FIELD VALIDATION OF CEMENTING SOLUTIONS FOR WELLS

WITH LOW BOTTOM HOLE TEMPERATURES S. Banerjee, K.M. Suyan, R.K. Meher and D. Dasgupta

Institute of Drilling Technology, ONGC, Dehradun – 248195 Email : banerjee_sbi@yahoo.com

ABSTRACT Cement slurry designing using conventional cement additives for primary casing cementation of

wells with low / moderately low BHCT (40 – 70 Deg. C) is an industry challenge. Cement slurries

for field implementation needs to possess adequate placement time, minimum fluid loss, good

rheological behaviour, minimum free water separation, short transition time, good stability and

early strength development capability under operational conditions. However, even with all the

recent advancements in cementing technology, till today simultaneously meeting all of above

requirements is extremely difficult for slurries designed with conventional cement additives.

In this study extensive investigations have been carried out to formulate suitable cement slurry

systems for low BHCT wells where all cement slurry parameters could be controlled

simultaneously. Studies have culminated in the development of novel slurry systems by

incorporating pozzolan spheres giving total control over all crucial slurry parameters which could

not be done earlier. The performance of developed designs have been elucidated and validated by

successful field implementation in an onshore field of ONGC. Case studies along with conclusions

and recommendations derived from this endeavor are presented in this paper.

KEYWORDS Low BHCT, Control Slurry Parameters, Pozzolan, Fluid loss control

INTRODUCTION

Bottom hole circulating temperature (BHCT) in the range 40 – 70 Deg C is encountered by almost

all operators across the globe quite frequently while drilling of oil and gas wells. Temperature in

this range is experienced mostly in shallow wells or in wells located in mature / depleted

reservoirs. However even today, cement slurry designing for casing cementation of wells with low

/ moderately low (40 – 70 Deg C) bottom hole circulating temperature is a challenging issue. The

situation becomes critical when the bottom hole temperature is lower than 50 Deg C.

Ideally designed cement slurries needs to be of desired density and should possess adequate

thickening time under field temperature and pressure conditions to provide sufficient operational

time to carry out cementation job. In addition the slurries used for production casing cementation

should have minimum free water separation, should be stable (no settling tendency) under

operational conditions, should possess minimum fluid loss and short transition time with right

angle set characteristics. For operational requirement they should also exhibit good rheological

1

behaviour. The good rheological properties facilitate efficient placement and effective mud

removal during cementation. The designed cement slurries should also have early strength

development characteristics and sufficient compressive strength development to meet mandatory

requirements within least waiting time. Additionally the designed cement slurry should be gas tight

for inhibiting gas migration / inter zonal communication and very low set cement permeability when

used against gas zone and should be financially attractive, eco-friendly and non-hazardous.

However, even with all the recent advancements in cementing technology, till today meeting all of

above requirements simultaneously is extremely difficult and a practical approach, with the least

acceptable variance from ideality, is adopted without compromising on crucial parameters.

In many onshore fields of ONGC, wells are being drilled where the BHCT is in the range of 40 – 70

Deg C. With the existing facilities and with available conventional cement additives, problems are

being experienced in these fields for designing suitable cement slurries for casing cementation

without compromising in one or more of crucial parameters (eg. fluid loss / rheology / thickening

time / compressive strength).

While cement compositions have been developed and are in use for casing cementation of wells

with low BHCT, the intricacies on satisfactory slurry designing for such wells is limited.

Thus, there is a pressing requirement for finding a feasible solution to this perennial problem and

have been consequently been investigated at the Institute of Drilling Technology (IDT) in ONGC.

This paper presents the laboratory data showing unique feature of the developed formulations

along with several recent field case histories demonstrating the effectiveness and advantages.

PROBLEMS THAT LEADS TO THE DEVELOPMENT

Many wells with low / moderate BHCT are presently being drilled in several onshore fields of

ONGC. However in many of such wells poor cementation have been observed possibly due to

job execution with cement slurries unsuitable for the purpose. The reason for implementation of

such cement slurries is the limitations in available technology.

A review of casing cementation details of some typical low BHCT wells in the onshore fields of

ONGC it is observed that the field implemented slurries are not adequately designed (Table– 1).

