aticollision schlumberger

165 Drilling Office X (DOX) Technical Manual / Anti-Collision 165

6 Anti-CollisionThis section contains the following topics:

6.1: Introduction

6.2: Anti-Collision Analysis

6.4: Drill Ahead Rules and Alert Zone

6.5: Graphical Outputs

6.6: Reports

6.8: Reference

PrivateCopyright 2012 Schlumberger, Unpublished Work. All rights reserved.BG

C-Documentation\NanNanZhang,PaulBolchover,XinChen,YuTao,HendrikSuryadi,RichardHarmer,ElenaMcMorris,HuangJian\\2.6\ReleaseDate:19-Nov-2012\EDMSUID:1654259735\Produc


6.1 IntroductionCollision with neighboring wells must be avoided, especially when adjacent wellsare producing. Any unplanned well to well collision is a D&M Zero Tolerance rule.To protect safety of people at the rig site and the environment, every well designand execution follows the Anti-Collision analysis performed in Drilling Officesoftware. The results of these computations shall be included into Well DesignFile (WDF) and rigorously checked during the well design process.

To ensure a good understanding of the wellbore objectives and Anti-Collisionconcerns at the planning phase, it is important for Directional Driller to participatein pre-job meeting. While drilling the Directional Driller must follow Anti-Collisionmonitoring plan and report any issues before the violation takes place.

Anti-Collision planning begins with accurate surveys of the position of thesubject well and all existing wells in the vicinity, as well as a complete set ofproposed well plans for the vicinity. The surveys and well plans are used tocarefully map the relationship of the proposed new well to all existing wells andany proposed future wells. Drilling Office performs an Anti-Collision proximityanalysis of a proposed/planned well (subject well) against the surrounding(offset) well(s). In addition to the proximity analysis, Drilling Office allows theuser to output proximity maps and reports.




6.2 Anti-Collision AnalysisAnti-Collision analysis consists of the following three steps:

Step 1 Definitive database

Anti-Collision planning begins from collection of the definitive directionaldatabase, that represents the most accurate and current description of all wellpaths within the working area. In addition to definitive directional surveys,Anti-Collision analysis also has consideration for future well plans and stillempty slots. That is why the Drilling Office allows to perform Anti-Collisionanalysis against proposed future well plans along with the existing offset surveys.Based on the proximity calculations, the engineer prepares the Anti-Collisionreports and plots that are used to analyze collision risks.

NoteDrilling Office flags the close approach situation for the wells that containsurveys within the database only. Anti-Collision analysis is not complete if anyof the existing wellbores contain incorrect, incomplete or missing surveys. This isbecause according to D&M wellbore surveying and Anti-Collision standard, theDirectional Driller has a special responsibility to ensure the count of the well invicinity matches the Drilling Office well count.

Figure 6-1: Drilling Office Anti-Collision calculations

Drilling Office differentiates between definitive and non-definitive surveys. Whilemany surveys might be available in the same borehole (e.g. MWD and Gyro),only one survey is selected to describe the wellbore profile. More accurate oneis usually selected as definitive. Non-definitive surveys usually are not used forAnti-Collision analysis. To account for ongoing operations, the Drilling Officedifferentiate between Final Definitive and Working Definitive surveys - a finaldefinitive survey is the most accurate description of the entire well path, whilethe "working" definitive surveys are the most accurate description of the wellpath to the present TD position.

Step 2 Perform Global Scan




Global scan is the initial scan made to search the entire database. Duringthe global scan the Drilling Office identifies the wells that can be reachedconsidering the technologies of the current days. That way the Drilling Officefilters out the wells that are impossible to collide with. Global scan is performedat the surface location; subsurface survey data is not used during this step. Thearea of 12,500m + MDs* radius is selected around the subject well. All the wellswithin this area are called nearby. All the rest wells a classified as single. Onlynearby wells are participated in the next step of Anti-Collision calculations.

Figure 6-2: Global Scan. The DOX Scan radius is automatically set to [12,500m + measured depth of the subject well]

The dummy survey created for all empty slots during the previous step (DefinitiveDatabase) and future well plans can also be identified as nearby if their surfacelocation fall into selected area. Particular care needs to be taken that all recentlycompleted or still drilled wells are updated with the latest Directional Data andincluded into Anti-Collision analysis. The nature of the global scan makes itimperative that field data is stored in the logical order in the database projects.Nearby wells that have been stored in different database projects cannot bescanned against each other.

