well drillability – horizontal well...

Sanggi Raksagati, 12204002, 2nd Semester 2007/2008 1

WELL DRILLABILITY – HORIZONTAL WELL

TORQUE AND DRAG PREDICTION

AND ITS APPLICATION FOR ERD WELLS By :

Sanggi Raksagati*

Abstract Torque and drag analysis is critical when well planning especially in Extended Reach Drilling (ERD) and Horizontal wells. One of the important part of predicting torque and drag is determining the correct friction factor. Friction factor combines all unknown factor that are un-measurable in the wellbore in drilling condition. Such as the uncertainty of the hole geometry (Dogleg Severity and Trajectory) , pipe stiffness, cutting beds, mud lubricity, pipe weight errors, Mechanical Equipment, and other interactions. This study will analyze the drillability of the horizontal wells due to impact of torque and drag. Using actual data from field X : well X6, X7 and X9 a number of well models is created using wellplan software, the torque and drag actual data from the model will be matched with the torque and drag data from the actual field data by modifying the friction factor. The model with the valid and matched torque and drag data will be used to study the effects certain scenarios to improve well drillability such as drillstring configuration, mechanical friction reducer device, fluid lubricant, the use of OBM and trajectory optimization (minimize dogleg severity). In the end this study will give a certain input how to improve the drillability of an ERD well. Keywords: Torque, Drag, Friction Factor Sari Analisa torsi dan drag sangat penting dalam perencanaan pengeboran terutama pada pemboran horizontal dan pemboran Extended Reach (ERD). Dalam melakukan analisa Torque dan Drag penentuan input factor friksi rata-rata yang akurat dan representative sangat penting. Faktor friksi mewakili variabel-variabel yang tidak dapat dihitung secara pasti didalam lubang bor ketika operasi pengeboran. Faktor tersebut diantaranya ketidakpastian geometri lubang bor (Dogleg dan Trajektori), rigiditas pipa, tanah sisa pengeboran, kemampuan pelumasan lumpur, kesalahan pengukuran dan standarisasi berat alat pengeboran dan interaksi lainnya. Dalam studi ini akan dikaji pengaruh torsi dan drag terhadap drillability pengeboran horizontal. Dengan menggunakan data lapangan X : sumur X6, X7 dan X9, sejumlah model sumur dibuat menggunakan software bernama Wellplan. Data torsi dan drag dari model akan dicocokan dengan data torsi dan drag dari lapangan dengan memodifikasi factor friksinya. Model dengan torsi dan drag data yang cocok akan digunakan untuk mengkaji pengaruh metode-metode yang menambah drillability seperti : konfigurasi pipa dan alat pengeboran, alat mekanis pengurang gaya friksi, penambahan aditif pelumas, penggunaan lumpur berbasis minyak dan optimisasi trajektori. Pada akhirnya studi ini akan memberikan input yang akan menambah drillability dari pengeboran Extended Reach (ERD) Kata Kunci: Torsi, Drag, Faktor Friksi *Student of Petroleum Engineering ITB


I. INTRODUCTION 1.1 Background The X field is located in the Block B, South Natuna Sea. Due to producing this field X, nine development wells were drilled and were conducted in batch drilling mode in a single platform to improve efficiency in materials handling and drilling operations. Rig E-67 was used in the operation. All conductors were driven pre-drilling operation. This study analyzes all the horizontal wells in Field X which is Well X6, X7 and X9. Well X6 and X7 is drilled to produce the west portion of the field and X9 is drilled to produce the east portion. Well X6 (slot-9) is a horizontal well with a complex tangent and horizontal hold starting KOP at 800ft MD. The 8820 ft MD/4067 ft TVD well has a horizontal section landed at 5448 ft MD and was kept entirely in the GM layer until reaching TD at 8820 ft MD with 6064 ft of vertical section and 3350 ft of horizontal section. The uniqueness of this well is it has a long vertical departure relative to the TVD and a short vertical section. This brings a new challenge in reaching the TD.

Well X7 (slot-4) is another horizontal well but unlike X6 this well is a “pregnant lady “well. It has a 7612 ft MD/4033 ft TVD with a 2173 ft of horizontal section. This well is relatively “twisted” if we compared it to the other two well. The X9 well (slot-2) is located at the east portion of field X. Well X9 has a 9348 ft MD/4086 ft TVD. This Complex Tangent and horizontal hold well has a 1556 ft of horizontal section. Figure 1 and 2 shows the positioning of the 3 wells in X field.

