spe_77752

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 Cement pulsation is a novel technology for enhancing zonal isolation by applying low frequency, hydraulic, pressure pulses to the wellhead immediately after cementing. The treatment maintains the slurry in a liquid state, which transmits hydrostatic pressure downhole, and keeps the well overbalanced thus preventing early gas flow after cementing.

The paper summarizes several stages in the development of cement pulsation technology including comparison to other methods, physical principles, process analysis, mathematical modeling, computer-aided design, laboratory testing, and field performance.

The paper supports published information on cement pulsation with data from research and field studies that was instrumental in developing the technology. Emphasis has been given to the analysis of the pulsation process, description of design model and software, and an updated account of field applications.

Described is the MS Windows software for pulsation design. Two examples demonstrate the computer-aided design. The examples show that the software could be used to find the pulse size and treatment duration for a constant-pressure treatment. Alternatively, a variable-pressure treatment with controlled treatment depth could be designed.

Data is presented from pulsation of over 80 wells in drilling areas notorious for early gas migration after cementing. Field applications of the technology in 80 wells provided significant evidence of the success of cement pulsation in preventing early gas leakeage in cemented wells.

Introduction Top Cement Pulsation In 1982, a landmark field experiment performed by Exxon revealed hydrostatic pressure loss in the annuli after primary cementing in wells1. Since then, hydrostatic pressure loss after cement placement has been considered a primary reason for gas migration outside wells. As the annular cement still in liquid state - loses hydrostatic pressure, the well becomes under-balanced and formation gas invades the slurry and finds its way upwards resulting in the loss of wells integrity.

Cement slurry vibration using a low-frequency cyclic pulsation is used by the construction industry for improving quality of cement in terms of better compaction, compressive strength, and fill-up. (Cement gelation or transmission of hydrostatic pressure is not a concern in these applications.)

In the oil industry, the idea of keeping cement slurry in motion after placement has been postulated a promising method for prolonging slurry fluidity in order to sustain hydrostatic pressure and prevent entry of gas into the wells annulus. The idea was based upon experimental observations that cement slurries in continuous motion remained liquidous for a prolonged period of time2,3.

Manipulating the casing string would move the cement slurry. Thus, early concepts considered keeping cement slurry in motion through casing rotation or reciprocation4,5,6. The motion should improve displacement of drilling mud and placement of cement slurry in the annulus.

The use of forced casing vibrations for gas flow control has become subject of several inventions in the 80's and 90's7,8,9,10,11,12. For example, enhanced filling of annulus with cement slurry without rotating or reciprocating the casing" was considered the main advantage of the first casing vibration method with mechanical vibrator placed at the bottom of the casing string7. All these methods have been already experimentally studied and patented. However, none of them have been used commercially because of difficulty involved in manipulating the entire casing string. Apparently, heavy equipment and installatioin needed to vibrate a long and heavy string of casing makes these methods not feasible, even onshore.

In 1995, Texaco patented a technique based on pulsation of the cement top13,14. In this method, low frequency and small-amplitude pressure pulses are applied at the top of the cement by cyclic pumping of water or air to the wellhead. The

SPE 77752

Cement Pulsation Treatment in Wells Andrew K. Wojtanowicz, SPE, John Rogers Smith, SPE, Djuro Novakovic, SPE/Louisiana State University, V. S. Chimmalgi, SPE/ONGC, Ken R. Newman, SPE/Coiled Tubing Engineering Services, Dale Dusterhoft, SPE/Trican Services, Brian Gahan, SPE/Gas Technology Institute

2 WOJTANOWICZ, SMITH, NOVAKOVIC, CHIMMALGI, NEWMAN, DUSTERHOFT, & GAHAN SPE 77752

treatment continues for sufficiently long time to keep cement in liquid state, reduce transition time, and maintain hydrostatic pressure overbalance.

Texaco field-tested a number of shallow (up to 4,700 feet) wells in the Concho (Queen) field of the Permian basin, Texas. The tests demonstrated that pulses could be transmitted through the slurry in the lab and that the bond logs of pulsed wells were superior to those that were not pulsed.

The Coiled Tubing Engineering Services, and the Louisiana State University have jointly further developed the cement pusation technology in a project sponsored by the Gas Technology Institute. Field testing of instrumented wells (with downhole pressure gauges) demonstrated that annular pulses could be transmitted to a significant depth in excess of 9,000 ft and that hydrostatic pressure in the annulus was maintained by pulsing the slurry15,16. Full-scale laboratory pulsation experiments with a thixotropic slurry in an LSU well showed how small pressure pulses would progressively break gel structure and deliver pressure to the wells bottom17,18. They also revealed that pulsation should have an additional advantage versus application of a constant pressure18. Another laboratory study showed that pulsation did not reduce final compressive strength or shear bond of cement19.

Development and commercialization of the technology required a method for designing the treatment. Mathematical modeling, performed at LSU, provided theoretical basis for the treatment design and diagnostic analysis methods and software3,17,20,21,22. Industrial use of the technology has been carried out by Trican Well Services Ltd. and Husky Energy in three oilfields of Eastern Alberta, Canada23,24.