Step 3 Proximity Calculations

A proximity scan must be performed on all wells that have been identified asnearby. At this step, the subsurface offset directional data is used to calculatethe distance between subject and each nearby well, called center-to-centerdistance. There is a number of ways to calculate the center-to-center distance;however, not all of them are accepted for use in Anti-Collision calculations. TheDrilling Office uses two methods to calculate proximity to the offset well:

Normal Plane method

3D Least Distance Method

Figure 6-3: 3D Least Distance and Normal plane




The center-to-center is used to analyze the risks during an Anti-Collisionanalysis. Different directional companies and some operators have come todifferent conditions that would dictate stop the job point, a point where the risksof collision are considered significant to stop drilling, assess the risks and makea new plan of actions that would allow to drill a well safe for the rig personneland the environment. These conditions are usually referred to as Rules.Schlumberger Drill Ahead rules are covered in the following topics and basedon D&M Wellbore Surveying and Anti-Collision Standard 002.




6.2.1 Normal Plane MethodThe normal plane method of computation calculates the close approach bystepping down each offset trajectory at the user-specified depth intervals.

A measured depth (MD) interval is recommended for the normal plane analysismethod because in horizontal wells, a true vertical depth (TVD) interval mayhave several positions within the offset trajectory, creating discontinuous results.The stepping is performed down the offset trajectory to ensure that the proximityof the entire offset trajectory is analyzed, and to ensure proper analysis ofperpendicularly approaching wellbores.

At each step (interval) down the offset trajectory, this method scans the subjecttrajectory to determine where a plane normal to the subject trajectory intersectsthe offset trajectory at the interval point. This scanning method can result inmultiple planes that are all normal to the subject trajectory and all intersectthe offset trajectory at the same point. Multiple solutions usually only occur inextremely tortuous well paths but are not limited to this type of trajectory.

The minimum (least) distance of all will be reported by the Drilling Office ascenter-to-center distance for a given scan point.

The proximity line, lying in the normal plane and connecting the intersection pointsof the subject trajectory and the offset trajectory, defines the center-to-center(ct-ct) distance between the trajectories. The azimuth of the proximity line willreference either north or the high side of the well. If a north reference is used,the azimuth is computed as the angle between the proximity line (which lies onthe normal plane) and the projection of north onto the normal plane. If high sideis used, the azimuth is computed as the angle between the proximity line andthe projection of high side onto the normal plane at that point in the subjecttrajectory (see Figure 6-4).




Figure 6-4: Azimuth proximity

NoteThe normal plane proximity analysis method is the only method that providesundistorted close approach results on a travelling cylinder diagram (referencedto the subject well) for all possible well profile geometries.




6.2.2 Horizontal Plane MethodThe horizontal plane method steps down the subject trajectory at theuser-specified depth interval.

A true vertical depth (TVD) interval is recommended for the horizontal planeanalysis method because in highly deviated wells (with inclinations at orexceeding 90 inclination) it is possible to have multiple penetrations with thehorizontal plane and multiple distances from the same point in the subjecttrajectory. Multiple penetrations are also possible within the subject trajectory. Ameasured depth interval can also be selected for this analysis, in which case thecorresponding TVD at that MD in the subject trajectory is used as the depth ofinterest. A measured depth interval analysis is subject to the same defects asthe TVD interval but, in some instances, can provide smoother results (due to aconsistent interval). However, a given TVD interval in a highly deviated well canresult in rapidly changing proximity distances. If no depth interval is selected,Anti-Collision Analysis will calculate the proximity at each survey or planningstation in the subject trajectory. At each step down the subject trajectory, thehorizontal line that intersects the subject trajectory and the offset trajectorydefines the ct-ct distance between the trajectories.

In the horizontal plane analysis, the azimuth of the proximity line can referenceeither north or the high side of the well. If the north reference is used, theazimuth is a true north referenced azimuth. If high side is used, the azimuth iscomputed as the angle between the proximity line and the projection of highside onto the horizontal plane.