Fig.1 – vertical section view of well X6,X7 and X9

Fig.2 – plan view of well X6,X7 and X9

Torque and drag analysis was conducted before drilling the wells to determine the drillability of the wells. Although the wells are already drilled and didn’t encounter any significant problems, the torque and drag analysis is simulated by using a friction factor number from field Z. However, using the accurate range of friction factor will enhance the predicting of designed well drillability more accurately. Moreover, using the accurate range of friction factor in Torque and Drag analysis will be useful in the future when planning for extended reach drilling (ERD). Understanding the causes of torque and drag and use it to minimize the effects of torque and drag is the key for a successful drilling. 1.2 Objectives Generally the objective of this thesis is: To analyze the drillability of horizontal wells due to impact of torque and drag (T&D) and indicate the potential factors and it application to maximize the drillability on ERD wells.The modeling will be run using Wellplan Software and compare with the actual T&D data. Specifically the objectives are:

• Analyze the actual torque and drag data and match with the torque and drag data simulation from wellplan software to predict the range of actual friction factor (Pick up, Slack Off, Torque)

• Understanding and determining factors impacting on Torque and Drag analysis

• Simulate scenarios with the valid range of friction factor and how far the existing wells could reach and factors that increase drillability

• Indicate potential factors and it application to maximize ERD wells drillability


II. TORQUE AND DRAG ANALYSIS 2.1 Basic Theory

Torque and Drag Theory Torque and drag is an important analysis in planning a well, but in a vertical well “theoretically” there is no torque and drag force occur because the pipe is only hanged in the well and no interactions between the wellbore and string. So what appears is only the torque on bit and compression/tension of the string. But in a non vertical wellbore the contact between the string and wellbore occurs, because of that additional forces to the string is detected. The forces are torque and drag. Here below in figure 3 is the picture of the forces that occurs in the wellbore.

Fig.3 – Wellbore and Drillstring Forces

Torque As mentioned before torque only occurs when the drillstring has a point contact with the wellbore and is being rotated. Torque analysis is important regarding the drill limitation we have such as the rig torque limit (rotating system), rig capacity and the string fatigue limits. The source of the rotational force can be caused by friction, mechanical torque and the rotation of the bit.

Frictional Torque occurs when the string rotates and the force will occur in contact areas between the string and wellbore. Frictional Torque will be detected when there is no axial movement, rotating off bottom in a perfectly clean hole. The frictional Torque is affected by the following:

• Compression/tension in the drillstring Tension can occur and force the string towards the wellbore even harder than gravity force. This will increase the contact force between the string and the wellbore.

• Dogleg severity

A diverse change of dogleg severity will result in an increase of contact area, and if tension occurs contact force will also increase.

• Hole and pipe size The difference between the inner diameter of the hole and outer diameter of the string results a clearance, the smaller clearance occurs the more contact force and contact area will occur.

• String effective weight The string effective weight will affect the normal force (force of the string to the wall) therefore will affect the contact force.

• Inclination The effect of inclination is similar with dogleg but not also the same. The higher inclination occurs the contact force will increase. But at high inclination, the string tends to rest in the hole, therefore the tension will decrease and the contact forces will decrease.

• Lubricity Lubricity of the mud will affect the friction caused by the string and hole contact.

Mechanical torque occurs by the contact of the drillstring with cutting beds or unstable formation. Bit Torque is the result of an additional torque needed when the bit is rotated to drill the formation. Drag In vertical well drilling ideally there will be no drag force occur. But as the inclination of the well increases contact force between the string and the hole will occur and drag will be detected. Drag appears when the string is manipulated in the axial direction and no rotation is applied. Like in ordinary physics drag force will appear like and additional force but in a direction opposite the drillstring movement. The magnitude of drag is dependent with the friction factor which embraces the uncertainties in the wellbore. Drag analysis is important in well design, in order to prepare rig capacities, prime mover to manipulate the string and to calculate the weight on bit resultant with the effect of drag. Drag can reduce the drilling efficiency when transferring the weight to the bit. Sinusoidal and Helical Buckling could occur when tripping in compression axial force exceeds the critical sinusoidal or helical buckling load. This could end in pipe sticking. Friction Factor In simple physics, the coefficient of friction is a


dimensionless scalar value which describes the ratio between the forces of friction between two bodies and two forces when interacting. The resulting force acts in the opposite way as from the movement of the object. The higher the friction factor the more force resists the object to move. Here below in figure 4 is a simple illustration of a simple effect of friction force.

Fig.4 – Simple Effect of Friction Force

But friction factor in drilling activities is much more complicated than the ordinary friction factor shown above. The friction factor in drilling covers the uncertainties in the wellbore so it can be exerted as a single dimensionless magnitude in calculations. There are many factors that could impact the friction factor in the drilling process beside the interaction between the drillstring, such as:

Cutting beds Dogleg severity and well trajectory Mechanical equipments (BHA, stabilizer,

OD tool joint, etc) Mud lubricity, etc

Here below in figure 5 is the illustration of the factors that impact the magnitude of friction factor.

