Journal of Unconventional Oil and Gas Resources 6 (2014) 28–33

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Journal of Unconventional Oil and Gas Resources

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Optimization of drilling parameters in Raniganj Formation,Essar coal bed Methane Block – A case study

2213-3976/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.juogr.2013.09.006

⇑ Corresponding author. Tel.: +91 9732004722.E-mail address: Pallab.Mazumdar@essar.com (P.K. Mazumdar).

Dipanjan Maiti, Pallab Kumar Mazumdar ⇑, Soumen SarkarDepartment of Geology and Geophysics, Essar Oil Limited (E&P), WEBEL IT Park, Surya Sen Sarani, Near Gandhi More, Durgapur 713208, West Bengal, India

a r t i c l e i n f o a b s t r a c t

Article history:Received 4 April 2013Revised 26 July 2013Accepted 24 September 2013Available online 13 October 2013

Keywords:Raniganj FormationROPWOBRPMMud parametersTorque

Coal bed methane is one of the proven and most accepted unconventional energy resources. Commercialviability of coal bed methane play depends on minimum investment during exploration as well as indevelopment and production phases. To develop a coal bed methane field more number of wells isrequired. Therefore, during development phase one of the important aspects is to bring down the drillingtime by optimizing rate of penetration and other related drilling parameters without compromising onwellbore stability. In this paper an effort has been made to understand the drilling parameters to achieveoptimized ROP (Rate of Penetration), based on real time drilling data gathered from directional wellswithin Raniganj Formation in Essar Oil Limited Raniganj Coal bed Methane Block. In this paper data usedare from three directional wells. Conclusions are based on relationship between different drillingparameters.

� 2013 Elsevier Ltd. All rights reserved.

Introduction

Energy is fundamental to the economic development in any na-tion and India is no different. High level of growth in the demandfor gas in India on the back of economic growth, coal bed methane(CBM) an unconventional source of natural gas is becoming analternative source of energy to meet part of the growing demand.The environmental, technical and economic advantage of CBMhas made it a global fuel of choice.

Having the 4th largest proven coal reserves and being the 3rdlargest coal producer in the world, India holds significant prospectsfor commercial recovery of CBM. Being a low profit business, com-mercial viability of CBM plays depend on minimum investmentduring exploration as well as during development and productionphases. Depletion of conventional resources, and increasing de-mand for clean energy, forces India to hunt for alternatives to con-ventional energy resources. Intense importance has been given forfinding out more and more energy resources; specifically non-con-ventional ones like CBM, shale gas & gas hydrates, as gas is less pol-luting compared to oil or coal. With growing demand and rising oiland gas prices, CBM is one of the most feasible alternative supple-mentary energy sources.

The Raniganj CBM block of Essar is located in the eastern mostpart of Raniganj Coalfield, in the state of West Bengal, India. Casestudy has been done from three directional wells in Raniganj For-

mation to find the best possible drilling parameters to drill a stablewell in Essar CBM block achieving optimum ROP. Only rotating sec-tions are taken into consideration and not the sliding section. Themain parameters that affect drilling are mentioned in Table 1 (RezaEttehadi Osgouei, 2007).

Method

In this paper case study has been done with the data generatedduring drilling of the wells. Different plots have been prepared fordifferent parameters and compared with the classical plots availablein the literature to find the best possible drilling parameters toachieve optimum ROP to drill a stable well in Raniganj Formation.

Parameters considered and their effect observed in the casestudy

The main controlling parameters for ROP that has been dis-cussed in this paper are Lithology, WOB, Bit, RPM and Mud Weight.The units that are being used are mentioned in Table 2.

Lithology

International standard drilling practices ROP is one of the oldestyet still valuable tools to recognize the lithology, if other parame-ters remain same. Lithology is a major and uncontrollable factor.The elastic limit and ultimate strength of the formation are the

Table 2Units used in the paper.

Parameter Unit used in the paper Unit in SI system

ROP m/h m/hWOB klbs ton (1 klbs = 0.5 ton)Mud weight ppg kg/m3 (1 ppg = 119.83 kg/m3)Torque ft.lbf Nm (1 ft.lbf = 1.3558 Nm)

Table 1Drilling controlling factors.

Environmental factors Controllable factors (Alterable)

Depth Bit wear stateFormation properties Bit designmud type Weight on bitMud density rotary speedOther mud properties Flow rateOverbalance mud pressure Bit hydraulicBottom hole mud pressure Bit nozzle sizeBit size Motor/turbine geometry

D. Maiti et al. / Journal of Unconventional Oil and Gas Resources 6 (2014) 28–33 29

important formation properties affecting penetration rate (RezaEttehadi Osgouei, 2007).

Compaction of the rock is another major factor affecting ROP(Walker et al., 1986). It is mentioned that the crater volume pro-duced beneath a single tooth is inversely proportional to boththe compressive strength of the rock and the shear strength ofthe rock, e.g. – Igneous or metamorphic rocks are much tough todrill than sedimentary rocks.