At least one parameter among thickening time / fluid loss / rheology is substantially above the

desired level. This is possibly one of the major factors which is responsible for poor casing

cementation and associated problems. The problem deserves immediate attention

In the effort to resolve the issue, the first attempt in designing such a cement slurry system for low

BHCT wells is naturally oriented towards a recipe containing a blend of conventional dispersant /

fluid loss control additive / accelerator.

2

Thickening time of cement slurry should be sufficient to enable an operator to safely place the

slurry to the predetermined depth. However, unnecessarily long thickening time should be avoided

as excessive thickening time resulted in annular rings against permeable zones to cause gas

migration, causes water pockets and severely affect the quality of cementation. In the results

presented in Table–1 it is observed that at moderately low BHCT of 60 Deg C, conventional

formulation with only fluid loss control additive & dispersant does not set upto 350 minutes. Hence

there is requirement of addition of a set accelerating additive. Calcium Chloride is mostly used in

the industry as set accelerator and has been used for this study. From the Table-2, it is observed

that addition of CaCl2 for cement slurry formulation had profound undesirable effects (increase in

fluid loss and poor rheology). The minimum dosage requirement of CaCl2 is 1.5% BWOC in

combination with additional fluid loss control agent & dispersants to attain the desired thickening

time of around 200 mins at this temperature. At these dosage the rheology of the cement slurry is

very poor (Vc = 12.2ft / sec in 3“ equivalent annular diameter) and it is very difficult to attain

turbulent flow regime with such poor rheology. With higher dosage of CaCl2 (eg. 2.0 % BWOC)

though the thickening time achieved near the desired value but both the rheology and the fluid loss

increases beyond control / acceptable limits. Therefore, while designing cement slurries at low /

moderately low BHCT by addition of set accelerator, it was essential to add high dosage of fluid

loss control additive to counter the effects of increase in fluid loss of the cement formulation.

However addition of high dosage of fluid loss control additive also again resulted in increase of

both the thickening time and rheology. Since the effect of set accelerator and fluid loss control

additive on thickening time and fluid loss control is diagonally opposite, it was nearly impossible to

design slurry with adequate parameters for production casing cementation at low / moderately low

BHCT using both the additives simultaneously.

Poor rheology results in poor cementation, as rheology of cement slurry governs mud

displacement in the annulus. Similarly, if fluid loss is not controlled it can cause premature

dehydration of slurry which can lead to annulus plugging resulting in incomplete displacement or to

a lesser extent change in slurry rheology and consequent decrease in mud removal efficiency. All

this thing have serious consequences and may ultimately lead to cement job failure or poor

cementation job. Therefore, to bring all these parameters under control simultaneously, a simple

but different approach is obviously required.

TURNAROUND INVESTIGATIONS

On the basis of experiments detailed above it is well realized that by following conventional

methodologies it may not be possible to design a cement slurry system where there will be total

control over all the crucial slurry parameters.

It is also realized that to meet the technical requirements, it is necessary to incorporate a suitable

material in the slurry design which will have profound influence on the control of all these crucial

3

parameters. Furthermore the selected material should have high pozzolanic reactivity so as to

able to participate in the chemical reactions towards reduction of thickening time at low bottom

hole temperature. The selected material also should be compatible with other conventional cement

additives, be low cost, non-hazardous and eco-friendly.

From previous experience of this laboratory in designing high performance slurries1, it is

understood that if a sub micron size particle is chosen it can fill up the void space between the

solid cement particles and thereby enhance mixability at reduced water and in the process lead to

reduction in fluid loss of the designed slurry.

Amorphous silicon based artificial pozzolanic material though have been used in slurry design for

cementation of wells for various purposes but its use for slurry designing for low BHCT wells is not

a established practice2. But considering the advantages of high reactivity of amorphous silicon

based artificial pozzolanic material and its fine particle size3, it was realized that incorporating this

material in slurry designing for low BHCT wells may provide a simple but effective solutions for this

problem.

Inspired by this concept, cement slurry designing for low BHCT wells incorporating artificial

pozzolanic silicon based material is viewed as an alternate for the total control of crucial parameter

of the cement slurry required for obtaining good zonal isolation.

