Jinish Shingl ,Nikshit Vor ,Mnn Shh
a School of Petroleum Technology,Pandit Deendayal Energy University,Gandhinagar,382426,India
b Department of Chemical Engineering,School of Technology,Pandit Deendayal Energy University,Gandhinagar,382426,India
Keywords:Dual string drilling Geothermal energy Kick detection
ABSTRACT The challenges found in geothermal well drilling have forced the industry to develop new significant cost effective,time saver technologies to go for deeper geothermal resources beyond their traditional limits.To overcome these challenges,the new Dual String Drilling (DSD) technology which is based on penetration pathway of drilling fluid and rock cuttings during drilling a well.The principle of DSD employs drilling fluid flows through the annulus of inner and outer string pipe while cutting from the bottom of the well through inner pipe.The paper describes the novel Dual String Drilling (DSD) technology,predicting their occurrence and advantages on drilling of geothermal well.The outcome shows that with DSD approach a lot of time will be saved to circulate the kick out of the well.Additionally,the characteristic response of DSD enables to improve the hole cleaning capacity,to prevent pipe stuck,better well stability,reduction in torque and drag,to remove the dynamic equivalent circulating density gradient in geothermal wells.Moreover,the paper also depicts the system and method for dual coil tubing drilling of horizontal well which follows the same principle as DSD.The comparison of DSD with conventional drilling assesses that the DSD technology is affordable and appropriate for geothermal well drilling.
Geothermal energy resources are an adequate,reliable and environment friendly source of energy for the future,considering the growing global energy demand and the growing needs to replace nuclear energy and coal-fired energy with more environmentally friendly choices.As a renewable energy resource,geothermal energy offers considerable resource potential,low carbon emission,and widespread distribution [1].The geothermal industry is nowadays encountering several problems as deep geothermal drilling,economic feasibility and it"s time to start investigating for new challenging areas,which represent a high economic risk,and other technical problems.Geothermal resources are found to be in a deeper and harder geologic formation than typical conventional hydrocarbon reservoir and on other hand drilling and completion process and associated cost tend to be the key factors for making geothermal resources economically feasible[2].Costs associated with the exploration and drilling stages of a geothermal project can account for at least 42% of overall project costs [3].Deep geothermal drilling in harder formation and lost circulation are the growing and recurring challenging areas.Since increased target depth results in the narrower operating window between pore and fracture pressure,well-control aspect is becoming more important in these challenging areas[4].One of the important aspects of well control operation is early kick detection and circulation out of the well[5].
The concept of two drill strings has been explained in this framework named as Dual String Drilling (DSD) with the aim of drilling in deep geothermal resources.The DSD technology is a closed circulating system,where the dual drill string(DDS)has two separate flow channels.The drilling fluid is pumped down through the annulus of DDS,and directed from the DDS through the BHA.The return flow which includes cuttings,is transported through the inner string.The annular isolator may dispose in an annulus of the outer pipe wall and the borehole wall.The method may include static control fluid in the annulus of the outer pipe and borehole wall called passive fluid [14].Flow control unit(FCU) installed for drilling operation allows precise control of the returning fluid flow and pressure,which immediately detects any small amount of influx or losses of fluid [6].
The concept of DSD technology is also applicable for coil tubing drilling namely dual coil tubing drilling which uses concentric coil tube instead of DSD.Coiled tubing (CT)is a long flexible steel pipe string reeled around a large drum for storage,transport and deployment.The DualCTD system using underbalanced drilling techniques can enable to solve significant limitations to go for drilling a geothermal resource.The DualCTD system is also intended to drill through challenging pressure regimes,depleted zones and loss zones.The method and system of Dual Coil Tubing Drilling(Dual CTD) has been reviewed in a horizontal well by utilizing underbalanced drilling technique.Due to presence of passive fluid inside the inner annulus above the sliding piston,it provides a hydraulic force on bit which leads easily and safely to drill in dryhot rock formations with higher rate of penetration.
To gain the practical experience of technology,Vestavik [7]has developed new drilling method and verified the full-scale test..Their main goal had the capability of hole cleaning capacity,hook load experienced by the drill string and downhole pressure control.The tests showed the significant potential to drill a well with dual string.
This paper focuses on the well control aspect for DSDand comparison of DSD with conventional geothermal drilling.The DSD and dual CTD technology provides faster,cheaper and safer drilling than the existing conventional geothermal drilling technologies.
Some of the important components are:
Dual drill string is dual wall drill pipe facilitates the suspension of inner string inside the outer string.A specially designed hanger carries the weight of inner string on the outer string.The outer conduit is used to pump down the drilling fluid into the hole while cutting with the fluid from the bottom of the well is brought back to surface by inner conduit.Drill bit is fixed at distal end of the lower outer pipe and an inner pipe is disposed within the interior passageway of drill bit.