The objective of this paper is to support published information on cement pulsation technology with data from research and field studies that was instrumental in the technology development. Emphasis has been given to the analysis of the pulsation process, description of design model and software, and updated account of field performnce.

Cement Pulsation Process and Equipment After cement placement, the well annulus is intermittently pressurized-depressurized by cyclically pumping water from the cement pulsation unit to the wellhead. A portable cement pulsation unit consists of an air compressor, water tank, hoses to connect to the well, instrumentation and a recording system. Pulses are applied to the annulus by water that is pressurized by the air compressor. After charging the well, the water is bled back to the tank. The system schematic is shown in Fig.1.

An air compressor continuously pressurizes an air tank. To pressurize the annulus, the control system opens a valve between the air tank and a water tank. The air pressure forces the water into and pressurizes the casing annulus. To release the pressure, the control system closes the pressurization valve and opens the exhaust valve. As the pressure is released, water returns from the casing annulus to the water tank. Once the pressure is fully released, water is added to the water tank if needed, to keep the water tank full.

The volume of water displaced to the well for each pulse is determined by measuring the water level in the tank. From this measurement a compressible volume is derived using a data -smoothing algorithm with corrections for water loss in the well and compressibility of surface installation.25. As the cement sets, the compressible volume of the casing annulus should decrease as shown in Fig. 2.

The pulses are quite slow, with built in delays. The pressure is applied and held for up to 10-25 seconds (design parameter). After pressure is released, there is a dormant period of up to 10- 25 seconds (design parameter). The pulsation frequency is low, of the order of 1-2 cycle/minute (design parameter).

Recorded parameters of the pulsation process are shown in Fig. 3. Each cycle includes three periods, pre-pressurization, pressurization, exhaust. During the pre-pressurization period, the air-tank and water-tank are not in communication with each other. The water that goes into the annulus during the previous cycle will continue to come back and will result in water level increase. At this time the annulus pressure can be in the range of 2 to 5 psi. The compressor during this period is used to compress only the air tank. The water tank pressure or annulus pressure will be very low during this period.

When the pressurization is started, the use of pressurized air to compress the water will reduce pressure in the air tank and increase the pressure of water in the water tank. Once the air pressure and the water pressure reach equilibrium, both the pressures will continue to increase together, but at a lower rate as some of the water is getting pumped into the annulus. During this time, the pressure in the water tank will force the water out of the tank into the annulus. The water level will be the lowest at the maximum annulus or water-tank pressure.

When the air tank is cut off from the water tank and water pressure is bled off during the exhaust period, the water pressure will fall suddenly allowing the water that went into the annulus to come back to the water tank. This will make the water level rise. The compressor will continue to increase the air pressure in the air tank during this period.

Physical Mechanism of Cement Pulsation The first question asked about this technique is typically about energy efficiency. Unlike other vibration techniques, this method uses small input energy that is very efficiently transmitted over a long distance despite the opposing friction. Thus, frictional losses must be small.

Efficient transmission of small top pressure pulses over several thousand feet down the annular column of Non-Newtonian fluids with yield stress could only be efficient if the column yields only at the walls while the bulk fluid (a plug) remains not sheared. From the analysis of flow of Bingham fluid in the annulus it has become clear that, for the plug flow, energy required to reciprocate the slurry is much smaller than that needed to shear the entire bulk slurry.

The analysis required revisiting the theory of Bingham fluid and deriving exact formulas for plug flow17,24. Figure 4 demonstrates the concept of a minimum velocity needed to shear the bulk slurry (reduce plug size to zero). It has been

SPE 77752 CEMENT PULSATION TREATMENT IN WELLS 3

shown that for typical pulstion parameters and annular sizes the slurry motion is well within the plug flow regime24. Moreover, the exact model of plug flow gives small values of shearing rates and pressure losses (energy loss) at velocities representing cement pulsation. The shearing rate relation is shown in Fig. 5, and the pressure loss in Ref. 17.

As the pulsed slurry moves only as a plug, its reciprocating motion can be simplified as an equivalent slow continuous motion in the plug flow regime. Thus, the relationship between average velocity and displacement amplitude becomes,

fzyzv *)(21)( = (1)

Displacement amplitude, y(z), is distributed along the slurry column. Cement in the upper annulus undergoes greater displacement than the deeper cement - the displacement amplitude reduces with increasing depth so does the friction opposing the displacement as the velocity is also reduced with depth.

After a few modeling attempts aimed at pressure wave propagation and other effects of pressure transient, the modeling focused on a pseudo dynamic concept where the velocity of the fluid and the deformation of the annular walls is considered, while the transient effects are neglected. The simplification is based on evaluation of the pressure pulse propagation velocity in the annular system for different annular sizes24. It was found that the velocity of pressure wave in the annulus would range between 2500 ft/second to 4200 ft/sec. As the typical pressure pulse duration used in cement pulsation is of the order of the 10s of seconds and the velocity of pulse application is low, the pressure transients are negligible.

