Copyright 2004, IADC/SPE Drilling Conference This paper was prepared for presentation at the IADC/SPE Drilling Conference held in Dallas, Texas, U.S.A., 2–4 March 2004. This paper was selected for presentation by an IADC/SPE Program Committee following review of information contained in a proposal submitted by the author(s). Contents of the paper, as presented, have not been reviewed by the International Association of Drilling Contractors or 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 International Association of Drilling Contractors or Society of Petroleum Engineers, their officers, or members. Papers presented at IADC/SPE meetings are subject to publication review by Editorial Committees of the International Association of Drilling Contractors and Society of Petroleum Engineers. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes without the written consent of the International Association of Drilling Contractors and Society of Petroleum Engineers is prohibited. Permission to reproduce in print is restricted to a proposal of not more than 300 words; illustrations may not be copied. The proposal 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.

Proposal Conventional means of primary cement placement pump the cementing fluids down the casing and well returns are taken from the annulus. This is the most common way of cement placement for the industry and has been used for more than 80 years. Much less commonly used by the industry, but recently gaining in use is the Reverse Circulation of Cement (RCC) technique. When using the RCC technique, the cementing fluids are pumped into the annulus of the well and returns are taken through the casing.

The recent acceptance of the RCC technique is mainly driven by economics and state-of-the-art technology bringing an alternative technique. Benefits of the RCC technique can include lowering bottom-hole placement pressure, reducing cement retarder concentration, lowering the time for cement placement, and increasing location safety. The main drawback to the technique is determining when uncontaminated cement is at and around the casing shoe.

This paper discusses the benefits and shortcomings of the RCC technique in relation to fluid friction, cement slurry design, location safety, and zonal isolation. The paper illustrates, through a case history, how RCC technique’s strengths are obtained while shortcomings are minimized. Field data from a recent job using the RCC technique on a 3100-m gas well in Alberta, Canada, as well as lessons learned from the job, are presented. Introduction One of the common concerns in the industry when it comes to cementing is the potential for lost circulation. While a few different approaches can be taken to address this problem, a viable alternative in reducing equivalent circulating density (ECD) is reverse cementing.1 RCC is a process in which

spacer(s) and cement slurries are pumped down the annulus and returns are taken through to the surface casing string (Fig. 1). Cement slurry’s location can be determined by two common methods:

• Utilizing a logging tool and radioactive tracers • Fluid markers

Fluid markers have been the favorite system because no

environmental issues are involved and it is more economical. Logging tools and radioactive tracers are more attractive when top of cement (TOC) inside the casing needs to be limited; i.e., when foamed cement systems are used in conjunction with RCC.

However, not every well is a prime candidate for RCC. Certain conditions should be present to indicate the necessity of this method.

RCC is not an entirely trouble free system. Some disadvantages exist in this system just like any other system that is in use. In the remainder of this paper, the advantages and disadvantages of RCC are discussed in detail. Also, a process is examined to aid with the decision-making when an operator is evaluating the options available to cement a string of casing.

Evaluation The following sections detail important advantages, disadvantages, and challenges of using the RCC method.

Advantages of RCC RCC can provide the following advantages in wells meeting the requirements for the method:

• Reduced EDCs • Improved mud displacement • Shorter slurry thickening times • Improved compressive strength development • Improved safety and environmental management • Easier cement slurry selection • Improved formation production due to less risk of

cement invasion into the producing zone

Reduced ECDs. ECD can be significantly reduced in RCC method. The gravity force is working in favor of the slurry flow; therefore, the hydraulic horsepower required to place the cement slurry is greatly reduced, which in turn reduces the friction pressures and yields low ECDs. Lower ECDs can provide the following benefits:

IADC/SPE 87197

Reverse Circulation of Primary Cementing Jobs—Evaluation and Case History Jason Davies, Kim Parenteau, Anadarko Canada Corporation; Guy Schappert, Farzad Tahmourpour, and James Griffith, Halliburton

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• Higher TOC • Elimination of stage tool(s) • Higher flow rate

Higher TOC is a desirable feature because the targeted

zone(s) are covered with cement. This advantage can also help the abandonment procedure since more of the annulus is filled with cement. Elimination of stage tool is another benefit. By not running a stage tool, the operator does not have to deal with possible complications of this method, thus saving the cost of additional equipment and rig time.

