The main requirement in sidetracking a well is the development of a lateral force to allow the bit to cut on the side of the hole.
BHA Geometry
The required lateral force shall be provided by the geometry of the BHA. The most effective BHA configuration to achieve a successful sidetrack kick-off is a mud motor with bent housing.
Kick-off Point Selection
If possible, the kick-off location shall be selected where a soft or soft-medium formation is present to increase the chances of obtaining a successful kick off and to reduce operational difficulties and minimise time.
A controlled ROP time-drilling circa 0.5 ft shall be maintained until confirmation of a successful kick-off is achieved.
Kick-off Plugs
If a series of plugs have to be set, the TOC of the last plug shall be established before POOH for kick-off assembly.
The kick-off plug cement shall have a higher than normal density to provide a firm kick-off surface. If possible it is better to batch mix the plug and rotate string whilst displacing cement. Drilling out of this plug shall not be attempted until the plug is capable of taking 12 000 - 20 000 lbs or string weight at 50 rpm as a maximum.
Casing Windows
When preparing to cut a casing window, the following guidelines shall be considered:
- Start out just below a collar if possible.
- When making the initial cut, care shall be taken to avoid casing collars and centralisers. The cut shall be initiated a minimum of 8 ft above a casing collar.
- Generally the lengths of window to facilitate an effective side-track are given in the following table.
|
Casing Size |
Minimum (ft) |
Optimum (ft) |
|
7in |
60 |
90 |
|
9 5/8in |
60 |
90 |
|
10.3/4in |
60 |
100 |
|
13 3/8in |
60 |
100 |
BHA Composition
A suitable assembly shall be agreed between the Directional Driller in consultation with the Drilling Supervisor. The number of non-magnetic components can usually be reduced by applying a survey correction algorithm to survey data. The following guidelines shall be considered:
The deflection of the bent sub will depend on the hole size, formation hardness and required deviation. The geometry of the assembly shall be checked to ensure there is lateral loading with the chosen motor length and bent sub for the hole size in which the assembly is run.
It is more economic to carry out the initial sidetrack with the mud motor/bent sub combination and then trip for a more optimum assembly for the remaining section.
If the assembly is to be used to drill ahead after the kick-off, a motor bearing bit shall be selected.
It is possible to run the MWD tool directly above the bent sub and utilise survey correction algorithms.
If side-tracking out of a casing window, the string shall be oriented using tool face high side readings or alternatively a universal bottom hole orientation (UBHO and gyro above the MWD).
Side-tracking on the Low Side of the Hole
The low side side-track can proceed to take off rapidly. There is a risk of creating high dog-legs coming off on the low side and it therefore may be preferable to come off low side to the right or left rather than purely low side.
Once the new hole has been cut, lateral separation shall be maintained to prevent collapse of the old hole onto the new hole.
Maximising Kick-off Success
In cases where external factors can reduce the chances of achieving a first time kick-off (Eg., hard formations, deep kick-offs, or restricted window lengths) the ideal assembly to maximise kick-off probability is:
PDC Side - tracking bit, mud motor, 1.5o bent housing, MWD, UBHO sub, DC’s.
Company’s experience suggests that a 1.5o fixed bent housing deflection is best. Initially, the assembly shall be time drilled until approximately 30 ft with 100 % formation returns, then the BHA shall be pulled to reset the deflection.