Keeping slurry in motion reduces static gel strength (SGS) development and delays the transition from liquid to solid. Several experiments have shown that the change of SGS development is practically independent from shearing rate.(3) Thus, for the design, one could assume that as long as the slurry is sheared at the wall, its gelation could be represented by a single altered SGS plot disregarding the shearing rate value. This also means that the pulsation treatment is effective when pressure value at depth exceeds the time-dependent value of the altered SGS of the slurry. The condition would define the depth of (effective) treatment.

Two sections could be visualized in the pulse-treated cement column. The upper, and usually very long, section is where the shear stress at the wall is larger than yield stress of the slurry and the slurry is in motion. In the bottom, typically short, section, the transmitted pressure pulse is smaller than yield stress so the slurry is motionless and not sheared. In this work we assume that the latter section column is negligibly small and all the pressure is expended when the displacement amplitude becomes zero. Hence in our analysis the terms treated depth and the depth of pressure transmission are equal.

During pulsation, the cement slurry at depth is sheared at the walls as it reciprocates upwards and downwards. As the annulus is a closed system, the slurry movement in the annulus

is caused by elastic deformation of the fluid and the annular walls. Thus, the displacement amplitude is caused by the pressure at a given depth and is controlled by the compressibility of the annulus below that depth. The annular compressibility represents isothermal compressibility of the slurry coupled with the elastic properties of the cased hole and a stratified open hole built of several layers of rocks having different properties and thickness. The annular system compressibility model has been derived and presented elsewhere17,26.

Designing pulsation treatment for a well involves determination of parameters such as the pressure pulse amplitude, pulse cycle duration, and depth of treatment as functions of time. Interestingly, the three parameters are somewhat dependent on each other as well as on the properties of cement, mud, and rock, and on the annular geometry of the well. Hence, mathematical modeling of pressure pulse transmission (attenuation), and displacement amplitude distribution becomes a basis for the design.

Mathematical Model of Cement Pulsation Process Derivation of the mathematical model has been based on the following assumptions: Reciprocating motion of slurry is represented by

equivalent continuous motion with average velocity given by Eq. (1);

The annular fluid motion follows Bingham plastic model in plug flow;

System compressibility applies and the annular system deforms elastically;

Pressure pulse duration is sufficiently long so inertial effects can be neglected;

Time dependent properties of stagnant and sheared slurry (yield stress and plastic viscosity) are known from pre-job testing;

Duration of time lapse between the two consecutive pulses is sufficently long so that the stress from the previous pulse fully diminishes; i.e. displacement amplitude is not affected by residual stresses;

There is an active mechanism of stress relaxation in the annular fluid column fluid loss to the rock;

The applied top pressure is attenuated by the total distributed friction due to slurry movement; i.e frictional pressure loss in plug flow controls pressure transmission downhole;

Displacement amplitude is distributed and controlled by compressibility and pressure distributions.

The top pressure pulse, p0, transmission formula is:

dzGvKdpz

po

)(0

0

+= (2) where:


12

12

212

ln0614.0936.0

)(200

)(1000

ddvC

ddYPC

G

ddPVC

K

f

f

f

+=

==

(3)

Displacement amplitude is described by the linear differential equation:

)]1)[exp(1)(5.0)exp( cGzcpzczKfycpdzdy

oo +=+(4)

which gives distributed displacement formula:

+

+=

3)exp(

3)exp())exp(1()(

32

32

0azzaz

aZZaZcpzy ppp

(5) where:

)exp(25.0 0cpcKfa = (6) From the model, the depth of treatment, Zp, and the top displacement amplitude, Y0, are calculated from the equation:

+=

3)exp())exp(1(

32

0p

ppo

aZZaZcpY (7)

Finally, the bottomhole pressure at any time is computed as:

))(4())(4()(1212

pp ZddYSgZZ

ddSGSgZp += (8)

The mathematical model was validated in full-scale pulsation experiments at the LSU Well facility, and by matching data recorded during cement pulsation in two wells in Texas17,26. We also used this model in a sensitivity study to evaluate relative effect of the parameters involved in the pulsation process. The study revealed that: Large well annuli improve pulse transmission

significantly; There is almost linear increase of treatment depth with the

size of top pressure pulse; Low-frequency pulses (f < 0.1) would significantly

increase treatment depth; Annular system with large compressibility would reduce

treatment depth.

Cement Pulsation Design Software Cement Pulsation Design software is a MS Excel based application integrating both spreadsheet calculations to handle the local mathematics and VB Macros to handle global task assignment and property distribution within the well of interest. The Main spreadsheet is shown in Fig. 6. The software consists of several spreadsheets, designed to separate several aspects of data. (User can navigate between

spreadsheets either by clicking the button containing particular aspect of data input or by using tabs in the bottom of the screen.) The input spreadsheet names and button titles for corresponding sheets are: ControlPanel, OpenHole, CasedHole, and Fluids. The output data is saved in the spreadsheet Results.

The ControlPanel spreadsheet is used to enable/disable the check for necessary Excel Add-Ins (Solver and Analysis ToolPak) or support software provided by Microsoft with original installation CD. Also using the ControlPanel, the user may request a customized listing of time-related properties at specific depths of interest. If the listing were omitted, the software would output the treated depth, top displacement and top pressure pulse for each timestep.