Permeability of the rock is also a controlling factor for ROP.More the pore space or void space is connected, more the perme-ability and as a result the rock becomes more easily breakable.e.g. – Shale has greater porosity by very less permeability and itis relatively hard to be broken.

The mineral composition of the rock also has some effect onpenetration rate (Walker et al., 1986).

Structural control also plays roll on the strength of the rock andin turn affects the ROP, e.g. in faulted region the rocks become brit-tle and very friable and as result in such zone ROP is generally veryhigh. Whereas, silicified faulted zone causes relatively much lessROP if other drilling parameters remain same.

Raniganj Formation is the most important coal-bearing forma-tion in RG (E)-CBM-2001/1 block. It is represented by light greyto dirty white micaceous fine, medium and coarse grained sand-

Fig. 1. ROP variation

stone, with alternating sequence of shale, carbonaceous shaleand coal. Six regionally co-relatable coal seams are present in Ran-iganj Formation (RN-1, bottom to RN-6, top).

MWD log recorded during drilling in Raniganj Formation is rep-resented in Fig. 1. Observed ROP is high in coal while ROP reducesin sandstone to shale to igneous bodies respectively.

Bit type, condition and hydraulics

Drilling bit selection is Bourgoyne et al. (1984) based on follow-ing considerations:

(a) Drag, or ‘‘fishtail,’’ bits work by scraping or shearing the bot-tom of the hole. For poor to moderately consolidated clays,silts, and sands fishtail bits are preferred because they peelthe rock or make a mush.

(b) Tri-cone roller bits with chisel- style, longer teeth are used infairly drillable rock, where tooth penetration and crushing isadequate. Roller bits with carbide buttons are generally usedto crush harder rock. TCR bits can be of different grade andthey are to be selected based on the formation property.

(c) Polycrystalline diamond compact (PDC) bits cut by a shear-ing action similar to that of drag bits, but PDCs can handleharder rock.

The Panchet Formation (Triassic) which overlies Raniganj(Upper Permian) is light green, fine to medium grained soft mica-ceous sandstone with thin bands/intercalations of green shale inthe lower part, and chocolate brown claystone and siltstone inthe upper part. Generally, in this section 1-1-7 TCR bit is foundto be most effective, while for Raniganj Formation 4-3-7 TCR bitor PDC bit gives better meterage (Fig. 2).

Weight on bit

Weight on bit (WOB) plays a critical role in formation drillingcontrolling both the inclination of directional well and ROP.

To drill the formation a threshold WOB is required (Point-a,Fig. 3a). When all other drilling parameters are kept constant theROP behavior is determined by WOB (a–b–c–d, Fig. 3a). In somecases, the decrease in ROP is observed at very high WOB (segmentd–e, Fig. 3a). This type of bit behavior is called bit floundering (Ira-wan et al., 2012).

with lithology.

Fig. 2. Decrease in ROP (m/h) with increasing bit hours for PDC bit.

Fig. 3a. Classic plot of WOB vs. ROP.

30 D. Maiti et al. / Journal of Unconventional Oil and Gas Resources 6 (2014) 28–33

Poor ROP at high WOB is attributed to less efficient bottom-holecleaning, because wellbore does not get properly cleaned and de-creases bit life.

ROP variation is seen with respect to WOB while keeping rota-tion per minute (RPM) and mud weight constant in Raniganj For-mation. TCR 4-3-7 bit was used in this section and increase inROP was observed with increase in WOB from 5 klbs to 6 klbs(Fig. 3b).

In Fig. 3c, at surface RPM = 60 and Mud Weight = 8.60 ppg, it isobserved that with 4-3-7 TCR bit ROP increases due to increase inWOB from 6 klbs to 7 klbs.

Likewise for PDC bit with surface RPM = 60 and MudWeight = 9.00 ppg, ROP increases at greater rate due to increasein WOB from 4 klbs to 4.8 klbs. Increment of WOB beyond 4.8 klbs

Fig. 3b. Change in ROP (m/h) with increasing WOB (klbs).

to 5.8 klbs ROP increases but at relatively lesser rate, beyond 6 klbswith increased WOB, ROP drops as shown in Fig. 3d.

In Fig. 3e, change in ROP is observed with respect to WOB atsurface RPM = 60 irrespective of bit type. ROP increases due to in-crease in WOB from 4.0 klbs to 6.0 klbs and drop in ROP is observedwith increasing WOB beyond 6 klbs.

Revolution per minute (RPM)

In directional wells bit RPM is contributed by both the surfaceand down hole mud-motor which is part of directional BHA.

Fig. 3c. Relationship with ROP (m/h) with increasing WOB (klbs) for TCR bit.

Fig. 3d. Change in ROP (m/h) with increasing WOB (klbs) for PDC bit.

Fig. 3e. Relationship between ROP (m/h) and WOB (klbs).

Fig. 4c. Variation in ROP (m/h) with increasing RPM.