Under the present study slurries have been developed using class G oil well cement and

conventional cement additives in conjunction with amorphous silica based pozzolanic material

where all crucial slurry parameters could be brought under control simultaneously (Table 3).

UNIQUE FEATURES OF SMART CEMENTING SOLUTIONS The developed slurries inspite of being designed with conventional additives had desired

thickening time (which can be adjusted to any predetermined time), low fluid loss, excellent

rheology, early compressive strength characteristics, zero free water and high stability (Table- 3)

as compared to conventional presently field implemented slurries (Table -1). These slurries were

also gas tight and exhibited very short transition time with near right-angle setting (Figure -1).

Obtaining all of the above mentioned parameter in the same manner was not possible through

earlier industry practices.

FIELD IMPLEMENTATION - CASE HISTORIES In early 2006 production casing cementation had been successfully performed as field trial of the

developed cement slurries in two development wells having low BHCT in onshore Cauvery basin

of ONGC. Later two other production casing cementations have also been successfully performed

in the same basin using the slurry formulation developed at IDT.

4

CASE HISTORY - 1

The initial field trials were executed in Well # KPDK and Well # VJDA for 5-1/2” casing

cementation. The details are given in Tables 4 & 5.

In both the wells KPDK and VJDA the actual cementing operations were performed as per industry

best practices. Wells were conditioned prior to cementation, IDT designed slurry was batch mixed

and casing reciprocated. Preflush (SAPP / Water) and spacer (Ultraflush-II) were used on

requirement. During operations turbulent flow could also be attained as the slurry had good

rheology and mud removal was efficient. Enhanced slurry properties coupled with concerted

efforts resulted in perfect cementation as evident from excellent CBL / VDL for both the wells

(Figure 2 & 3)

In the Well # VJDA, object-1 has produced gas 210000 M³ / day & condensate 50 M³ / day through

12 mm bean during production testing. In the Well # KPDK, Object- 1 during testing has produced

15000 cu.m gas /day and 50 cu.m oil with no water cut through 6mm bean at FTHP 950 psi and

CHP 1600 psi.

CASE HISTORY - 2

After successful initial field trials, the next field trial was carried out in an exploratory Well TVAT

(Tables 4 & 5) for 5-1/2 inch production casing cementation. The well details are given below.

Well name : TVAT, Depth : 3088m, Float collar : 3075m

Casing size : 5.5”, Bit size : 8.5”, Cement rise : 3088m-2500m

Zone of interest Obj-1: 3065-61m, 3037-34m, 3029-26m, 3022-20m, 3018-16m, 2991-89m, 2973-

70m, 2944-40m, and 2896-2894m; Obj-2: 2759-55m and 2735-32m

Max recoded temperature 241º F aT 3088 M; The average hole size was 9.5inch in this well.

Slurry was designed at 65º C BHCT using 10 % Pozzolan , 0.4% FLA and 0. 6% Dispersant. TT

was 190min and API Fluid loss was 180ml.

Execution

The job was carried out by pumping 8M³ water as pre-flush followed by 17 M ³ smart solutions

slurry of 1.90 gm/cc and displaced with 37.2 M ³ mud of 1.30gm/cc . Bumped plug at 1200psi and

raised to 1700psi. Job was normal and the CBL-VDL recorded at 700psi is found excellent

throughout the cemented interval (Figure 5).

CASE HISTORY- 3

The fourth successful field trial was carried out in exploratory Well # UGAB in the same onshore

basin (Tables 4 & 5). The well was having drilled depth of 3005m but the 7inch casing was short

landed at 2075m by placing a cement plug at 2200m. Here cement rise was planned 1700m from

surface. A SAPP solution was pumped as preflush and which was followed by cement slurry of

1.88 sg. The cement bond log shows excellent cement bondage.