Higher flow rate (pump rate) is another benefit to lower ECDs. When the critical velocity targets are approached and met, a higher percent of mud cleaning efficiency is realized.2 Meaning, when pumping speed increases, the “circulatable hole” value increases (Eq. 1), which in turn causes a better cement job and better cement bond log (CBL) results because more mud can be removed from the hole.3 Improved Displacement. Since RCC can be placed at higher rates, an inherent improvement of mud displacement over conventional jobs placed at lower rate (to reduce the ECD of slurry placement) can be obtained. In addition to the above discussion of higher flow rates, Griffith et al, indicated through large-scale tests that mud displacement is slightly improved by the RCC method under similar rates and conditions.4 Briggs also saw improvement of mud displacement on multi-string completions.5 This is because of the larger drilled particles falling to the bottom of the hole which are lifted into the casing, as opposed to the high particle slip condition of being lifted in the annulus.

Spacer and marker design is extremely important and entirely dependent on the drilling fluid properties. Density, plastic viscosity, yield point, and rheology of the drilling fluid are the basis for designing the proper spacer and marker fluids. If an oil-based mud is used, the system wettability determination will become very important as well. A series of tests should be performed to help the operator select the optimum surfactant package. A wettability meter is a good tool for determining the effectiveness of a surfactant package in the spacer and marker fluids (Fig. 2).

Shorter Thickening Time. The concentration of retarder used in these slurries can significantly be reduced or even eliminated because fill slurries will not be required to go around the shoe. If necessary, sections of the fill cement blends may contain accelerators where the shoe cement and the deeper parts of the fill contain retarders. In addition to the engineered design of cement blends based on interval temperatures that they will be exposed to, the minimized displacement volume can provide a shorter pumping time. On most RCC jobs, the displacement volumes are less than 1m3 (6.9 bbl). The displacement volume is a calculated small volume of water that is pumped behind the fill cement to clear the surface lines and to provide enough clear casing-to-casing length to ensure the setting of casing slips. Rig time saving could add up to tens of thousands of dollars on deep well rigs with larger deep casing strings. Also, when using the RCC

method, the need for cement plugs is eliminated because the cement slurries are pumped on the backside. Improved Compressive Strength Development. Compressive strength development of fill slurries has always been an area of concern when dealing with cooler temperatures in the uphole sections. Compressive strength of shoe slurries when dealing with higher densities and at moderate to high temperatures is typically not a concern.

However, in moderate to deep wells that have under-pressured and potential upper hydrocarbon-bearing zones, compressive strength development becomes a concern. Shallower potential producing zones in a deep well have to be cemented properly and according to regulatory organization requirements. In Alberta, the Energy Utility Board (EUB) requires a compressive strength development of 500 psi (3.5 mPa) in 48 hours for any cement slurry that is to be placed across a hydrocarbon-bearing zone.

In conventional cementing, fill cement is retarded to provide enough pumping time to pass around the casing shoe. However, the retarded fill slurry may be unable to meet the regulations if it has to cover shallower zones of interest in the upper hole sections. In the RCC process, the fill cement will not see the shoe bottomhole temperatures (BHTs); therefore, a faster setting cement can be designed.

Improved Safety and Environmental Management. In general, RCC can be a safer method of cementing oil and gas wells. Placement and displacement pressures are much lower than pressures observed on conventional cement jobs.

RCC is also an environmentally friendly process. Positive environmental impact is realized through less time and equipment used on location. Cement Slurry Selection. Cement slurry design parameters and considerations are crucial to the successful completion of an RCC job. All potential producing zones and all under-pressured zones should be clearly identified, and cement slurry properties should be tailored to suit the zones. Water tables should also be of primary concern if they are present.