Fig.5 – Friction Factors Illustration

Friction factor could also be different even in the same wellbore with the same conditions. Different types of manipulation of the string exerts different friction factor, example when pick up, slack off and

rotate (will be explained later) occurs at the drillstring. This must be a concern in torque and drag analysis. Torque and Drag Equations To estimate and calculate the torque and drag, nowadays engineers use computer models, in this thesis computing torque and drag is used with the WELLPLAN 2000 PC based software. But before proceeding with the modeling understanding the basic physics equations is needed to know the basics. The drag and torque model is based on a simple mathematical model, first developed by the Exxon Production Research (Johancsik et al., 1984). This model assumes that the load on the drillstring is only dependent to the effects of gravity and frictional drag mentioned before as friction factor. The force that indicates the magnitude of interaction between the string and the hole is the normal force. Normal force in this model is contributed by: 1) effects of gravity and 2) effects of compression and tension in the welbore cuvatures. Here in figure 6 is a simple free body diagram of the string with the wellbore.

Fig.6 - Drillstring Free Body diagram

(Taken from “Drilling design and Implementation for Extended Reach and Complex Wells” by K&M Technology Group Texas)

From the simple free body diagram above, researchers derivate it into equations that could calculate torque and drag as follows:

( ) ( )[ ] 2122 sinsin αααθ ettN LwFFF Δ+Δ+Δ=

Net FLwF μα ±Δ=Δ cos

rFT Nμ=Δ

ttt FFF Δ+=

TTT Δ+= Where

NF = Normal force, lbf

tF = Pipe Axial Load, (+) tension, (-) compression, lbf


T = Torsion in the pipe

tFΔ = Axial Load Change, lbf ΔT = Torsion Change, ft-lbf μ = Coefficient of friction r = Radius of the pipe, ft α = Average wellbore inclination,

degrees Ө = Average wellbore azimuth, degrees Δα = Wellbore inclination change,

radians ΔӨ = Wellbore Azimuth change, radians The calculation is not too complicated but to simulate the real condition the calculation is divided in segments regarding the different condition in the drillstring and wellbore. Starting from the bottom of the string until the top (rig), and accumulate that to obtain the total load and torsion. This process nowadays is easier when computer software simulation take place. 2.2 Torque and Drag Analysis Methodology The thesis work is broken into three parts, which is: Part I: Reanalysis the Existing Torque and Drag Data

1) Reviewing drilling report and available data to understand and to collect needed data such as

a. Field X subsurface data related to well target (geological and reservoir)

b. X6,X7 and X9 Bottom Hole Assembly data for 8.5” hole

c. Mud properties d. X6,X7 and X9 Well trajectories e. Drilling parameter including actual

Torque and Drag data 2) Loading all available softcopy data into

Wellplan (PC software build to simulate Torque and Drag).

3) Cross checking each parameter of the model in wellplan with each well final well report to validate 8.5” hole model.

4) Inputting actual torque and drag data into spreadsheet (a lot of this has been done already) and inputting asci torque data from LWD surveys

Part II: Generating Torque and Drag Data from Wellplan and Match with Actual Data

1) Generate T&D data for the wells casing section and match it with the actual data to obtain the valid range of casing average friction factor for Pick Up, Slack Off and torque.

2) Regenerate the wells T&D data with valid casing friction factor until TD and match it with actual data to obtain the valid range of friction factor for the horizontal open hole section.

Part III: Modeling the Maximum addition of horizontal length and indicating the factors that affects the drillability of wells

1) Increase the horizontal section using the valid model for each well and check the drillability from the T&D limit and determine the maximum horizontal length addition without any variable change.

2) Simulating the matched model with several scenarios to indicate the potential factors that increase drillability, in this case the length of the horizontal section.

2.3 Benefits of Torque and Drag Analysis Torque and Drag analysis have an essential part in drilling design. Torque and Drag analysis although often uses data from other field and with many uncertainties is nearly always conducted, this is because there are benefits that can be obtained from this analysis, such as:

• To determine the drilliability of the well and improve the design

• To prepare rotating system rig capacity, the obtained surface torque data generated in the model could be a useful reference in determining it

• To prevent buckling limitation when drilling in well, if the axial force/tension/compression shown in the model exceeds the critical buckling force then the drillplan must be readjusted, and when its valid model can be a reference when drilling operation

• To prepare the rig capacity when maximum hookload from model is known and plus a certain of safety factor and margin overpull

• To know the magnitude of torque on bit • To prepare the block weight and string

configuration needed • To be an additional data in casing and

completion design III. TORQUE AND DRAG MODELING AND

MATCHING 3.1 Torque and Drag Modeling


As mentioned before T&D modeling in this thesis is using WELLPLAN 2000. In WELLPLAN 2000 there is Torque Drag option and cases and parameters that need to be inputted. The data is obtained from X6, X7 and X9. Well report and Maxwell data in share drive. As mentioned before, the analysis scope is only

for 859 ” casing and 8.5” open hole section. Here is

a brief explanation what data has been inputted and what data is assumed.