NoteHorizontal plane method is NOT to be used as an Anti-Collision Analysis tool.




6.2.3 Strengths and Weaknesses of the ScanningMethodsBelow, compare two methods used by the Drilling Office to define theCenter-to-center distance:

Figure 6-5: 3D Least Distance scanning method




Figure 6-6: Normal Plane scanning method

Both, 3D Least Distance and Normal Plane methods suffer from different butdistinct weaknesses, and therefore, both methods must be used during theAnti-Collision scanning process to investigate the potential of collision. As aresult, today the Normal Plane method is used to produce the Traveling Cylinderplot (discussed later) for graphical visualization of the no-go areas that the bitshall not cross, while 3D least distance is used to monitor the Anti-Collisionworking with the numerical results. The visualization of the Anti-Collisionsituation using 3D Least Distance method might distort the scale, and that iswhy is not used.

Below is the summary of strengths and weaknesses for each scanning method:




Table 6-1:

Normal Plane

Strengths Weaknesses

Travelling cylinder plots are undistortedand depict the true 3-D relative position ofsurrounding wells; these plots are especiallyuseful for forward projections.

Scanning down the offset survey ensures thatno portion of the offset well is missed. Thisability is especially important for detectingclose approaches between wells orientedperpendicularly.

Will not detect a close well that is passing justbeyond the end of the subject wellbore.

Table 6-2:

Horizontal Plane


Limited use as an anticollision tool withhigh-angle, designer wells. A spider plotprovides more information for anticollision,while depicting a horizontal perspective.

Useful for determining relative positions offormation penetrations that are essentially flat.

Extremely distorted travelling cylinder plots.

Table 6-3:

3-D Least Distance


Provides the true closest distance betweenwellbores for a given position, which is usefulin additional applications (well injection/fluidflow analysis). Anticollision uses are forblowout intersection for well killing proceduresand for active magnetic ranging anticollisionprocedures.

Distorted travelling cylinder plots.

Example #1:

In this example, the subject and offset wells are within 120ft away at the Subjectwell TD. While Normal plane does not show any indication of close approachsituation, the 3D least distance method does. Compare results for both scanningmethods below:




Figure 6-7: Offset Well Analysis using 3D Least Method

Figure 6-8: Offset Well Analysis using Plane Method




6.3 Scan MethodThe Anti-Collision analysis provides the following three standard methods forcomputing separation distances between the subject well and the offset well(s):

3-D least distance

Normal plane

Horizontal plane

The following illustration is provided to assist in understanding the three standardmethods of computing separation distances (see Figure 6-9).

Figure 6-9: Three methods of computing separation distances




6.3.1 3-D Least Distance MethodThe 3-D least distance analysis method calculates the nearest distance to eachoffset well by stepping down the subject trajectory at user-specified depthintervals. A measured depth interval is recommended for this method because inhorizontal wells, a true vertical depth (TVD) interval may have several positionswithin the subject trajectory, creating discontinuous results. Once the depthinterval is selected, at each step (interval) down the subject trajectory, this methodscans the offset trajectory to determine a plane that is perpendicular/normal tothe offset trajectory and intersects the subject trajectory at the interval point.Perpendicular to this plane is a tangent point of a spherical radius, centered onthe interval point in the subject trajectory. Mathematically, this distance is theshortest (least) distance between the subject trajectory and the offset trajectory.

This process can be visualized as if, at each interval point, it computes theradius of a sphere centered on the subject trajectory that just touches the offsettrajectory (see Figure 6-10), and defines the center-to-center (ct-ct) distancebetween the trajectories.




Figure 6-10: 3-D least distance method of scanning

The scanning can result in multiple planes all normal to the offset trajectory, whileintersecting the subject trajectory at the same point. In this case, DOX will reportthe minimum (least) as a center-to-center distance, as shown in below example.




Also considered as a possible solution among the multiple solutions are theend-points of the offset trajectory - surface location and well total depth (TD).These points are considered solutions even though the end points may notnecessarily fit the definition of a plane normal to the offset trajectory.