The top drive adapter is a dual conduit swivel positioned at the top of the drill string provides clockwise torque to allow rotation of dual drill string by means of connection through the rig"s top drive unit to the dual drill string.TDA contains two flow ports for in/out flow to pump the drilling fluid down into the well and another port for return flow from the well.
The flow control unit (FCU) includes valves,chocks,ECD component and actuators to allow safe &controlled in/out flow in DSD and to maintain constant downhole pressure during drilling operation.
RCD must be installed on the top of the blowout preventer(BOP)to cap the annulus and to seal against the drill string and for management of back pressure in the well during the rotation of drill pipe.
Downhole valve assembly comprises of Dual Float Valve (DFV)which is patented surface-controlled valve,Flow X-over (FXO),Booster valve(BV).These valves are positioned at the lower end of dual drill string.The DFV terminates the DDS into a conventional BHA.The DFV controls the return flow by simultaneously closing and opening of both channels of the drill string.The DFV includes a flow cross-over from the well annulus into the inner conduit of the DDS.During close position of DFV,isolation will be occurred in the inner pipe from the well bore and allow the pressure to be less connected with.While in open position in DFV,the flow of drilling fluid to the surface by the connection of inner pipe to the well bore.
It is a tool can be installed as a part of DDS outside the drill string to allow isolation of DDS and well annulus.It can prevent the lost circulation and mixing of two fluids with different density by providing the down hole hydraulic force to the system in the well annulus.A well-designed piston slide inside casing and allows drill pipe to rotate.It facilitates flow with cuttings in upward direction in the inner annulus.It provides enough pressure in the well annulus between the sliding piston and the rotary control device (RCD) to give additional weight on drill bit when required.
Cleaning the hole is an important factor in conventional geothermal drilling in order to prevent plugging of the hole.Cutting accumulates at the bottom of the hole in the well annulus when active circulation is stopped which can lead to stuck pipe situations.Further,in case of extended reach drilling (ERD) to drill long horizontal well cutting accumulates at lower side of the Hole.
DSD has ability to efficient hole cleaning capability by cutting brought back rapidly to surface via inner string rather than through annulus of the well.Thus,sticking of pipe problems can be eliminated by the DSD(see Fig.1).
In the conventional geothermal drilling method,ECD is the gradient of pressure loss in the well annulus due to friction of returning fluid with cuttings.ECD component increases with the depth which leads to power consumption of pump.DSD eliminates the ECD component by filling the annulus with passive fluid to overcome this challenge.It maintains ECD component equal to pressure working window to reach maximum horizontal depth in ERD (Fig.2).The ECD is calculated by following formula:
Where TVD is true vertical depth(ft),MW is mud weight(ppg),and Psis the pressure drop in the pipe annulus between depth TVD and surface,psi.
Fig.1.Schematic of DSD technique with active and passive drilling fluid.The blue colour represents the active circulation fluid with low density,low viscosity in the DDS and the green colour represents static passive fluid for downhole control [7].
It is necessary to provide the sufficient and stable weight on the drill bit for adequate penetration rate in order to time savings.The sliding piston pushes a bit forward,by the pressurization of passive fluid,which is above sliding piston to provide hydraulic weight on bit.It also facilitates long reach of horizontal drilling well with high penetration rate.
The string in front of sliding piston will be under compression by hydraulic WOB as well annulus is filled by heavy static fluid and behind the piston,it will be held in tension.Thus,buckling problems can be significantly reduced by DSD especially in curved and horizontal section while dealing with ERD.
The heavy static fluid around the dual drill string in the well annulus than the active circulation fluid inside the DDS provides heavy over light solution which causes positive buoyancy effect on DDS due to differences in densities and significantly reduces torque and drag in the well annulus to allow long reach of the horizontal well to drill.
Fig.2.Precise pressure control unit which can enable to drill in narrow pressure window [8].
Pressure profile can be maintained through the heavy static fluid in the well annulus,which allows to drill of longer section with the setting of fewer casing in the well.
As the lighter mud with cutting is pumped up from the annulus of string with low rates,power consumption for pump is very less in order to cost savings.Whereas in conventional drilling technique owing to drag forces acting on a wall of formation,drilling mud with cutting requires a high pump rate to lift to surface&so the power consumption is high.
The DSD is a closed loop circulation system,where drilling fluid is injected through inner annulus and return flows up the inner string.During well control condition the flow paths in the DDS must be isolated from the well.This is done by valve assembly which is placed at the bottom of the DDS.The valve assembly is controlled by flow control unit which allows accurate control of fluid flow in/out in DSD.