• General In this section inputting the general data from the well such as Origin N, E, Azimuth, well depth MD and reference point is inputted.

• Wellbore Editor Wellbore editor enables the user to input the wellbore information for casing and open hole such as Length, ID, and friction factor. The friction factor is assumed or could be matched later with thethe actual data.

• String Editor String and BHA data can be inputed in the string editor. The real data is from the well report written by the drilling contractor and from the rig inventory. It includes the OD, yield strength, torsional strength, weight, etc. • Survey Editor In Survey editor MD, Inclination and Azimuth are inserted from the survey file in well report. The TVD, dogleg, Vertical section, etc is calculated automatically as the MD, Inclination and Azimuth data inserted.

• Fluid Editor Fluid editor options enable the user to input the fluid used in the drilling such as: rheology properties, mud base and other mud properties. For all three wells the mud type is assumed the same base on the fact from the data there is no significant difference. The mud type used is RDF with a PV 14 cp and YP 34 lb/100ft2.

• Other Inputs Other inputs such as traveling block weight, stiff string model, MOP, WOB, torque on bit, tripping speed, pumping rate and pressure could also be inputted based on the actual data.

• Cross Checking the Model To make sure the model is valid, data validation is needed. There are many sources of data, mainly from

the well report since well X6, X7 and X9 are already drilled. The crosschecking is shown below:

859 ” casing setting depth is crosschecked

with the casing run report in the executive summary

BHA assembly is crosschecked with the well BHA report and the bitrun summary written be the drilling contractor of the rig

Well Trajectory Azimuth, Northing and Easting is with the definitive survey and the directional survey data released by the drilling contractor

Fluid data is cross checked with the hydraulics summary in the well report, etc

Note that friction factor is still assumed here regarding there is no actual T&D data involved yet, the friction factor range will be determined in the matching section.

3.2 Torque and Drag Matching Previously in the static model, all the parameters are validated. But as mentioned before, friction factor cannot be precisely determined. The matching process is done to obtain the actual valid range of friction factor. Notice here that the matching is not a perfect match from the real world, this happens because a certain factor such as:

• The computer model is an ideal model with a mathematical calculation. The actual data is a data with a lot of variables and inputs from operating condition that mathematical model cannot comprehensively be represented

• The matching standard from the writer • The uncertainty of the data collected from

the field In the T&D analysis there are the PRS (Pick up, Rotate off and Slack off) data and also the torque data rotating off. (all the figures and chart of the matching process is attached in the appendix)

Axial Friction Matching (Drag Matching) The cased hole friction factor number will affect the matching in the open hole section. So the cased hole friction factor is matched first. Before we match the cased hole friction factor, first we have to calibrate the traveling weight. Initially the traveling block weight from the data is 65 kip (kilopounds). But there are uncertainties in the traveling block weight and also in the string weight. The way to calibrate the traveling weight is to match


the rotating off bottom hookload with the model. The rotating off bottom doesn’t detect the hookload addition due to drag and dependent with axial friction factor. Generating hookload/drag chart from the model and do trial and error by modifying the traveling weight until getting the perfect match is a way to obtain the traveling weight. After matching the traveling weight of the string then casing matching can be done. The output from the casing matching is a range of friction factor and one average friction factor to determine the open hole friction factor. There are three series of data is PRS test, the rotate off bottom hookload is dependent from friction factor, so we only match the pickup and the slack off. The friction factor of the two is not similar due to the difference resultant force when slack off and pick up. Regarding that, pick up and slack off has to be matched individually to find the correct friction factor. Here from figure 7 trough 9 below is the matching charts process. (the other charts is in attached in the appendix)

Fig.7 - Well X7 Traveling Weight Matching

Fig. 8 - Well X9 Pickup Cased Hole Friction Factor

Matching

Fig 9 - Well X9 Slack Off Cased Hole Friction Factor

Matching

There is no perfect match in the matching process, but a certain range of friction factor for Pick up and Slack Off is obtained. So an average friction factor is picked and fixed to the model. Open hole friction factor is matched by using this. All of the process is done by trial and error. Here below in figure 10 is the matching of the open hole for the PRS test.

Fig. 10 - Well X9 Open Hole PRS Test Matching

When one friction factor for casing and open hole is approached and obtained, actually it represent the average friction factor that oocurs. Rotational Friction Matching (Torque Matching) Torque matching is relatively simple than drag. First match the rotating off bottom torque generated with the actual data. To match the rotating on bottom, the torque on bit is inputted into the model. The torque on bit is determined from matching to the actual data with the data generated from the model. The difference between rotating off bottom and rotating on bottom is the torque given to the bit or torque on bit. Crosschecking with the asci files from the LWD device also can be done. The asci files records the torque generated for a certain depth. The torque data is ranged between the rotate off and rotate on because


it resembles the actual drilling condition. Here in figure 11 is the rotating off and on bottom Torque and matched with the actual and asci file.