The proximity line connects the center of the sphere to the tangent point. Theazimuth of the proximity line can reference either north or high side. If the northreference is used, the azimuth is computed as the angle between the proximityline and the projection of north onto the normal plane of the subject trajectory. Ifhigh side is used, the azimuth is computed as the angle between the proximityline and the projection of high side onto the normal plane of the subject trajectory.




6.4 Drill Ahead Rules and Alert ZoneSchlumberger D&M Anti-Collision Standard follows Drill Ahead Rules asbelow. Each of them applies to the planned and drilled well trajectories at alltimes.

Rule 1: Surface rule

Rule 2: OSF (Oriented Separation Factor) rule

To avoid hitting other wells, we apply a minimum allowable separation (MAS)from offset wells for the well path in both rules with the largest MAS value beingdominant. At or near surface, OSF values are unrealistically high, because theEOUs are very small. The Surface Rule has been implemented to impose aminimum separation between wells until the OSF Rule MAS value becomesdominant. The MAS is applied along the offset well and defines the edge ofNo-Go Zone as well. If the trajectory or any of the projection indicates center tocenter distance is less than MAS, an exemption must be raised.

The Drill Ahead rules are based on minimum allowable separation (MAS) thatmust be maintained between subject and the offset well. The calculation ofMAS depends on the service provider or operator and usually programmedinto the directional software. Schlumberger Drilling Office contains a numberof pre-defined sets of rules, including some that were developed by the clients.If client requires their Anti-Collision rules are used, they will be applied alongwith the Drill Ahead rules used by Schlumberger; the more conservative of thetwo will apply.

The set of Anti-Collision rules is manually selected in Drilling Office by theDrilling Engineer (see below). If no Drill Ahead rules were selected, theAnti-Collision computation will not be possible.

Figure 6-11: Proximity calculations require a set of Drill Ahead rules selected.Ensure the correct revision of Drill Ahead rules is used

D&M Anti-Collision Standard S002 defines two rules for MAS calculation: Rule#1 Surface Rule and Rule #2 OSF Rule. Both rules are applied at all times,with largest MAs being dominant. Two rules have been used due limitation of




the OSF near the surface when the EOUs are very small. The Surface Rule hasbeen implemented to impose a minimum separation between wells until theOSF MAS becomes dominant.

While drilling the deviations from the plan are possible, the Anti-Collisionanalysis identifies the area (drilling tunnel) where the chances of collision arelow. The Drilling Office calculates the distance, called allowable deviation fromthe plan (ADP) that represents a safe zone of deviations in the direction to theoffset well.

= ADP CtCt MAS

whereCtCt is a center-to-center distance.

Negative or zero ADP means a violation of the Anti-Collision rules.

The results of MAS calculation may vary depending on the proximity scanmethod:

Figure 6-12: Anti-Collision calculations using 3D least distance method

Figure 6-13: Anti-Collision calculations using Normal Plane method




Where the Anti-Collision analysis indicates a violation of the drill-aheadconditions, the well trajectory must be redesigned and additional analysisperformed. If no other design options are reasonably available, an expertapprover may approve an exemption, depending on specific circumstances andthe risk assessment. Please refer to D&M Anti-Collision Standard 002 andD&M Appendix to SLB-QHSE-S010 Management of Change and ExemptionStandard for more details.




6.4.1 Surface RuleSurface Rule stipulates a single distance value that represents the MinimumAllowable Separation (MAS) between two wells. MAS for Surface Rule isalways calculated at Well Reference Point (WRP) which is located at thetrajectory position at its point of ground penetration. This will be ground-level(GL) or mud-line (ML) as appropriate.




6.4.1.1 MAS Definition Based on Surface Rule

The surface rule applies to all offset wells and allows drilling tunnel or ADPequal 20% of the well-to-well clearance. The surface rule MAs is capped at 10meters.

For all offset wells with a MAS@WRP > 10 m, a maximum MAS value of 10 m isto be used (see Figure 6-14).