Fig.3 shows the DDS in closed position,no entrance of fluid is permitted through inner annulus or inner string.
PIA=PIP＜PBHP
To open the DDS,the first step is allowing the circulation by pumping into an inner annulus and inner pipe which results in equalize pressure in inner annulus,inner string and BHP [18].
Fig.3.DSD downhole valve assembly in close position.
PIA=PIP=PBHP
To further pumping into an inner annulus creates a higher pressure in inner annulus than BHP,which opens the IPV.
PIA ＞PBHP
The flow into an inner annulus is passing through the following stages,which opens the NRV and BV allowing the circulation through the system.
Fig.4 shows the DDS in open position,allowing the circulation through inner annulus and returning up with inner string.
To close the system,the first step is to close NRV and BV which are controlled by equalizes BHP and inner annulus,inner pipe pressure.
PIA=PIP=PBHP
When the pumps are slowly ramped down to zero,the pressure equalizes which causes closed NRV and BV.In next step,when pressure in the inner annulus bleeds off giving pressure in the inner annulus lower than the BHP [11].
PIA＜PBHP
The IPV will close the access from the wellbore into the inner string.
The final pressure is,PIA=PIP＜PBHP
Fig.4.DSD downhole valve assembly in open position.
The kick is observed when the pore pressure of the rock is exceeded the pressure of static mud(fluid)in the well annulus.The formation fluid enters the wellbore and the pressure of returning fluid is increasing as the additional influx in the well.The casing must be designed in order to tolerate this exceeded pressure in the conventional method while in case of DSD influx is circulated out through the inner pipe,it must be able to withstand with this additional increase in pressure due to influx in the inner pipe[12].
During first circulation,the opening sequence of DFV allows lighter mud(active circulation fluid)to pump down through inner annulus and return fluid with the additional influx is taken up to the surface through original mud weight by means of inner pipe.
The bottom hole pressure is kept constant to control well by using constant choke pressure until the total influx is circulated out.The choke pressure is remained constant until the influx is circulated out of the well(Fig.5).
The choke pressure is determined by,
Pchoke=SICP+SM+Pfriction
Where,SICP=shut-in casing pressure.
SM=safety margin
Pfriction=friction pressure loss
During second circulation,the kill mud (heavy mud) pumped down through the kill line to displace the lighter static mud from the well annulus in order to kill the well.
The casing pressure is held constant during the pumping of kill mud in the well annulus.
Fig.5.First circulation in well killing process.
As the kill mud pumped down the well annulus,it enters the inner pipe in order to circulate the lighter static mud out from the inner pipe.
Fig.6.Second circulation in well killing process.
The choke and/or flow rate is regulated to maintain constant BHP in order to keep inner annulus pressure stable throughout second circulation (Fig.6).
During third circulation,the light mud is pumped down through inner annulus of Dual drill string to displace the kill mud out from inner pipe.
Fig.7.Third circulation in well killing process.
Fig.8.Schematic view of the dual CTD in horizontal well by underbalanced drilling technique.1) Tubing reel 2) Inner tube 3) Outer tube 4) Coil tubing 5) Gooseneck 6)Injector head 7)Stripper 8)Blow out preventer stack 9)Slips 10)Rig floor 11)Rotating drill head 12)Hydrill 13)Stack 14)Inner pipe valve 15)Bottom hole assembly 16)Drill bit 17) Inner annulus 18) Well annulus 19) Whipstock
The process kept continuing until the totally kill mud is circulated out from DDS.Rig choke can be adjusted to keep well annulus pressure and flow rate stable [15](Fig.7).
The Dual Coil Tubing Drilling(DualCTD)system is based on the same principle as performing in DDS method.The system uses concentric coil tube instead of a DDS.The DualCTD is the continuous flexible steel pipe circulating of drilling fluid through the CT string.The CT has one continuous longitudinal seam,electricwelded with high-frequency induction welding.The normal CT string diameter ranges from 0.75 in.to 4 in.and single strings with lengths up to 30000 ft have been manufactured [9].There are no connections in CTD,as in conventional jointed-pipe operations and it leads to no interruption of circulation.
Fig.9.Dual coil tube working reel [10].
The system consists of two concentric tubes having two separate flow paths.The first outer casing which is lining the wellbore.The second inner casing which is placed inside the outer string.The coiled tubing drill string is a continuous pipe having a drill bit and mud motor assembly,at the distal end of string for rotating the drill bit.This separated flow conduit with managed pressure drilling is intended to solve some of the challenges with coiled tubing drilling(CTD),which involves hole cleaning capacity,low resistance to buckling,stuck pipe,lost circulation,wellbore stability,well control etc.The dual coil tubing drilling system is also intended to drill through challenging pressure regimes,depleted zones and lost circulation zones [13].