The OpenHole spreadsheet takes input data for up to 20 different rock strata composing the openhole section of the well annulus. For each strata the input is similar to that for the CasedHole section and includes: bottom depth of the strata, size of vertical gridblocks, sizes (open hole diameter, inner and outer casing diameter), Poisson's ratio for rock and steel, Young's modulus for rock and steel, and compressibility of the fluid opposite the strata.

Values of Youngs modulus and Poissons ratio for the openhole strata can be estimated from empirical correlations. Poisson ratio values for sedimentary rocks vary from 0.2 for a hard and fragile rock to 0.5 for most soft rocks. Correlations of Poison ratio vs. confining/overburden pressure have been developed for specific areas. For the Gulf coast area, the Eaton correlation gives the Poison ratio vs. depth directly. For other areas, the Poissons ratio can be calculated using Eaton correlation for Gulf coast area with variable overburden pressure. The procedure for finding the overburden pressure is described elsewhere27. Once the overburden pressure is known, Youngs modulus can be computed from the empirical formula,

naa pppKE )/(0= (9)

where, K0 = rock dependent empirical modulus number n = rock dependent exponent pa = atmospheric pressure p = overburden pressure Values for K0 and n for elastic rocks are given in Table 3.1

in Ref. 28. The remaing input data are saved in the Fluids spreadheet.

The spreadsheet accepts two types of time-dependent data, annular fluids properties (drilling mud, tail, and lead slurry), and pulsation data (pressure pulse size, and cycle time).

The Results spreadsheet returns two types of output data: time- dependent and depth-dependent. The time dependent results are time, minutes; treatment depth, ft; top displacement, gal; and top pressure, psi. The depth-dependent results are controlled by a user-provided listing in the ControlPanel and they include a header containing timestep and pulse cycle; depth, ft; pulse pressure, psi; and, displacement, gal.


Algorithm By definition, average fluid properties for the annular system are computed from Eqs. (10) and (11). Equation (12) gives the first approximation of depth of pressure pulse transmission, which is a function only of the average yield point. In the subsequent iterations the treatment depth is computed from Eq. (13) derived from Eq. (2).

ip

iN

iavg PVZ

lPV )()(1=

= (10)

ip

iN

iavg YSZ

lYS )()(1=

= (11)

avgp YP

ddpZ )(300 120 = (12)

dzGvKppZ

)(0

0 += (13) The equations (3,5,6,10,11, and 13) are iterated until the

depth of treatment, average Yield Point, and Plastic Viscosity converge.

Compressibility of the total annular system (down to the cement bottom) remains constant throughout the pulsation process3. However, the compressibility of the treated annulus is not constant. Most of the time, the treated annular system will have both open hole and cased hole portions, with the openhole section having larger compressibility than the cased hole section. (By definition, the system compressibility is the volumetric averages of the open hole compressibility and the cased hole compressibility.) Thus, with continuing pulsation and increasing gelation, the depth of pressure pulse propagation (treatment depth) and the treated system compressibility will both reduce. Consequently, the treated system compressibility values must be updated after each series of iterations.

The software algorithm flowchart, presented in Fig. 7, can be summarized as follows. For the previous value of treatment depth, the average PV and YP (and system compressibility) is computed from Eqs. (10) and (11). Then, the top displacement amplitude is computed from Eq. (7). Since the correction factor Cf is velocity dependent and we do not know the velocities of the cement column, we assume unity value for Cf (simplified plug flow model) and calculate the top displacement amplitude and velocity from Eq. (1).

The annular column is divided into large number of small grids to enable us to take into account the variation in the annular geometry and the fluid properties. The displacement amplitude, velocity, correction factor, Cf, and frictional pressure loss is calculated across each grid together with cumulative pressure attenuation described by the integral in Eq. (2). The calculation for different grid blocks is continued till the transmitted pressure pulse becomes zero. The corresponding depth becomes an updated depth of treatment

and is substituted back to calculate once again the average property of the fluid, the corresponding system compressibility and top displacement. Once this operation is carried out for the grid-wise pressure distribution, the spreadsheet automatically updates displacement amplitude distributions. These iterations are repeated until the depth used to calculate the top displacement and the depth of pressure pulse transmission become equal. Normally, the calculations converge within two to three iterations. The above computations represent the calculation procedure for a single time step.

For next time step, the fluid properties are updated and the procedure of calculation is repeated. The top displacements and the depth of treatment for each of the time step are saved and plotted against time or number of pulses. Also, as the iterations are carried out for a non-linear system of equations, the safest way to do it is using MS Excel's Add-in Solver.

Examples of Computer-aided Design Two types of cement pulsation treatment are demonstrated below, constant-pressure operations and controlled-depth operations. The treatments were designed for the same well in Texas shown in Fig. 8. (The subject well was actually treated using the constant-pressure pulsation pattern.) The properties of annular fluid used in the design were as follows: Drilling mud:

Depth = 6,900 ft Density = 12.5 ppg YP = 10 lb/100 sq ft PV = 18 cp

Lead cement: Depth = 7,700 ft Density = 12.5 ppg YS = ( )200198.09681.8 + te PV = ( )200169.09917.9 + te Tail cement: Density = 15 ppg YS = ( )200053.0674.70 + te PV = 65.1491124.210510 2336 +++ ttt Where t is elapsed time in minutes.