Fig. 11 - Well X7 Rotating off and on bottom Torque

Matching 3.3 Torque and Drag Matching Results From the Torque and Drag matching process, friction factor range and approximation are obtained. This will validate the model to analyze it in the next step. Anomalies and uncertainties will also be discussed. The output from the modeling and matching are:

Cased Hole Axial Friction Factor (CHFFs) Range and Open Hole Axial Friction Factor (OHFFs) Range for Drag analysis. This includes Pick up and Slack off Friction Factor.

Cased Hole Rotating Friction Factor (CHFFr) Range and Open Hole Rotating Friction Factor (OHFFr) Range for Drag analysis. This value is also dependent with each other.

Torque on bit range when drilling operation occurs.

Minimum Weight that can result helical buckling

Maximum Weight that exceeds pipe yield The friction factor for the PRS test is in a certain range due to the actual data itself. The minimum and maximum friction factor for the model is determined by the data span. Matching although a range of friction factor is obtained but one average friction factor for CHFF and OHFF is determined to be inputted to the model. Here below in table 1 summarizes the results and original data from the matching process of well X6, X7 and X9.

Table 1-Well X6, X7 and X9

the number inside ( ) is the average friction factor

3.4 Discussion While doing the trial and error for the models, for a different friction factor applied the pick up and slack off hookload responds differently. With the increase of the friction factor the pick up hookload increases, this is due the additional force from the drag increases. Drag force is the normal force times friction factor in other word when the friction increases the drag increases too, therefore the hookload needed to pick up the same drillstring is heavier due to the increased drag force. Note here that drag force is always in the opposite direction from where the object is moving (drillstring). In other hand slack off hookload decreases when friction factor is increases. In slack off the drillstring is moving downwards relative to the hole, therefore the drag force is moving upwards. The hookload needed to “push” the string downwards needs to be increased to overcome the additional drag force due to the increased friction factor. Therefore the hookload detected at the surface decreases. In rotating of bottom, the change of friction factor doesn’t change the hookload, this is because there is no axial motion in rotating off bottom.

Mentioned before that friction factor in the drilling scope a number that is not only the represent the roughness or interaction between the string and the wellbore, but also many factors related to it. For the well X6, X7 and X9 of field X, the drilling was performed in a nearly same condition, such as:

The fluid used to drill is relatively the same, so the lubricity factor for friction factor can be assumed equal for each well

The BHA and casing used to drill the 8.5”


well is relatively the same and the drillpipe is the same.

The open hole section of the three well is in the GM. This fact leads to the assuming that the effect of the formation roughness is the same, although not for the presence cutting beds in the wellbore.

Here below is the plot of the friction factor range between the wells for pick up and slack off shown in figure 12 trough 13

Fig.12 - Cased Hole Friction Factor Range

Fig. 13 - Open Hole Friction Factor Range

Shown above that in the well X6 has a wide range of friction factor, this is because the well X6 open hole is longer than the other well, the open hole drag highly deviates above and under the average friction factor model. It also could be seen that well X6 has the highest friction factor for open hole. Shown also at figure 11 that the asci files from the LWD and the actual data doesn’t match. The test data after 6500 ft MD freezes and result a straight line. But the asci data matches the simulation data perfectly. This brings a question whether the test data for this well and other well is correctly taken. The “not so smooth” data for the wells results also to the wide span of the friction factor range. The relatively high average of friction factor in the three wells is also because the use of the RDF type drilling fluid. This water based drilling fluids

generates high friction factor due to the low lubricity. The low lubricity, beside because of the water based but also because the large amount of CaCO3 used. CaCO3 is an inert solid that reduces the lubricity of the mud. Drilling torque is essential to analyze regarding the planning for the rig capacity. The bit design could also be determined by knowing the torque on bit magnitudes that vary for each depth. Indicating and analyzing the torque could enable better torque projection and plan torque reduction planning. IV. POTENTIAL FACTORS TO MAXIMIZE

ERD WELLS DRILLABILITY In this section, the valid model with the fixed average friction factor will be simulated in certain scenarios. The objective is to find out whether the scenario improves the drillability of the well. The indication that the well drillability increases is by viewing the change in the horizontal section. The drillability improves when the horizontal section increases without any problems (such as helical buckling, pipe yield limit, surface torque limitations, etc). The X6 well will be the model used to simulate the scenarios and how to maximize the factors that in the end well X6 could be an ERD well in the simulator. Here below at figure 14 are the X6, X7 and X9 well Reach and TVD Plotted with other offshore Wells released by K&M.

Fig 14 - K&M Wells Depth vs Reach Plot

with Field X Wells

From the plot above, the horizontal wells in field X is not considered an ERD well but studying the behaviors of the wells could enhance the ERD well understanding. First the wells will be simulated for how far the well actually could reach. Of course the reach is only based on the computer model, computer simulator assumptions and the available data.