D. Maiti et al. / Journal of Unconventional Oil and Gas Resources 6 (2014) 28–33 31

A typical plot of penetration rate vs. rotary speed with all otherdrilling variables kept constant is shown in Fig. 4a. Penetration rateusually increases linearly with increase in rotary speed (Segmenta–b). When the rotation speed increases above a threshold value,ROP comes down (Segment b–c), and after point-c rotation speedslows down the ROP (Reza Ettehadi Osgouei, 2007).

Relationship between ROP and surface RPM keeping WOB at3 klbs and 2.5 klbs are shown in Figs. 4b–4d. Increase in ROP is ob-served with increasing RPM from 20 to 40. However, drop in ROP isseen while increasing RPM beyond 60 keeping WOB at 3 klbs.

Fig. 4a. Relationship between ROP and SRPM (N).

Fig. 4b. Change in ROP (m/h) with increasing RPM.

The poor response of penetration rate at high values of rotaryspeed usually affects wellbore stability and induces enlargementof the well bore. Increasing RPM increases torque resulting in dropin effective ROP (Fig. 4e).

Acquired real time drilling data of wells A, B & C, show that opti-mum surface RPM (SRPM) during drilling Raniganj Formation is40–60 to get better ROP without compromising on wellborestability.

Mud weight

Following physical characteristics of drilling fluid affect theROP: (1) density, (2) rheological flow properties, (3) filtration char-acteristics, (4) solids content and size distribution, and (5) chemi-cal composition. (Fear, 1996), Afsari (2003), Beck et al. (1995).

Fig. 4d. Increase in ROP (m/h) with increasing RPM.

Fig. 4e. Relationship of Torque (1000 ft.lbf) with RPM.

Fig. 5c. Relationship between Mud weight and Torque at 50 RPM.

32 D. Maiti et al. / Journal of Unconventional Oil and Gas Resources 6 (2014) 28–33

1. Increased plastic viscosity decreases rate of penetration.2. Increasing mud weight decreases rate of penetration.3. At constant plastic viscosity, penetration rate is decreased

by increased solid content.4. Increase in drilling fluid density increases bottom-hole

pressure causing increase in pressure differential betweenborehole and formation fluid.

Fig. 5a shows that at constant WOB and surface RPM, stable ROPis achieved by keeping mud weight between 8.5 ppg to 8.7 ppg.When mud weight increases more than 8.7 ppg, ROP decreasesremarkably.

Likewise decrease in effective ROP has been observed with in-crease in torque due to increase in mud weight. The relationshipbetween torque (1000 ft.lbf) and mud weight (ppg) is shown inFigs. 5b–5d respectively.

Case studies show that 8.5 ppg to 8.7 ppg mud weight (withplastic viscosity 39–41) is best for drilling through Raniganj Forma-tion to achieve optimum ROP without compromising on well borestability.

Result and discussion

Based on the study of three directional wells following are theresults:

1. In Raniganj Formation 5 klbs to 6 klbs WOB gives bestresult both in terms of wellbore stability and ROP.

2. 40–60 surface RPM is optimum for drilling both in terms ofTorque and ROP. Bottom hole RPM is sum total of bothsurface RPM and mud motor RPM.

Fig. 5a. Mud weight (ppg) vs. ROP (m/h) plot.

Fig. 5b. Relationship between Mud weight and Torque at 40 RPM.

Fig. 5d. Relationship between Mud weight and Torque at 60 RPM.

3. Mud weight 8.5 ppg to 8.7 ppg is efficient enough forwellbore cleaning, minimizing torque and optimizing ROP.

4. Mud viscosity 39–41 (funnel viscosity) is favorable forwellbore cleaning keeping torque within safe limit.

All the three directional wells taken for the case study had mud-motor of same specification in their BHA. The mud pump outputwas more or less constant for all the wells individually. Polymerbased mud was used for all the three wells. The study has beendone based on the data generated during drilling through RaniganjFormation of Essar CBM Block, so the result may vary for differentformations within the block and it may also vary for Raniganj For-mation in different blocks.

Conclusion

It is concluded that 5 klbs to 6 klbs (2.5 ton to 3.0 ton) WOB, 40to 60 surface RPM, mud weight 8.5 ppg to 8.7 ppg (848 kg/m3 to868 kg/m3) having funnel viscosity of 39–41 should be used toachieve optimum ROP while drilling a stable well through RaniganjFormation in Essar CBM block in Durgapur.

Acknowledgments

The authors express their sincere gratitude to Mr. Iftikhar Nasir,CEO-E&P, Essar Energy Plc.; Dr. Shailendra Kumar Singh, Vice Pres-ident & Head Technical-Unconventional; Mr. Apoorva Ranjan, Pro-ject Director-Raniganj CBM Project for giving the opportunity topublish this paper. The authors would also like to thank the entiresub-surface and drilling team of Raniganj CBM project at Durgapur,West Bengal including Dr. Prafulla K. Jha and Mr. Anil K. Singh fortheir continuous support and guidance while writing this paper.

D. Maiti et al. / Journal of Unconventional Oil and Gas Resources 6 (2014) 28–33 33

The views and opinions expressed are solely of the authors and donot necessarily reflect those of Essar Oil Limited.

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