Predicted results of the constant-pulsation process are shown in Figs. 9 and 10. Over the first 50 minutes, the top 300 feet of the lead slurry column is pulsed followed by continuing pulsation of the lead slurry until 2 hr and 10 min of the treatment. After that time, only drilling mud column is pulsed; the treated depth becomes constant and equal to 6,900 ft. Also, the top displacement amplitude becomes constant and equal to 10 ft Fig. 10.

The controlled-depth pulsation was designed in two stages. During the first 120 minutes pulsation is carried out with constant pressure of 100 psi Fig 13. During that time, the depth of treatment reduces from the initial 8,000 ft to 7,000 ft, i.e. to the top 100 feet of the lead cement. To treat more cement for longer time we plan to increase the treatment depth from 7,000 ft to 7,100 ft and keep the depth constant until 3


hrs of pulsation as shown in Fig. 11. The increased and constant treatment depth requires progressive enlargment of the top pressure pulse from 100 psi to 230 psi Fig. 13. The increase of top pressure compresses mostly the 6,900-foot long mud column so the top displacement amplitude should rise. In fact, figure 12 clearly demonstrates the increase of the amplitude from 11 to 25 feet.

Laboratory Testing of Pulsed Cement Slurries The proposed testing protocol for cement pulsation is analogous to, but different than, the pre-job tests performed for primary cementing operations. This testing is not mandatory, but it is useful for determining the feasibility of, and for making both simple and comprehensive performance predictions for, a specific job. This is particularly important for deep jobs, for those with a high viscosity drilling fluid, or for diagnosing problems during a job.

The most important properties are a pulsation-specific yield point, conventional gel strength measurements, and the expected pulsation time analogous to thickening time. Modified cement test methods can also be used to demonstrate how pulsation controls the development of cement gel strength. The protocol uses conventional lab equipment to measure the required mud and cement properties. The specific procedures for measuring all these properties, the basis for such measurements, and an example application to an instrumented field cement pulsation job were previously described in detail by Smith et al29.

Tests of Mud Properties. The mud properties that must be measured for use with cement pulsation are the plastic viscosity, the yield point, and the gel strength versus time. Viscometer speeds of 3, 6, and 100 rpm were selected to represent the range of fluid velocities expected in the well annulus during pulsation. The following definition of yield point was validated by previous work18. The pulsation specific mud rheology parameters are: YP = 3 6 3( ) (14) PV YP= 3 100( ) (15) The mud gel strength is measured using conventional definitions and procedures for drilling fluids27.

Tests of Cement Properties. The cement properties that should be measured are the same as for mud. However for cement, all of the properties vary with time and prior shear history. The viscometer measurements are made with a specially modified Fann viscometer. Measurements are made at 3, 6, and 30 rpm with a standard F1 spring and a 1.2276 cm radius bob that has been knurled to minimize cement slippage. The speeds were selected to represent the range of annular velocities expected in the cement column. YP k k k= 2 11 3 6 3. ( ( )) (16) PV YPk= 29 7 2 1130. ( ( / . )) (17) Gel Strength k peak= 211 3. , (18)

The validity of these measurements was confirmed by comparison of simple predictions of job feasibility and performance to downhole pressure measurements reported in a previous paper29.

Measurement of the maximum cement pulsation treatment time is performed by operating a MACS Analyzer at 8 rpm to simulate pulsation. A consistency reading of 25 to 35 Bc at 8 rpm is proposed as the limit which determines the maximum treatment time. The actual static gel strength can also be measured when the consistency reaches this level by stopping rotation and switching the device to its static gel strength measuring mode.

The measurement of conventional static gel strength versus time under normal conditions is also potentially useful. Comparing the consistency versus time under simulated cement pulsation with static gel strength versus time can give an approximate indication of whether pulsation will suppress gel strength development for a particular slurry.

Field Results The final phase of the research was to test the pulsation theory in the field to determine if pulses would, in fact, prevent gas flow into the annulus. These tests could confirm that the ability of cement pulsation to maintain bottom hole pressure demonstrated in previous instrumented field tests15,16 does supress flow after cementing. Therefore, cement pulsation has been applied to a total of eighty wells in Canada in areas with previous gas migration problems. A more complete description of the test program is given by Dusterhoft et al24.

Wells in the Eastern Alberta area of Canada were chosen to test the technique. This area was considered ideal since it contains a number of wells that experience gas migration to surface, which is easy to measure by monitoring surface casing vents. Another advantage of this area is that it has a solid history of recorded vent leaks to compare to.

A pulsation cementing project was undertaken with Husky Energy in three fields: the Tangleflags, Wildmere and Abbey fields. All of these fields have experienced various levels of gas migration in the past and numerous techniques have been used in an attempt to control the problems.