The DualCTD system with surface and sub surface equipment are illustrated in Fig.8 for drilling a horizontal well.A concentric coiled tubing drill string is inserted through a gooseneck and an injector head,at the top of the coil tubing assembly.The coil tubing is lowered through the stripper and blowout preventer stack where it goes down through the rig floor slips presenting to hold the inner string.The stripper is mounted on the injector head,which provides a hydraulic seal around the string.A slip is a device used to grip and hold the upper part of a dual coil string to the rig floor.There are a number of systems presenting below the rig floor including the rotating drill head,the hydrill,and the lower BOP stack,through which the coil tubing extends as it moves down the inner string [16,17].
In the underbalanced drilling operations,the drilling fluid is pumped down the annulus of dual CT string which,in turn,rotates the mud motor and drill bit.The swivel joint“A” is used for active fluid supply and the return fluid is circulated out through the swivel joint “B” [10](Fig.9).
The mud motor uses the hydraulic power from the drilling fluid to rotate the bit.Drilling fluid then cleans the bit and transports the cuttings to the end of the BHA,where the DFV guides the drilling fluid with cuttings into the inner string where it is transported back to the rig floor.Return drilling fluid with cuttings in the annulus between the two CT strings can cause problems with cuttings getting stuck and blocking the flow path because of the smallannular clearance between the two pipes.The inner string will not be centered in the outer string and therefore lay on the low side of the wellbore.The annulus between the Dual CT string and the borehole wall/casing will be a passive annulus that is not a part of the circulating system during drilling.This will be called the outer annulus.A heavier fluid is injected into outer annulus called the passive fluid.This can be used to drill longer horizontal sections by reducing the frictional drag the CT string.The passive fluid should secure the borehole wall and eliminate formation damage by limiting fluid invasion and by not reacting with unstable formations.The pressure of the annulus fluid column has to be steady within the pore-and fracture pressure of the formation for the whole drilling section.
At acertain depthwhencoiltubingencounters thewhipstock,and horizontal wellisdrilledupto certainpoint.Whendrilling isdone,the whole well has to be changed to the heavier passive fluid to keep the borehole stable.Passive fluid can be circulated down the dual CT string and the lighter drilling fluid will be returned through the inner string.Passive fluid can also be supplied through the supply line.
DSD can enable to reduce the rig operation time and with efficient drilling,well control can also find to be safer in this technique(Table 1).The table brings to light the total CAPEX+OPEX for DSD would be less than the conventional geothermal drilling (Tables 2 and 3).
Table 1 Comparison of CAPEX and OPEX for conventional Geothermal Drilling and DSD.
Table 2 Comparison of CAPEX including tangible and intangible for conventional geothermal drilling and DSD.
Table 3 Comparison of OPEX for conventional geothermal drilling and DSD.
When the kick is detected by the primary and secondary indicators,kill mud is pumped down to the well annulus and initial influx is circulated out through inner pipe by using rig choke which can significantly reduce the time to kill the well.Thus,the time from the detection of kick to the kill the well for DSD is less than the conventional geothermal drilling.
As the flow control unit (FCU) has precise control of flow and pressure in order to detect immediately any small amount of influx or loss of fluid,the time to detect the kill can be improved.
Overall cost can be reduced as the required volume of active circulation fluid is less than the conventional x" drilling than the existing conventional geothermal drilling technologies.
In short,return drilling fluid with cuttings through inner pipe has the great potential of solving several well control problems such above mentioned.As coin has two different sides it has some following challenges:
(1) This method might be not-effective in shallow drilling operations in terms of cost prospective.
(2) Due to a complex arrangement of instruments,it requires educated manifold.
(3) DSD method is not able to resolve the hole deviation problem facing during drilling.
Owing to a Procurance of safety with DSD and economic feasibility;DSD method can have the ability to open the door in offshore geothermal drilling.The additional characteristics response of DSD could enable longer horizontal drilling and may open the door for ease drilling and exploitation of geothermal field in offshore.
Authors contribution
All the authors make a substantial contribution to this manuscript.JS,NV and MS participated in drafting the manuscript.JS,NV and MS wrote the main manuscript.All the authors discussed the results and implication on the manuscript at all stages.
Availability of data and materials
All relevant data and materials are presented in the main paper.
Declaration of competing interests
The authors declare that they have no competing interests.
Acknowledgements
The authors are grateful to School of Petroleum Technology,Centre of Excellence for Geothermal Energy,and Department of Chemical Engineering,School of Technology,Pandit Deendayal Energy University for the permission to publish this research.
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