Some of the cement slurry properties that need to be engineered are as follows:

• Thickening time (proper pumping time) • Compressive strength (500 psi or more in 48 hours) • Static gel strength (crucial to pressure maintenance) • Fluid loss (as per conventional method) • Free water (as per conventional method) • Rheology (as per conventional method)

RCC can allow the design engineer to select different

slurry densities with shorter pumping time as the designer works their way towards the upper sections of the annulus. This usually means less or no retarder in cement slurries that will cover upper sections of the hole.

Good understanding of the dynamics, fluid behavior, cement slurry properties, and drilling fluid properties will help ensure a successful RCC job.

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Less Risk of Cement Invasion into the Producing Zone. RCC is also a good alternative when producing zones are not strong enough to withstand the ECD. Production rates may suffer if cement invasion into a producing zone occurs. This situation can be more severe if the well is not to be hydraulically fractured. Disavantage and Challenges of RCC The previous sections have covered most of the RCC method’s benefits. However, just like any other system, RCC also has a disadvantage, and some challenges also have to be faced when this method is used. Disadvantage. The main disadvantage of the RCC method is determining when uncontaminated cement is at and around the casing shoe. Pinpointing the location of the cement slurry is easier if the logging tool mechanism is used; however, if a fluid marker system is used, the location of the uncontaminated cement becomes more difficult to determine. Challenges. The RCC method requires a more complex rigup, greater design effort, and more experienced personnel.

Rigup. Rigging up to perform a RCC job is not complex; however, more iron and a special kit are required to rig up for the RCC technique. Part of the rigup is designed to handle contingency situations.

Design Effort. Every job should be carefully designed, monitored and executed. All parameters in this process should be studied in detail before and during every job. Cooperation and communication between the operator, drilling fluid company representative, and cementing company representative should be continuous.

Experience. Having experienced staff and crew is

important for performing each job from design to execution. Case History The following sections contain an overview of a recent Canadian job where RCC was chosen due to its advantages in reducing ECDs while cementing.

Well Details. Well properties were as follows:

• Casing shoe at 3074 m/Hole TD 3075 m • Hole/Casing size: 200 mm/114.3 mm • Oil-based drilling fluid system at approximately

1340 kg/m3 Pv = 25, Yp =5 • Designed top of fill cement 40 m inside casing/casing

annulus • Designed top of shoe cement 2400 m (annulus) • Designed top of shoe cement 250 m inside casing • Under pressure zone (s) present • Seepage to moderate loss of drilling fluid during

drilling • Fill cement slurry density 1500 kg/m3 • Shoe cement slurry density 1726 kg/m3

Results. Fig. 4 demonstrates the pump pressure observed on a recent RCC job performed in Alberta. Initial pump pressure

was recorded at approximately 6 MPa and decreased to 2.6 MPa as the job proceeded.

The expected displacement pressures on the same job if it was to be performed conventionally would be approximately 25 MPa. The pressure difference of approximately 20 MPa can help make the operation safer for all the personnel on location.

Cement Bond Log Evaluation. Using conventional cement bond logging techniques may lead to an improper evaluation on lightweight (and foamed) cements. Traditional cement bond logging (CBL) tools run in combination with the modern ultrasonic scanning tools are the recognized tools of choice. These tools provide waveform images and acoustic amplitudes that describe cement-to-pipe and cement-to-formation bonding. Ultrasonic scanning tools provide circumferential images and cement-to-pipe bond information.

The major problem with evaluating lightweight (and foamed) cements is that the impedance values can be below those of annular fluids, such as mud and water. Thus, conventional interpretation of ultrasonic images may provide an incorrect foamed and lightweight cement interpretation.

The service company’s advanced cementing evaluation technique was used to analyze the bond log data. Comparing the 6.9-mPa (1,000-psi) pressure pass over the corresponding zero pressure pass main section, a very slight change appears to exist in the cbl amplitude and the derivative bond index curves. Formation signals are present on the cbl waveform over the entire logged interval, suggesting good formation-to-cement bonding.