Figure 6-14: MAS calculated from a subject well (grey)

Mathematically it can be represented through the following equations:

= + ADP CtCt R R )0.2 (WRP 1 2where R1= largest hole radius of the offset well

R2 = subject well radius




CtCtWRP = center-to-center distance at the well reference point

Figure 6-15: Surface MAS

In this case, the following is true:

= + = + +MAS CtCt CtCt R R CtCt R R[ ( )] ( )0.2 0.8 0.2WRP WRP WRP1 2 1 2This makes sure there is always separation between offset and subject wells.The effect is that the offset wells within 10 m of the subject well have a no drillzone / No-Go Zone of 80% of the initial clearance @WRP (see Figure 6-16).

Thus,

ADP = ct-to-ct MAS

ADP is short for Allowable Deviation from Plan. The MAS is individuallycalculated for each offset well. (For example, if there are 12 offset wells on aplatform within 10 m ct-ct of the subject well, there will be 12 individual MASvalues calculated.)




Figure 6-16: Clearance @WRP and No-Go Zone based on Surface Rule

Below is the example of calculations performed by Drilling Office:

Figure 6-17: Example of the Drilling Office output

Well 1:

For 30 holes and =CtCt ft17.30( ) , MAS is calculated by the following way:

= + + = + +MAS CtCt R R( ) ( )0.8 0.2 0.8 17.3 0.2WRP 1 2 30"2 30"2= + = + =+MAS ft( )13.84 0.2 * 13.84 0.5 14.3415 1512

Compare this result to the results in Figure 6-15 above.




Well 2:

For the holes radius, = =R R 30"1 2 and = CtCt 79.29 , MAS is calculatedby the following way:

= + + = + +MAS CtCt R R( ) ( )0.8 0.2 0.8 79.29 0.2WRP 1 2 30"2 30"2= + = + =+MAS ft( )63.43 0.2 * 63.43 0.5 63.9315 1512

Due to capped surface rule MAS, the final MAS for the Well 2 equals to 10meters or 32.81 feet.

From above, it is clear that MAS is calculated separately for each of the offsetwells. Calculated at the surface, Surface MAS of the same value is applied tosub-surface depths (see Figure 6-15) until OSF rule becomes dominant.




6.4.2 OSF (Oriented Separation Factor) RuleThe OSF rule uses a probabilistic approach for Anti-Collision analysis.Historically, the industry came to a number of different calculations that werebased on the ratio between well-to-well clearance and the cumulative uncertaintybetween two wells:

=Separation Factor_ ClearanceCumulative Uncerta y_ int

For example, traditional separation factor (often SF) used the semi-major axesof two borehole EOUs to calculate the cumulative uncertainty:

=+

SF ClearanceSemi Major Semi Major_ _well well1 2

Unfortunately, this method of calculation did not provide the same probability ofcollision for the same separation factor used.

Compare scenario below. On the right, we turned one of the EOU semi-majoraxis by 90 to be positioned away from us. Note the difference in EOUseparation; the greater it is-the less chances of collision.

Figure 6-18: The probability of collision based on the same traditionalseparation factor may vary

To analyze the risks of a collision, Schlumberger uses an Oriented SeparationFactor (OSF). Regardless, of situations, provided the same OSF, it guaranteesthe same probability of a collision. Instead of geometrical sum of EOUs, the OSFuses relative positional uncertainty between subject and the offset well:

=OSF Clearancelative Positional Uncerta yRe _ _ int




While it is often believed that RELATIVE positional uncertainty above is ageometrical sun of two EOUs along the probability line, this is not true. OSF is aprobabilistic analysis. Defining the boundaries of relative positional uncertainty,the calculation does not deal with the sizes of two ellipsoids of uncertainty. TheAnti-Collision is concerned with the possible position of the subject and offsetwell RELATIVE to each other; that is why ABSOLUTE uncertainties are notused for Anti-Collision analysis.

To comply with Wellbore Surveying and Anti-Collision Standard, RelativePositional Uncertainty in Drilling Office is calculated using 95% of confidencelevel and 3D. These setting are default as long as set of latest Schlumbergerdrill ahead rules was selected.

The boundary of the drill ahead rules are selected based on OSF=1.5 To complywith the standard, all offset wells must satisfy OSF greater, but not equal, a valueof 1.5 at all analysis points.

