Wellbore Filter-Cake Breakout System
Design Specification
Water-Based and Oil-Based Mud Filter Cake
System A A-6S-W System B B-6S-O
1. Scope and System Basis
1.1 System definition
This specification defines two wellbore breakout systems. System A, code A-6S-W, treats water-based mud filter cake: six displacement stages, total fluid volume 8.0× openhole volume (29.287 m³ per 100 m). System B, code B-6S-O, treats oil-based mud filter cake: six displacement stages, total fluid volume 10.0× openhole volume (36.61 m³ per 100 m). Stage volumes are quoted per 100 m of 8-1/2 in. openhole.
Both systems are designed from the scale composition of the filter cake upward. The stage program follows the priority order of the scale types, and the breaker class for each stage is fixed by that priority rather than by product preference.
1.2 System comparison
Table 1 Comparison of the two systems
|
Dimension |
System A (water-based, A-6S-W) |
System B (oil-based, B-6S-O) |
|
Number of scale types |
6 (S1–S6) |
6 (S1–S6) |
|
Soluble solids basis |
CaCO3 (soluble) |
Barite (insoluble) + CaCO3 (soluble) |
|
Total fluid volume |
8.0× = 29.287 m³/100 m |
10.0× = 36.609 m³/100 m |
|
Stages |
6 (A1–A6) |
6 (B1–B6) |
|
Largest single stage |
A3 polymer breaker main stage, 2.0× |
B5 carry-out flush stage, 2.5× |
|
Preflush nature |
Aqueous (mutual solvent + water-wetting surfactant) |
Oil-based (base-oil slug); aqueous preflush not permitted |
|
Primary breakout route |
Enzyme → delayed acid / chelant |
Demulsification → water-wetting conversion → delayed acid / chelant + internal oxidizer |
|
Temperature ceiling, fastest route |
135 °C (enzyme) |
88 °C (internal oxidizer) |
|
Temperature ceiling, maximum |
177 °C (generic chelant) |
146 °C (D-SOLVER HD) |
|
Mid-temperature acid grade |
90–130 °C (N-FLOW 408 grade) |
90–130 °C (N-FLOW 408 grade) |
|
Key incompatibilities |
Enzyme × mutual solvent; enzyme × oil phase |
Aqueous fluid × stripped oil phase (re-emulsification); D-STRUCTOR HD × non-ZnBr2 brine |
|
Primary acceptance criterion |
Removal rate + cement-bond interface |
Removal rate + solids D90 passability + carry-out completeness |
|
Capability gap |
Above 177 °C; S6 residue |
Above 146 °C; barite carry-out efficiency not field-measured |
2. System A - Water-Based Mud Filter Cake
2.1 Scale types and composition
Table 2 System A scale types
|
Scale type |
Composition |
Role in the filter cake |
Operational hazard |
|
S1 Polymer network |
Starch fluid-loss additive, PAC / PHPA encapsulator, XC or biopolymer viscosifier |
Mechanical skeleton binding all solids into one body |
While the skeleton holds, acid and surfactant reach the surface only; a barrier forms between cement slurry and formation and second-interface bond strength drops sharply |
|
S2 Carbonate bridging solids |
Ground CaCO3 (weighting / bridging) |
Soluble solids filling the polymer skeleton; the bulk of solids mass |
Occupies volume so the cake cannot spall as a whole; if not dissolved it enters the annulus with flow during screen running and plugs slots |
|
S3 Bentonite and drill solids |
Sodium bentonite, fine formation cuttings |
Supply cake volume and thickness |
Thick mud ring, higher drag, stuck-pipe risk on trips; thick and uneven mud residue at cementing |
|
S4 Oil film and oil-wet solids |
Lubricant (oil-based or synthetic-based), crude influx, diesel |
Hydrophobic film on the cake surface |
Aqueous treatment fluid cannot wet the cake; enzyme and acid do not reach the target; oil-wet interface prevents cement bonding |
|
S5 Lost-circulation material (LCM) |
Fibre, granular and flake material |
Fills large pores, local sealing |
Fibres longer than slot width cannot pass - the direct cause of screen sticking; mostly acid-insoluble or partly soluble |
|
S6 Thermally degraded polymer residue |
Polyacrylate degraded above its temperature limit |
Turns into insoluble gel-like residue |
Enzyme, acid and surfactant all ineffective (see 2.5); one root cause of breakout failure in high-temperature wells |
Scale priority: S4 → S1 → S2 → S3 → S5 → S6. The first four are treatable, S5 is partly treatable, S6 lies outside the capability of this system.