5

VALIDATION THROUGH FIELD IMPLEMENTATIONS In the year 2005-06, remedial jobs had to be performed in at least 6 Nos. of low BHCT wells in this

onshore field alone. Implementations of the smart cementing solutions presented in this paper

have solved the field problems of poor cementation in such wells. To establish the effectiveness of

smart cementing solutions, the results of cement job evaluation for four representative wells drilled

in the same area (Well # KMP # 36, KMP # 37, KMP # 40 and KP# 42) are compared (Table – 6 &

Figure 4). It can be seen from the CBL/VDL of these wells that much improved bondage has been

achieved in well KP#42 where cementation was performed adopting the smart cementing solutions

in comparison to the other wells cemented with conventional slurry. The cementation problems

faced in earlier wells have been completely eliminated by the implementation of smart cementing

solutions in Well KP # 42 resulting in no need for any remedial cementation job. The

implementation of this innovation has resulted in saving on OPEX, as expensive rigdays were not

required for any unproductive downtime. Thus this innovation has served dual punch of technology

upgradation as well as cost savings.

CONCLUSIONS

¬ Conventional cement slurry is not ideal for production casing cementation of wells with low /

moderately low BHCT (40 – 70 Deg C). In such formulations, Calcium Chloride is often used

as set accelerator. It adversely affects cement slurry properties. The present practice is to

compromise on any one parameter due to technical reasons.

¬ Cement slurry design for low / moderately low BHCT wells may be done with an alternate

approach of incorporating a suitable material in the slurry design which should have high

pozzolanic reactivity and small particle size to fill up the void space between the solid cement

particles for exhibiting easy mixability at reduced water.

¬ Amorphous silica based pozzolon material has been successfully used for cement slurry

design for low / moderately low BHCT wells. It has shown excellent results with simultaneous

total control of all crucial parameters such that cement slurry can now be designed for any

specific requirement within this range which was earlier nor possible.

¬ Use of amorphous silica based pozzolan material in cement slurry design helps in early

compressive strength development and results in reduction of WOC period leading to cost

savings.

¬ Comparison of properties of presently used cement slurry designs being implemented in fields

with formulated cement slurries under this study clearly show that the formulated cement

slurries are superior than presently used cement slurry on all crucial parameters.

ACKNOWLEDGMENTS

6

Authors are highly thankful to Sri V. K. Jain, GGM (D), Head-IDT for his inspiration & valuable

guidance during this study and the management of Oil & Natural Gas Corporation Limited for the

permission to present this paper. Thanks are also given to all associated with the Cementation

and Cementing Material Laboratory of IDT, ONGC for their help and co-operation rendeded for this

Project.

REFERENCES 1. Dasgupta, D, et all,: “Lightweight slurries - A success story of application in low fracture

gradient and depleted fields for improved cementation”, SPE paper 101810 presented at the

2006 SPE/IADC Indian Drilling Technology Conference and Exhibition held in Mumbai, India,

16–18 October 2006.

2. Nelson, Erik B,: “Well Cementing”, Edited by., Elsevier Science Publisher B.V., The

Netherlands.

3. Ghosh, S.N.: Cement and Concrete Science & Technology”, Volume I, Part I, ABI Books

Private Limited, New Delhi - 110019 (1991) 361.

7

TABLES: Table 1: Table 2: Typical field conditions and slurry Parameters with Calcium Chloride Acclerator Parameters of field implemented Slurry (Sg. 1.90, BHCT 60 Deg C)

conventional slurries

*Critical Velocity Vc, through 3” equiv. annular dia

Composition

C W FLA Disp-ersant CaCl2

TT (mins)

Fluid Loss (ml/30 min)

Vc * (ft/sec)

100 44 0.7 0.4 - Not Set

upto 350 min

- -

100 44 1.0 0.2 1.0 285 190 10.50

100 44 1.0 0.4 1.0 295 175 9.83

100 44 1.0 0.4 1.5 260 234 12.20

100 44 1.0 0.5 1.5 332 - -

100 44 1.0 0.4 2.0 230 290 14.3

WELL KKDB KADH KPDI VJDB

Slurry Density 1.90 1.90 1.90 1.90

BHCT, Deg. C 55 63 60 67

BHP, PSI 3500 4500 3400 4500

FLA % BWOC 0.2 0.3 0.3 0.4

Dispersant % BWOC 0.1 0.3 0.2 0.3

Thickening Time (mins) )

at BHCT

205

215

215

250

Fluid Loss (ml/30 mins)

at BHCT

1379

1583

1050

1279

Rheology at BHCT

Vc (ft/sec)*

10.55

9.48

8.78

8.33

*Critical Velocity Vc, through 3” equiv. annular dia

Table 3 : Performance of smart cementing solutions slurries

Table 4 : Wells Parameters for Development Wells # KPDK, # VJDA

and Exploratory Well # TVAT and # UGAB

KPDK (KP # 42)