Tangleflags Area. A typical Tangleflags well is described in Table 1. This area has had a history of moderate gas migration problems with an average of 10.5% of the wells drilled experiencing gas migration problems. A total of 24 wells were included in the study: seventeen were pulsed and two were abandonment plugs that were also pulsed. None of the wells pulsed experienced any leaks.

Wildmere Area. A typical Wildmere well is also described in Table 1. It has been more difficult to control gas migration in Wildmere, which has had an average of 25% of the wells leaking. Twenty wells were cemented in this area: four were plug jobs and 16 were production cement jobs. Of the 20 cemented, 18 had no leaks while two wells leaked. Both wells that leaked experienced equipment freezing problems during pulsation so the pulses were not transmitted to the annulus.


The above test results were taken from the information available from Huskys gas migration test database and may not be inclusive. Not all wells drilled in the years specified may be reflected in the numbers - some wells do not have tests on file. The leaker/non-leaker status is based on the most recent test conducted.

Abbey. Table 2 outlines a typical Abbey well. Abbey is in a river valley and wells in this area have serious gas migration problems. The wells here are shallower due to the lower surface elevations. Gas migration is believed to originate from the Milk River zone at 360 m and 3,800 kPa. This depth reduction, in conjunction with higher pressure gas zones, has resulted in more than 80% of the wells leaking in this field. Eight jobs have been performed in this area. Six of the wells have not leaked and two wells have leaked. One well is leaking through the vent and the second is leaking through the ground around the surface casing. Additional Field Testing. Since the initial Husky tests, an additional 28 wells have been cemented in other serious gas migration areas of Alberta. To date, the pulsation system has been 100% successful in preventing vent leaks in these wells. The deepest wells pulsed to date have been 1,300 m. Four wells were cemented to this depth in an area where 75% of the offset wells had vent leaks. All four pulsed wells had no vent leaks.

One two-stage well was pulsed. The second stage was cemented from 1,000 m to surface and was pulsed. The previous two offset wells experienced surface vent leaks from a gas zone above 1,000 m. The pulsed well had no vent leak. A summary of the additional wells is contained in Table 3.

Conclusions The paper summarizes several stages in developing cement pulsation technology, from comparison to other methods, to physical principles, to process analysis, to mathematical modeling, to computer-aided design, to laboratory testing, and, to field performance. Several aspects of the technology have been supported with new data leading to the following conclusions: 1. Keeping the cement slurry in motion prolongs its liquidity

and ensures well pressure overbalance. The motion can be induced with several techniques: casing/cement vibration, casing rotation/reciprocation, or cement pulsation. The latter method is the simplest, most convenient to use without disrupting rig routines.

2. Low-frequency and small-amplitude pressure pulses require small input energy but can be transmitted deep downhole with little attenuation. The reason for high energy efficiency is the plug flow motion of the slurry with small energy loss for shear friction outside the friction-less plug.

3. The pulse cycle time is long and greater than the transit time. The pulsation process involves a series of individual applications of pressure to the top of the annulus. In each application the pressure is held long enough so it could be

felt deep downhole. This principle not only defines the designed duration of the pressure holding time, but it also allows elimination of pressure transient effect for mathematical modeling of this process.

4. The mathematical model describes effect of a single pressure pulse at a time. (The rectangular pulse shape has been verified with the monitoring system of the pulsation unit.) The model relates the applied pressure to the length of the annular slurry column in motion (treatment depth) for known dynamic (in-motion) properties (PV, YS) of the slurry at that time. As YSdynamic < SGSstatic; the hydrostatic pressure at depth is greater than that for a static column at any time.

5. The design software simulates cement pulsation process by recurrent pulse-wise applications of the mathematical model over the entire treatment time involving hundreds of pulses. At each pulse, the software searches for iterative solution to the system of non-linear equations describing pressure/dispalcemnt transmission along the heterogenous annular column comprising several sections of different fluids and up to 20 layers of rocks in the open hole.

6. As demonstrated in examples, the cement pulsation software could be used to find the pulse size and treatment duration for a constant-pressure treatment. Alternatively, a variable-pressure treatment with controlled treatment depth could be designed.

7. The fluid property definitions proposed herein provide a reasonable basis for predictions of cement pulsation feasibility and performance.

8. Field application of the technology in 80 wells provided statictically significant proof of cement pulsation performance in preventing early gas leaking in cemented wells.

Nomenclature a = flow parameter defined by Eq. (6) c = compressibility, 1/psi Cf = parameter defined by Eq. (3) d1 and d2 = annular diameters: inside and outside, respectively E = Youngs modulus of rock, psi f = frequency, 1/sec G = flow parameter defined by Eq. (3) K = flow parameter defined by Eq. (3) K0 = rock modulus number, 103 in. l1, li...lN = lengths of different fluid sections, ft N = number of fluid sections in annulus = viscometer dial reading PV (t) = plastic viscosity, cp p0 = top pressure pulse, psi p = frictional pressure loss t = time, min _v = average velocity of pulsed slurry defined by Eq. (1) z = depth, ft Z = depth of cement bottom, ft Zp = depth of pressure pulse travel (treatment depth), ft


y(z)= displacement amplitude at depth, ft Y0 = top displacement amplitude, ft YP (t) = yield point (mud: YP=YS), lbf/100sq.ft. YS (t) = dynamic yield stress of pulsed cement, lbf/100sq.ft. SGS = static gel strength, lbf/100sq.ft. = density, lb/gal g = acceleration of gravity, lb-ft/sec2 References 1. Cooke C.E. Jr., Kluck M.P., and Medrano R.: "Field

Measurements of Annular Pressure and Temperature During Primary Cementing", SPE Paper 11206, 1982.

