Extended Reach Difficulty Index Software - Information

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Well Path 1-4c

1. Adds complexity for greater veritcal depth, irrespective of the type of directional program or water depth.

2. Adds complexity for extended reach.

Very shallow-TVD and very deep-TVD wells cannot achieve the reach of aggressive moderate-TVD ERD wells. For aggressive shallow ERD wells, it becomes more diffcult to counteract frictional forces. For aggressive deep ERD wells, high mechanical loads become limiting (drill string and rig). Scoring reflects these trends. (drill string and rig).

Use of TVD below mudline eliminates deep water effects in this category.

3. Accounts for directional changes deep in the well, which add complexity beyond a simple build and hold plan. Use the ideal directional plan dogleg, not the actual-well survey cumulative dogleg (which will be much higher).

To calculate for 2-D well plans, simply add all of the inclination changes from spud to TD (e.g. for a 2-D horizontal well, this will be 90 deg). For S-turn wells, add the cumulative build and drop section doglegs, do not subtract.

For 3-D wells, do not add azimuth changes directly to inclination changes (i.e. at inclinations less than 90 deg, azimuth changes result in less total dogleg). Instead, use a survey calculation computer program to calculate cumulative dogleg from spud to TD.

For cases where a 3-D well ends in a horizontal section, 3 additional points are added to the score to account for the complexity of active steering at the bottom of the well.

4. Accounts for the complexity of hitting small targets in deep wells.

Since inclination is usually easier to control than azimuth in a deep well, the target width perpendicular to the well course (projected onto the horizontal plane) is used (e.g. the diameter for a circular target). Dipping targets typically rate no special consideration (special points can always be added in category 16 for unusually tough cases such as a steeply dipping target with strike nearly parallel to well azimuth). For multiple targets, use the smallest ratio of width/MD. (Horizontal wells are an exception to the above rules, since staying inside a narrow vertical window is the main challenge: use the total target vertical height tolerance in this case.)

Mud/Temp/Press 5 - 9c

5. Accounts for the complexity of managing a high weight mud system (e.g. solids control requirements, increasing viscosity, barite sag, materials logistics, etc.) without regard to mud type or well interactions.

6. Minimum complexity is associated with unweighted water mud (generally used in the simplest wells) and lower weight oil muds (which inhibit shale swelling). Complexity for higher pore pressures is covered in Category #5 above. Category #6 adds extra points for high weight oil mud (diesel, mineral oil, synthetic) since this increases the risk and cost of lost returns compared to water mud (oil mud tends to act as a "frac fluid" to propagate existing or induced fractures, which commonly occurs when drilling abnormal pressure wells).

7. Accounts for drilling through severely drawn-down sands and for underbalanced drilling (e.g. Austin Chalk) where formation fluids are flowing while drilling. For hole sizes of 8-1/2" and smaller, an accurate hydraulics model should be used to add circulating pressures (ECD), since these can be substantial (Caution: many hydraulics models do not account for rotational effects in ERD wells in smaller hole sizes)

8. Accounts for the complexity of managing mud systems and personnel safety due to high static well temperatures.

9. Used to identify challenges associated with small fracture gradient margins in hole sizes of 8-1/2" and smaller.

Shallow TVD ERD wells and deep TVD wells in these hole sizes can have high ECD's due to pipe rotation (ERD wells), cuttings pickup when circulation is initiated, and pressure surges due to pipe movement. A hydraulics program which accurately calculates ECD's is essential to make these calculations. ECD's in larger hole sizes are much lower and are usually not an issue. However, larger hole sizes can also be a challenge in deep water and when over-size casing is run (e.g. 11-3/4" inside 13-3/8"). For these cases, Category 16 "Points For Special Situations" should be used.

Casing/Redrill 10 a-11g

10. No points are scored for the very simplest wells which are drilled to TD below structural casing.

A single point is scored for running a long string to TD below surface casing.

Beyond this, there is a jump in complexity for each additional long string or liner that is run. For redrills or deepenings from existing "casing, count only the new strings run. For dry holes, do not count planned strings not actually run.

Points are added to the base score for each of the following special cases:

(a) one point for each liner that is tied back

(b) one point for each string requiring rollers, and/or full floatation (entire string empty), and/or addition of heavy pipe to the top of the string

(c) two points for high angle ERD wells for each casing string/liner requiring use of rotation and/or the differential floatation technique (mud over air) to get the string to bottom.

11. Accounts for the complexity of various types of redrill and slot recovery procedures.

For some wells, more than one type of "fishing" operation is required and multiple points are scored (e.g. cut/pull + section mill + cement plug = 6 points). Points should not be added to the basic score for a drive pipe whipstock slot recovery, since it includes an allowance for abandonment of the previous well. Note that redrills may result in a lower overall complexity than a new well due to fewer new casing strings, as accounted for in Category #9.

Well Type 12a-13d

12. Accounts for the complexity of drilling a particular well plan for the first time, and the subsequent improvements which occur for similar wells in the future.

Note that it might be appropriate to score points for challenging non-typical wells in an ongoing development program (e.g. a deep gas well drilled during a shallow oil development program). Exploration and Near-Field Wildcats (or "Outpost" wells) generally have a higher degree of risk and complexity, and are scored higher than the first well in a typical development program. When there has been no drilling in a field for more than two years, typically there are different engineering and operations personnel, and this case is treated the same as a first development well

13. Accounts for the complexity associated with various types of wellhead and BOP systems. Complex systems require additional planning and well time. They can adversely affect the probability of well success if they result in serious mechanical problems or lost time during open hole drilling operations (e.g. delays to pull a subsea stack for a hydraulics leak may result in a lost well section due to hole instability).

Program/Rig 14a-16c

14. The available rig can have a significant impact on ERD drilling. A "big rig" (e.g.3 mud pumps, 30,000' racking capacity) may place no constraints on well design or operations. Frequently, an existing rig must be used which was not designed with aggressive ERD wells in mind, and a limited number of operational constraints must be dealt with. The most challenging situation is a rig which cannot be used to drill a "full size" well (e.g. a long section of 12-1/2" intermediate hole), requiring downsizing of the casing program and significant compromises in operating parameters.

15. Accounts for the extra planning and trouble time associated with using a rig that is brand new or has been stacked for an extended period. There is also typically a "break-in" period when an active rig is new to the operator's drilling organization. This typically results in extra planning, the need to develop working relationships in the field, and possibly rig modifications.

16. This section allows you to add your own extra points for special situations which add to the complexity of the well. You enter a description and the points directly.