VJDA (VJ # 27)

TVAT UGAB

BHCT, Deg. C

60 63 65 60

BHST, Deg. C

90 101 107 90

BHP, psi

3500

4100 4000 3500

Depth (m)

1950

2690 3088 3005

Mud Sp.Gr.

1.22

1.23 1.30 1.24

Slurry Sp. Gr. (planned)

1.90 1.90 1.90 1.90

Sl. No Particulars Bottom hole conditions

1 BHCT (Deg. C) 55 60 63 67

2 BHST (Deg. C) 90 94 101 107

COMPOSITION

3 Cement 100 100 100 100

4 Water % BWOC

46 46 46 46

5 Pozzolan% BWOC

10 10 10 8

6 FLA % BWOC

0.4 0.4 0.4 0.5

7 Dispersant % BWOC

0.6 0.6 0.6 0.6

SLURRY PARAMETERS

8 Thickening Time (mins)

237 215 207 191

9

Fluid Loss (ml/30 mins) at BHCT

221 223 225 123

10 Free Water (ml) at BHCT

Nil Nil Nil Nil

11 Rheology at BHCT, PV

56 54 60 73

Rheology at BHCT, Yp

10 10 10 7

Rheology at BHCT, Vc (ft/sec) *

4.98 4.14 4.84 5.48

12 Stability Stable Stable Stable Stable 13 Comp. Strength (psi) at BHST / 3000 psi 8 Hrs 1250 - - - 24 hrs 3640 3710 4300 3850 96 hrs 5300 - 4900 4000

*Critical Velocity Vc, through 3” equiv. annular dia

9

Table 5: Slurry Composition & Parameters for Wells # KPDK, # VJDA, # TVAT and # UGAB Table 6: Comparison of Casing Cementation

results between Well KP # 42 (smart solutions) and KMP # 40

Targeted Values KPDK VJDA TVAT UGAB

COMPOSITION Cement - 100 100 100 100 Water % BWOC - 46 46 46 46

Pozzalan % BWOC - 8 8 10 10

FLA % BWOC - 0.5 0.4 0.4 0.4

Dispersant % BWOC - 0.6 0.6 0.6 0.6

SLURRY PARAMETERS Thickening Time (mins)

200 239 227 190 210

Fluid Loss (ml/30 mins) at BHCT

200 158 223 180 238

Rheology at BHCT, Vc ft/sec

6.0 6.9 6.4 6.7 6.5

Free Water (ml) at BHCT

Nil Nil Nil Nil Nil

Stability Stable Stable Stable Stable Stable Comp. Strength (psi) at BHST / 3000 psi

2000 Plus 3140 3230 3340 3175

Well Parameters

Well No. KP # 42 (KPDK) KMP # 40

Category Development Development

Depth (m) 1950 2500

Slurry Sp. Gr. 1.90 1.90 BHCT, Deg. C 60 67

Thickening Time (min) 228 242

Fluid Loss (ml/30 mins) 144 808

Remedial Jobs Nil Remedial operations (squeeze)

carried out against two

zones of interest

CBL / VDL Excellent (under 2 mV)

Moderate to Good

(Average near 20 mV all through)

FIGURES: Figure 1 : Consistency Plot Composition : C100 + Water - 46 + FLA – 0.4% + Dispersant – 0.6% + Pozzalan – 10% + TBP – 0.1%

10

Figure 2: Figure 3: Excellent CBL-VDL at well # KPDK (KP#42) CBL-VDL at well # VJDA excellent against throughout the entire cemented interval all the three objects

Figure 4: Comparison of CBL-VDL of well KP#42 completed with smart cementing solutions with that of completed wells with conventional Figure 5: slurry in the same field Excellent CBL-VDL at well # TVAT

11

KMP#36 KMP#37 KMP#40 KP#42

12