The MAS for OSF rule can further be calculated as follows:

= = +OSF 1.5 MAS R Rlative Positional Uncerta y

( )Re _ _ int

1 2

or

= + +MAS lative Uncerta y R R( )1.5 Re _ int 1 2where as previously R1= largest hole radius of the offset well, R2 = subjectwell radius

Since relative positional uncertainty is not a constant value, OSF based MASalso varies and represent an area that must not be entered unless approvedduring exemption process.




Figure 6-19: Along red NO-GO line the OSF=1.5

Alert System

To warn the Directional Driller about the approaching risks, Drilling Office wasbuilt using an Alert system. Any well that entered Alert zone must be monitoredwhile drilling.

The Drill Ahead rules consist of Surface and OSF rules; both are implemented atall times, however only one is dominant. The dominating Controlling rule willdefine the Alert Status in Anti-Collision Analysis (shown in red below).

Table Drill Ahead Rules are shown in red:

For example, if the Surface rule is dominant and MAS=8m, the drilling mustcease before center-to-center distance ahead of the bit equals 8m.




If the OSF rules is dominant and MAS = 25m for OSF=1.5, the drilling mustcease before center-to-center distance ahead of the bit becomes equal 25m.However, if you were to monitor Anti-Collision based on MAS computationsusing DD Toolbox, you would find that MAS value, unlike Surface rule, wouldchange from station to station. Instead, it is easier to monitor a threshold thatis a constant value, like OSF=1.5. MAS and ADP for OSF rule is monitoredgraphically by use of TC plot.

The Anti-Collision monitoring plan is always tied to alert status:

Under ALERT condition a details Anti-Collision report must be analyzed andincluded in the well design file (WDF). Traveling cylinder plot shall be used at theappropriate scale to identify the wells that are at the risk of the collision.

A MINOR risk well is an offset well where the 1


operations. The well must be re-designed to attain a minor risk status. Theexemption process must be repeated again and approval received beforeallowing a bit to cross that boundary.




6.4.3 The Drill ahead RulesThe Surface Rule and OSF rule each apply a MAS for the well path. SurfaceRule MAS is based on geometrical clearance between two wells. OSF RuleMAS is based on separation with relative positional uncertainty between twowells (see Figure 6-20).

Figure 6-20: Surface Rule MAS (Grey) and OSF Rule MAS(Red)




At or near surface, OSF Rule is not suitable, as EOU sizes are very small.However, the most common well collision problems are found at surface. That iswhere Surface Rule is often dominant (see Figure 6-21).

Figure 6-21: Surface Rule is dominant

If the wellbore trajectory fails either the Surface or OSF Rules by entering theNo-Go zone created by surface rule or OSF >1.5 rule is not allowed without anapproved exemption. Drilling MUST STOP (see Figure 6-22).This is applied tothe calculation at the survey point, projection to the bit and minimum 60m (180 ft)projection ahead.

Figure 6-22: Violation of Drill ahead Rule MUST STOP




6.5 Graphical OutputsThere are two types of graphical outputs from Anti-Collision Analysis results:

Traveling cylinder plot

Spider plot




6.5.1 Traveling Cylinder PlotTraveling cylinder plot is not unique to Schlumberger and used for Anti-Collisionpurposes by the oil industry since 1968. The main advantage of the travelingcylinder plot is its ability to clearly and accurately display drilling tolerances ordrilling tunnel. Drilling Office Traveling Cylinder is produced using Normal planescanning method. The azimuth of the proximity line on the traveling cylinder plotare referenced to the north. It is computed as an angle between proximity lineand north direction projected onto normal plane.




All depths that appear on the traveling cylinder plot are the measured Depths ofthe PLANNED trajectory. While drilling, the real trajectory is plotted relative tothe plan. The proximity to the offset wells is monitored by comparing the positionof the projections against the NO-GO zones. The NO-GO circles are plotted for aparticular depth around offset trajectories with the radius equal MAS that wascalculated using Normal Plane method. That is why NO-GO circles for travelingcylinder are based on either Surface or OSF rule, whichever dominates at thatpoint. The area between the center of the traveling cylinder plot and the NO-GOcircle is a drilling tunnel or ADP. The tolerance lines can be drawn connectingthe NO-GO circle edges for the same depths to form a NO-GO envelope. Referto Anti-Collision Procedures for more information on traveling cylinder plotand tolerance lines.



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