to-no cuttings out of the hole. Fig.5 shows the magnitude of the sweeps density and viscosity components relative to measured increases in ECD. This example demonstrates the need to look beyond the ECD increases in PWD logs to help determine the efficiency of a drilling fluid sweep.

Sweep 4 clearly brought the greatest amount of cuttings out of the hole. When the sweeps density is backed out of the calculations, a total of 929 lbm cuttings were calculated to be removed from the hole. Fig.6 shows the split between the sweep density and cuttings density as functions of the Delta ECD. Sweep Performance Issues Questions have arisen from these results concerning the poor performance of some of the sweeps compared to Sweep 4. Observations regarding their performance are given below:

Sweep size. Compared to the sizes of high-density sweeps 1 and 2, Sweep 4 was 67 100% larger in volume. In the open hole, the 10-bbl volume of Sweep 4 is predicted to cover 577 ft of annular length. While Sweep 4 had the highest density of the 3 sweeps, the differences in density were on the order of 2 to 5% less only. Clearly, sweep volume is a key parameter in their success.

Sweep viscosity. In none of the sweeps applied in the tangent section did the elevated viscosity levels help to bring more cuttings out of the hole. In the highly deviated hole sections, the circulating fluid will take the path of least resistance,


which in this case means that the highly viscous fluid will flow over the top of the coiled tubing. This flow diversion effect has been reported many times in the hole cleaning literature.

Combination sweeps. From the data presented in this paper, the use of combination high-density high-viscosity sweeps does not promote cuttings removal from the wellbore. Indeed, better performance was seen with the high-density sweep. The combination sweeps viscosity component only served to increase ECD and promote flow diversion of the sweep over the top of the coiled tubing.

Drill string rotation. Since drill string rotation was not possible in this coiled tubing application, these tests represent the worst cases for using sweeps to clean highly deviated wellbores. From the drilling literature, we know of the positive benefits rotation has on hole cleaning in highly deviated wellbores. However, the general lessons learned in this study of drilling fluid sweeps are valid for rotary drilling as well.

Sweep Circulation Time Differences in drilling fluid sweep performance can also be seen when the Delta ECD results are plotted vs. circulation time in units of bottoms -up. In this way, the progress of the sweeps through the hole is normalized for changes in pump rate and depth of sweep application. In Fig.7, the results for 3 sweeps used in the high-angle sections are plotted:

Sweep 4 high-density Sweep 5 combination high-density high-viscosity Sweep 6 high-viscosity A number of observations can be made for the data plotted in Fig.7: Of the 3 sweeps, the high-density Sweep 4 stayed the

most intact while being circulated out of the hole. The two sweeps having high-viscosity levels were much more strung out through the annulus. Stringing out is attributed to the flow diversion effect mentioned earlier.

None of the sweeps were circulated out of the hole after the first bottoms -up time period. Indeed, for Sweeps 4 and 5, they had only just begun to increase Delta ECD by this time.

To remove the sweeps completely out of the hole, circulation times ranged between 1.5 and 2.75 bottoms -up intervals.

In the high-angle sections, the high-density Sweep 4 was the most efficient of all the sweeps in terms of carrying out the most cuttings in the shortest circulation timeas measured in bottoms -up units.

Conclusions A number of conclusions can be drawn from the material presented in this paper: A numerical method has been developed to evaluate the

efficiency of drilling fluid sweeps to clean wellbores. This method uses PWD measurements, drilling parameters, and drilling fluid properties to quantitatively estimate the amount of cuttings brought out of the hole by the sweep.

Of the seven sweeps studied in this paper, the high-density Sweep 4 was the most efficient. It brought the largest amount of cuttings out of the hole in the least amount of circulation time / bottoms -up units. Linked to its efficiency were primarily its larger volume and secondarily its higher density level.

In the high-angled sections, the high-viscosity sweeps performed poorly, as did the combination high-viscosity, high-density Sweep 5. Poor performance is attributed to the effect of flow diversion.

These results are valid for non-rotating drilling situations and thus represent worst-case scenarios. In rotary drilling operations, drill pipe rotation can improve the situation significantly.

Sweeps can take a long time to circulate out of the hole. In this study nearly 2.5 bottoms -up intervals were needed to completely re move Sweep 4 from the wellbore. In actual drilling situations, large volumes of cuttings picked up by a sweep may remain in the annulus if sufficient circulation time is not allowed. In such cases, hole pack-offs, stuck pipe, and tight hole incidents may later occur.

To work efficiently, sweeps should be built with sufficient volume. In this study, best-performing Sweep 4 was formulated to cover 786 ft of length inside the casing or 577 ft of length in the open hole.

Acknowledgments The authors would like to thank their respective companies for permission to present this paper. Nomenclature PWD = pressure-while-drilling as measured with downhole

tools ECD = equivalent circulating density References 1. Power, D., Hight, C., Weisinger, D. and Reimer, C.: Drilling

Practices and Sweep Selection for Efficient Hole Cleaning in Deviated Wellbores, paper IADC/SPE 62794 presented at the 2000 IADC/SPE Asia Pacific Drilling Technology in Kuala Lumpur (11-13 September).

2. West et al.: Method and Composition for Sweep of Cuttings Beds in a Deviated Borehole, U.S. Patent No. 6,290,001 (Halliburton Energy Services).

3. Dalton, C., Paulk, M., and Stevenson, G.: The Benefits of Real-Time Downhole Pressure and Tension Data with Wired Composite Tubing,, paper 2002-220 presented at the Petroleum Societys Canadian International Petroleum Conference in Calgary (13 13 June 2002).

4. Hemphill, T.: Method for Determining Sweep Efficiency for Removing Cuttings from a Borehole, U.S. Patent Pending.


Table 1

Key Parameters for Drilling Fluid Sweeps Studied

Sweep Designation 1 2 3 4 5 6 7 Sweep Type*** HD HD HV HD HD/HV HV HV

Sweep Density [lbm/gal] 12.0 11.6 9.5 12.3 13.2 9.8 9.8 Sweep Volume [bbl] 5 6 5 10 8.5 7 7

Pump Output [US gal/min] 90 90 80 90 90 90 120 Bottoms -Up Time [min] 20.3 23 27 24 14.5 18 6.7

Mud System Density [lbm/gal] 9.45 9.45 9.5 9.4 9.9 9.8 9.8 Baseline ECD [lbm/gal] 10.7 10.5 10.55 10.75 10.8 10.8 10.8

*** HD = high-density HV = high-viscosity Fig. 1Mud weight and PWD measurements as functions of time during a typical drilling fluid sweep application.

Fig. 2 Key components of total ECD produced by a drilling fluid sweep bringing cuttings out of the annulus.

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Fig. 3 Delta ECD vs. time for high-viscosity sweeps Fig. 4 Delta ECD vs. time for high-density sweeps

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