2. Cooke, C.E., Gonzalez, O.J., and Broussard D.J.: Primary Cementing Improvement by Casing Vibration During Cementing Casing Time, SPE 14199, 1988.

3. Wojtanowicz, A.K., and Manowski, W.: "Pressure Pulsation of Cement for Improved Well Integrity - Field Method and Theoretical Model," Proc. 10th Int. Scientific & Technical Conference: New Methods and Technologies in Petroleum Geology, Drilling and Reservoir Engineering, Krakow, Poland, June 24-25, 1999, Vol. 2, 421-436.

4. Carter, G., and Slagle, K.: A Study of Completion Pratices to Minimize Gas Communication, SPE 3164, Central Plains Regional Meeting of the Society of Petroleum Engineers of AIME, Amarillo, TX (Nov. 16 -17, 1970).

5. Carter, G., Cook, C., and Snelson, L.: Cementing Research in Directional Gas Well Completions, SPE 4313, Second Annual European Meeting of the Society of Petroleum Engineers of AIME, London, England (Apr. 2-3, 1973)

6. Christian, W.W., Chatterji, J., and Ostroot G.: Gas Leakage in Primary Cementing - A Field Study and Laboratory Investigation, SPE 5517, 50th Annual Fall Meeting of the Society of Petroleum Engineers of AIME, Dallas, TX (Sept. 28 - Oct. 1, 1975).

7. Solum, K.W. et al.: Method and Apparatus for Vibrating and Cementing a Well Casing, U.S. Patent 3,557,875, Jan. 26, 1971.

8. Cooke, C.E. Jr.: Method for Preventing Annular Fluid Flow, U.S. Patent 4,407,365, Oct. 4, 1983.

9. Keller, S.R.: Oscillatory Flow Method for Improved Well Cementing, U.S. Patent 4,548,271, Oct. 22, 1985.

10. Bodine, A.G., and Gregory, Y.N.: Sonic Cementing, U.S. Patent 4,640,360, Feb. 3, 1987.

11. Rankin, R.E., and Rankin, K.T.: Apparatus and Method for Vibrating a Casing String During Cementing, U.S. Patent 5,152,342, Oct. 6, 1992.

12. Winbow, G. A.: Method for Preventing Annular Fluid Flow Using Tube Waves, U.S. Patent 5,361,837, Nov. 8, 1994.

13. Haberman J. P., Delestatius D. M., and Brace D.G.: Method and Apparatus to Improve the Displacement of Drilling Fluid by Cement Slurries During Primary and Remedial Cementing Operations, to Improve Cement Bond Logs and to Reduce or Eliminate Gas Migration Problems, US Patent 6,645,661, 1995.

14. Haberman J. P., and Wolhart, S.L.: Reciprocating Cement Slurries After Placement by Applying Pressure Pulses in the Annulus, SPE/IADC 37619, March 1997.

15. Newman, K., Wojtanowicz, A.K., and Gahan, B.C.: Improving Gas Well Cement Jobs with Cement Pulsation, Gas Tips, Fall 2001, pp. 29 33.

16. Newman, K., Wojtanowicz, A.K., and Gahan, B.C.: Cement Pulsation Improves Gas Well Cementing, World Oil, July 2001, pp. 89 94.

17. Chimmalgi, V.S., and Wojtanowicz, A.K.: Design of Cement Pulsation Treatment in Gas Wells Model and Field Validation, Paper 2002-240, Petroleum Societys Canadian Petroleum Conference 2002, Calgary, Alberta, Canada, June 11-13, 2002.

18. Martin, J.N., Smith J.R., and Wojtanowicz, A.K,: Experimental Assessment of Methods to Maintain Bottomhole Pressure After Cement Placement, ETCE01-17133, ASME Engiuneering Technology Conference on Energy, ETCE 2001, February 5-7, 2001, Houston, TX.

19. Shear Bond/Compressive Strength testing, CSI Final Report on Pulsation Project submitted to CTES, Houston, TX, 2001

20. Manowski, W.M., and Wojtanowicz, A.K.: Oilwell Cement Pulsing to Maintain Hydrostatic Pressure: A Search for Design Model, J. Energy Resource Technology-Transactions of the ASME, Vol 120, December 1998, pp 250-255.

21. Kunju, M.R., and Wojtanowicz, A.K.: Well Cementing Diagnosis from Top Cement Pulsation Record, SPE 71387, SPE Annual Technical Conference and Exhibition, New Orleans, LA, September 30 October 3, 2001.