In this study well X6 will be conditioned in several scenarios to know what impacts its drillability. The reason choosing well X6 is because it has the longest departure vs. TVD ratio. Analysis from the actual data and well report will also be conducted to analysis what happened during well X6 operation that enhance the drillability. 4.1 Maximum Horizontal Extension Section

Simulation with Current Well Model In the previous section, the models of the three wells are fixed with an average friction factor for each well’s CHFF and OHFF. The model is assumed valid regarding the matching and the data crosschecking. In this section the valid models are tested to reach the maximum addition length of horizontal section possible within the mechanical limits in torque and drag analysis. The mechanical limits are:

Maximum Torque available in the rotating system. In the E-67 Drilling Rig the limit is 45500 lb-ft for low gear maximum continuous torque.

Maximum Torque due to make up torque or the drillsting limitation when fatigue and failure occurs.

Minimum hookload weight before helical buckling occurs. Helical buckling will occur if the hookload detected at the surface is below the minimum weight of helical buckling.

Minimum effective tension for every section of part of the drilllstring cannot be under the minimum tension/compression for helical buckling. Where the helical buckling will occur in the part of the string could be known by this calculation.

The maximum hookload that could be provided by the hoisting system. For E - 67 rig is 750 T.

The procedure starts from the previous model but the trajectory, wellbore and string length is extended to a certain length until the mechanical limits occur. Here in figure 15 below is the well X7 drag chart in attempt for the maximum horizontal section extension.

Measured Weight (kip)-150 -100 -50 0 50 100 150 200 250 300 350 400 450 500 550 600 650 700 750 800 850 900

Bit

Dep

th (f

t)

5000

6000

7000

8000

9000

10000

11000

12000

L E G E N DRotate Off BottomRotate On BottomTripping OutTripping InMax Weight YieldMin Wt. Helical Buckle

Fig. 15 - X7 Drag Chart for Maximum Horizontal

Section

The slack off from well X7 encounters the helical buckling line shown in the picture above. The helical buckling occurs when drilling at depth approx 8550 ft MD. If the string is rotating using the geopilot system, buckling will not occur, but it could be a problem for the completion design. This limits the depth of X7 well from the drag charts perspective at 8550 ft. For safety the 8405 ft MD is taken. This is because in the effective tension chart below at figure 16, when tripping in at 8405 ft MD the hookload graph nearly passes the helical buckling limit at an area about 400 ft from the surface on the drillstring. Here below is the effective tension chart of well X7 when drilling at 8405 ft MD.

Effective Tension (kip)-35 -30 -25 -20 -15 -10 -5 0 5 10 15

Measured D

epth (ft)

150

200

250

300

350

400

450

500

L E G E N DTension LimitHelical BucklingSinusoidal BucklingRotate Off BottomRotate On BottomTripping OutTripping In

Fig 16 - Well X7 8405 ft MD Effective Tension

Chart

Here below at table 2 are the results of the maximum horizontal length simulation for the wells at field X.


Table 2 Maximum Horizontal Section Possible

Addition 4.2 Drilling Scenarios Case Studies Heavy Weight Drillpipe Arrangement To improve the drillability one of the solutions is to apply more weight or “down” force to the bit. This action enables the string to have the extra pushing power. In this case the extra push force is provided by the same type of heavy weight drillpipe (HWDP) used in the BHA of well X6 since we simulate well X6. The pushing force from the heavy weight drillpipe will be optimum when placed in a nearly vertical or in low inclination and as the drilling process occurs the HWDP will then move to the lower part of the well and enter the high inclination or horizontal section where the push force is not effective anymore. The HWDP in the horizontal section will become the part that must be pushed so it’s not effective anymore and needs to be rearranged so the push force is in the “sweet” spot again. The simulation for well X6 is calculated in several scenarios:

• Adding heavy weight drillpipe directly after the original BHA assembly

• Keeping the heavy weight drillpipe at the “sweet” spot

Adding heavy weight drillpipe directly after the original BHA assembly and continue to drill In this scenario (scenario A) the HWDP is added after drilling the well X6 original TD and continued until exceeding the mechanical limits. From the drag chart (Appendix) the helical buckling will occur when drilling at 15400 ft MD, but at the torque chart the rotate on bottom torque will exceed the make up torque of the heavy weight drillpipe at 14800 ft MD. The make up torque input of the HWDP is assumed by the grade of the drillpipe with HT55 joints at the E - 67 Rig Data Book.

Heavy Weight Drillpipe at the Sweet Spot In scenario B the HWDP is placed at the “sweet” spot. The location needs to be studied further for the effectiveness and the time efficiency. For this scenario the HWDP is placed 4000 ft MD until surface. Here below in figure 16 is the configuration of the scenario B.