Few potential zones were perforated individually. Both formation and casing integrity were tested successfully. Figs. 5 and 6 show the processed data and the impedance derivitve curves.

Well Fracture Treatment Results Reduction of cementing ECD on the producing zone is extremely desired, especially across low-pressure zones to prevent filtrate damage or to avoid lost circulation conditions. Productive zones may take fluid (cement), and therefore fracturing treatments are not entirely successful.

Fig. 7 shows a comparison between the fracture treating pressures for the RCC well and three offset wells. The RCC well had the same treating pressure as the offset wells, but had lower breakdown pressure and twice as much sand placed into the fracture. This result indicates that the cement filtrate invaded the formation less in the RCC well than it did in the offset wells.

Cement was tagged approximately 400 m from bottom inside the casing. Cement was drilled with weight on the bit of 20 daN and rotation rate of 60 rev/min. Most importantly, the casing shoe was successfully tested to 20 MPa.

Conclusions The first RCC job was attempted more than 75 years ago. Some of the countries that have successfully used this technique are the United States, Russia, Middle East, and Canada. Based on past and recent RCC jobs, the following conclusions can be reached:

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• RCC is a viable option available to the industry. Today’s technology in combination with the economical efficiency demand upon the oil and gas industry has made this technique an acceptable and common service.

• Economical, environmental, operational and practical benefits of this technique are very appealing to the industry today.

• If the appropriate conditions exist, RCC can be the best method used to cement a well.

A decision making tree (Fig. 8) has been included to aid in selection of the cementing technique. By following the steps and answering the questions on the flow chart, an operator can select the best cementing technique for a well. Acknowledgements The authors would like to acknowledge the management of Anadarko and Halliburton for their support and permission to publish this paper. References 1. Virginillo, B. and Tahmourpour, F.: “Reverse Circulation of

Primary Cementing Jobs – Evaluaiton,” CADE/CAODC Paper 2003-036 presented at the 2003 CADE Conference, Calgary, Alberta, Canada, Oct 20-23.

2. Haut, R.C. and Crook, R.J.: “Primary Cementing: The Mud Displacement Process,” paper SPE 8253 presented at the 1979 Annual Technical Conference and Exhibition of SPE/AIME, Las Vegas, Nevada, 23-26 September.

3. Griffith, J.E. and Ravi, K.M.: “Monitoring Circulatable Hole with Real-Time Correction: Case Histories,” paper SPE 29740

presented at the 1995 Production Operations Symposium, Oklahoma City, Oklahoma, 2-4 April.

4. Griffith, Nix and Boe: “Reverse Circulation of Cement on Primary Jobs Increases Cement Column Height Across Weak Formations,” 1993 SPE Productions Operations Symposium, Oklahoma City, OK, Mar 21-23.

5. Briggs, H.G.: “Abu Dhabi Triple Completion Wells are the First in the Mid-East,” Oil and Gas Journal, Volume 9, No. 6, June, 1969.

Appendix Vc = Calculated Annular Volume/Annular hole volume Vc = Critical velocity

VolumeHoleAnnular

VolumeAnnularCalculatedHoleleCirculatab = (1)

NR<2000 == Flow is laminar NR>4000 = Flow is turbulent NR = (VDр)/ч (1.2)

µρVD

N =Re (2)

NR = Reynolds number V = Velocity of fluid (m/s) D = Diameter (m) Р = Density Ч= Dynamic viscosity (pa.s)

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Fig. 1—Reverse circulating through to surface casing string.

Fig. 2—Wettability meter.

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Fig. 3—Displacement efficiency changes, flow rate and wellhead pressure measured over a circulation prior to a conventional cement job.

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Fig. 4—Pump pressure on a recent RCC job performed in Alberta.

Fig. 5—CBL evaluation, processed data.

Fig. 6— CBL evaluation, impedance curves.

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Fig. 7—Treating pressure comparison between the Alberta well and three offset wells.

Fig. 8—Conditions that can make RCC a viable option.

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