22. Novakovic, D., Wojtanowicz, A.K, and Chimmalgi V.S.: Cement Pulsation Design Software, LSU Final report submitted to GTI (August 2001) 42.

23. Dusterhoft, D., and Wilson, G.: Field Study of the Use of Cement Pulsation to Control Gas Migration, Paper 2001-01 presented at the C ADE/CAODC Drilling Conference, Calgary, Alberta, Canada, October 23-24, 2001.

24. Dusterhoft, D., Wilson, G., and Newman, K.: Field Study of the Use of Cement Pulsation to Control Gas Migration, SPE 75689, SPE Gas Technology Symposium, Calgary, Alberta, Canada, April 30-May 2, 2002.

25. Kunju, M.R.: Post-treatment Diagnosis of Cement Pulsation in Wells, MS Thesis, Chapter 4, Louisiana State University (May 2001) 33.

26. Chimmalgi, V.S.: Design of Cement Top pulsation to Avoid Gas Migration During Cementing, MS Thesis, Chapter 2, Louisiana State University (December 2001) 37.

27. Bourgoyne, A. T. Jr et al.: Applied Drilling Engineering, second edition, Society of Petroleum Engineers, Richardson, TX (1991) 42, 502.

28. Gidley, J.L., Hoditch, S.A., Nierode, D.E., and Veatch, R.W. Jr.: Recent Advances in Hydraulkic Fracturing, first printing, Society of Petroleum Engineers, Richardson, TX (1989) 452.

29. Smith, J.R., Martin, J.N., Newman, K. R. and Gahan, B.C.: Field Evaluation of Pre-Job Test Protocol for Cement Pulsation, CIPC 2002, Calgary, AB, June 11-13, 2002.


Table 1 Typical Tangleflags and Wildmere Wells

Tangleflags Wildmere

Location: 51-26-W3M 48-6-W4M

TD: Approximately 600 m Approximately 700 m

Casing: 177.8 177.8

Hole Size: 222.3 222.3

Cement Tops Surface Surface

BHST: 25oC 25oC

BHCT: 25oC 25oC

Surface Casing Depth 133 m 133 m

Gas Producing Zones Up and down the hole Up and down the hole

Table 2 Typical Abbey Well

Location: 22-17-W3M

Surface 244.5 m casing at 70 m

Intermediate: 177.8 mm casing at 130 m

222.3 mm hole size

BHST = 90oC; BHCT = 23oC

Potential gas zones along interval

Production: 114.3 casing to 500 m

158.8 mm hole size

BHST = 26oC; BHCT = 23oC

Potential Gas Zones Milk River at 360 m BHP of 3,800 kPa

Table 3 Additional Pulsed Wells

Area No. of Wells Avg. Depth (m) Estimate of % Vent

Leaks Prior to Pulsation

% Vent Leaks After Pulsation

Lloydminster 23 800 20% 0%

Red Deer 1 1000 100% on 2 offsets 0%

Whitecourt 4 1300 75% 0%


200 gal.200 psi V.P.

Water to Well Annulus

Water Input Water Tank

Air Tank

Air control valveAir Input

200 gal200 psi V.P.

Figure 1- Cement pulsation unit flowpath

2

3

4

5

6

7

8

9

10

11

12

0 30 60 90 120 150 180 210 240Time (Minutes)

Vol

ume

(Gal

lons

)

Figure 2 Change of compressible volume during cement pulsation


0

20

40

60

80

100

120

140

160

180

64 74 84 94 104 114 124 134Time (Sec)

Pre

ssur

e(ps

i) / v

olum

e (g

al)

Annulus/ Hose Pressure Air Tank Pressure Water Level

Pre-PressurePressurize

Exhaust

Pre-Pressure

Cycle 2Cycle 1

Figure 3 Recorded parameters of cement pulsation cycle

Figure 4 Plug size reduction with increasing flow velocity

12.25X9.625 Annulus; YP = 40 lb/100sq.ft.; PV=83 cp

0

20

40

60

80

100

120

0.03 0.05 0.10 0.25 0.30 0.40 0.50 0.75 1.00 1.25 1.50 1.75 2.00 3.00 4.00 5.00 6.00 7.00 8.00 Velocity, ft/sec

Plu

g S

ize/

Ann

ulus

Siz

e,%


Figure 5 Shearing rate in plug flow

Figure 6 Cement Pulsation Design Software

12.25X9.625 Annulus; PV = 80 cp: YP = 26 lbs/100 sq ft.

0

100

200

300

400

500

600

0 1 2 3 4 5 6 7 8 9 Velocity, ft/sec

Exa

ApproxiS

hear

Rat

e, 1

/sec


Figure 7 Algorithm of Cement Pulsation Design Software

Figure 8 Example Well Schematic


Figure 9 Treated depth vs. time for constant-pressure pulsation

Figure 10 Top displacement amplitude vs. time for constant-pressure pulsation


Figure 11 Treated depth vs. time for controlled-depth pulsation

Figure 12 Top displacement amplitude vs. time for controlled-depth pulsation


Figure 13 Top pressure vs. time for controlled-depth pulsation

spe_77752

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