Fig. 16 - Well X6 Configuration for Scenario B

After doing several calculations with Wellplan, the B scenario the mechanical limit is the make up torque. In section 4.1 for well X6 after running in 10600 ft MD without the additional HWDP helical buckling occurs, so in scenario B placing the HWDP in the “sweet” spot after drilling 10600 ft MD is not possible. So for this case after drilling 10000 ft MD, HWDP is inserted. From the drag chart the helical buckling will occur at 15500 ft MD. From the torque chart, the torque will exceed the make up torque at 15400 ft MD. Scenario B could reach deeper than scenario A with an advantage point from the torque side. The B scenario involves more less HWDP length and weight so the torque is less than scenario A. Improving the make up torque can be done by the use of high torque equipments and joint is essential in ERD wells to improve the drillstring endurance in high torque situations. Reducing the torque can also be done by using mechanical friction reducer device or lubricants. Lubricant Additive Scenario To improve drillability, in this case the length of the hole, the addition of lubricant in the drilling fluid could be in use. The lubricant itself reduces the friction factor of the case hole and the open hole. As the friction factor is reduced then the resistant force when tripping in, tripping out and rotating is also reduced.


For this case the lubricant that is used is the lube 776 produced by MiSwaco and has an average friction factor reduce by 33 %. After simulating the torque and drag, the result from the torque and drag analysis is that this model could reach 16500 ft MD. Here below in figure 17 trough 18 is the comparison between the actual T&D data, the model with the match FF data and the model with the lubricant.

Fig. 17 - Well X6 Drag Chart Comparison from

Actual Data, Matched Model and the Model with Lubricant

Fig. 18 - Well X6 Torque Chart Comparison from Actual Data, Matched Model and the Model with

Lubricant

This is an example of an ideal use of lubricant. In the real world several considerations is critical when using the lubricants such as the lubricants ideal working condition (pressure, temperature, chemical condition), the last of the lubricant effect due to time and condition, formation damage consideration, etc. In the ideal condition, the use of lubricant significantly improves the drillability. Mechanical Friction Reducer Scenario Mechanical friction reducer is a device attached to the drillstring to reduce the contact area of the drillstring to the wellbore and creates a stand off

between the string body and the wellbore. It also acts like a bearing nut that rotates relative to the string. So it doesn’t rotate relative to the wellbore, hence the torque will be reduced. Here below at figure 19 is the illustration of the mechanical friction reducer device and on table 3 is the specification of the HD Super Slider used to simulate the scenario.

Fig. 19 - Mechanical Friction Reducer Device

(taken from Western Well Tools)

Protector Size 5.5"

Sheeve Diameter 7.75"

Overall Length 20 "

Rotational Friction Reduce 85%

Sliding Friction Reduce 50%

Table 3 Mechanical Friction Reducer Specification Usually mechanical friction reducer is located in the build section, the placement in the build and deviated section is the most efficient due to the contact area and force is relatively large than the vertical section. The placement is also usually within the casing section, the device could reduce the wear of the casing due to contact with the drillstring. The placement is from 3400 ft MD until 4800 ft MD, for every three joints of drillstring one device is placed. Here below on figure 20 is the comparison between the matched model, the actual data and the model with the mechanical friction reducer device.

Fig. 20 - Torque Chart Comparison


From the chart above could be seen that as soon as the device is inserted for about 4026 ft MD from the bit, at 5800 ft MD the torque is reduced for about 30%. After simulating the torque and drag, the model is simulated doing drilling without tripping again to simulate the maximum horizontal section. The result from the torque and drag analysis is that this model could reach 11600 ft MD. Trajectory Evaluation Scenario The trajectory to be compared is the smooth planned trajectory with a constant dogleg and curvature with the actual trajectory drilled that has a variation of dogleg and curvature due to operational condition such as the constant adjustment to the trajectory and the characteristic of the motor and steering system. Here below on figure 21 is the dogleg comparison between the planning and the actual.

Fig. 21 - Dogleg comparison

From the dogleg chart we could see that the actual plan has a uniform dogleg than the actual condition. From the theory friction factor is affected by dogleg, the result from the torque and drag analysis is that this model could reach 11000 ft MD. Hole Cleaning at Well X6 Hole cleaning is essential in horizontal wells, in ordinary vertical well hole cleaning or cutting transfer is countered by pumping the mud exceeding the slip velocity of the cutting. This could solve hole cleaning in ordinary vertical or low inclination because the cutting falls downwards against the mud flow direction. In horizontal wells, cutting still falls downwards due to gravity, but the mud flow comes from the side. So the slip velocity could not be countered.

In 65-90 degree well, the cutting will settle in the low side of the well and will form a continous layer, the drilling fluid will only pass the upper side of the hole above the drillpipe like shown on figure 22. In these type of well mechanical agitation to transport the

cutting to the high velocity fluid movement is needed regarding the cutting is almost still in the lower part. The cutting is stirred to the high side by rotating the drillstring with certain RPM and will be moved to the high velocity fluid by certain mud characteristics and will be carried until eventually it will fall to the lower side. This process continues until the cutting reach the low inclination part.

Fig. 22 - Fluid Movements in Annulus

(Taken from K&M ERD Design book) Well X6 also used this process in term of cutting transport. The geopilot used in the drilling of the 8.5” hole enables rotating the string while steering to accomplish the trajectory. In well X6 the drilling encountered losses at about 7400 ft MD. LCM material was spotted and during the spotting, circulation was conducted for about two hours. The result of this process could be seen on the pick up, slack of test. The indication of the friction factor reduce is after the circulation the trendline changes. Here below on figure 23 is shown the drag chart change of trend for well X6.

Fig. 23 - Well X6 Drag Chart Change of Trend due to

Hole Cleaning


4.3 Case Studies Summary From the cases above, we could see that drillability could be improved. The well X6 in ideal condition could be optimized and done by certain scenarios to be an ERD well. Here below is the summary shown in depth vs departure plot and the table trough figure 24 and table 4.

Fig 24 - Depth vs Departure Plot of the scenario cases

No Scenario TD (ft MD)

Departure (ft)

Departure vs TVD Ratio

1 Original Well X6 8820 6064 1.49

2 Continue Drilling 10600 7844 1.93

3 Trajectory Evaluation 11000 8244 2.03

4 Mechanical

Friction Reducer

11600 8844 2.17

5 HWDP A 14800 12044 2.96

6 HWDP B 15400 12644 3.11

7 Lubricant 16500 13744 3.38

Table 4 Summary Table of the Scenarios V. CONCLUSIONS Torque and Drag Analysis

1. Torque and Drag analysis enables better projection for drilling facility preparation and torque and drag reduction action

2. Torque and drag should be analyzed by distinct open hole and cased hole friction factors and should be derived from actual field data

3. Drag analysis should be conducted with matched friction factor from pick up, rotate

of bottom and slack off test (PRS test). The friction factors from the PRS data may vary due to the resultant force so it needs to be matched separately.

Potential Factors to Maximize Drillability

1. Drag analysis is highly dependent from accurate friction factor and string mechanical limitation to buckle. Sinusoidal buckling could be tolerated but helical buckling should be avoided that could end to a lock up.

2. Rotating the string could avoid the helical buckling limitation, but completion consideration is needed regarding some completion could not be done by rotating.

3. Adding additional weight could improve the downward push force and increase drillability. Design optimization and tripping scenarios needs to be considered

4. Lubricant addition gives a significant impact on friction reduce and drillability although the effect on inflow performance, the working conditions and the amount of time the lubricants are effective is a concern

5. The use of mechanical friction reducer is more a protection for the casing and the string itself than as a direct length improvement. Casing protection is a concern in ERD well where the casing and string are encountered frequently.

6. Trajectory optimization in well X6 gives an improvement to the drillability.

7. Hole cleaning strategies is essential in horizontal and ERD wells due to the different cutting and mud flow characterization.

VI. RECOMMENDATIONS

1. The field data acquisition needs to be improved

2. When adding additional weight by HWDP, design optimization and tripping scenarios needs to be considered

3. The effect on inflow performance, the working conditions and the amount of time the lubricants are effective, is a concern on using lubricant

4. Casing protection is a concern in ERD well where the casing and string are encountered frequently so the use of mechanical friction reducer could be done

5. Hole cleaning is essential to improve drillability. Hole cleaning that involves


rotation must be planned to minimize wellbore erosion

REFERENCES 1. Aadnoy, B.S., Stavenger, U., and Andersen,

Keith, Stastoil. : “Friction Analysis for Long-Reach Wells” SPE Drilling Engineering 1998, IADC/SPE 39391

2. Lesage, M., Falconer, I.G., Wick, C.J. : “Evaluating Drilling Practice in Deviated Wells with Torque and Weight Data” SPE Drilling Engineering 1998

3. Rae, G., Lesso, Jr., W.G., Sapijanskas, M. : “Understanding Torque and Drag : Best Pratices and Lessons Learnt from Captain Field’s Extended Reach Wells” SPE/IADC 91854

4. Aarrestad, T.V., Blikra, H. : “Torque and Drag – Two Factors in Extended-reach Drilling” SPE Drilling Engineering 1994

5. Quigley, M.S., Dzialowski, A.K., Zomora, M. : “A Full-Scale Wellbore Friction Simulator” IADC Drilling Conference 1990

6. Mimms, M. Krepp, T. : “Drilling design and Implementation for Ectenden Reach and Complex Wells”, 2nd Edition 1999. published by K&M Technology Group Texas

7. Rubiandini S, R. : “TM-3133 Perancangan Pemboran”, ITB, 2007

8. Rubiandini S, R. : “TM-2231 Teknik Operasi Pemboran”, ITB, 2007


APPENDIX

FINAL THESIS LEGAL RELEASE LETTER

well drillability – horizontal well...

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