Background
Well collisions are avoided by maintaining suitably conservative separation distances from offset wells. These distances are often referred to as Minimum Allowable Separation Distances or MASDs, and they take into account the measurement uncertainties associated with the well trajectories. Offset wells that could represent a health, safety or environmental (HSE) risk are subject to the most stringent MASDs.
Recommendation
The Collision Avoidance Work Group recommends that dispensation from MASD rules should not be allowed for HSE risk wells. In particular, the probability of the drilling assembly failing to penetrate the offset well in the event of a collision cannot be reliably quantified and is therefore not a valid justification for allowing contravention of a HSE risk MASD.
Rationale
The probability of drilling through an offset well's producing or injecting string can be split into two components:
- P1, the probability of contact between the bit and the offset casing, which is a function of well separation and the uncertainty associated with the relative positions of the two wells - normally managed using MASD rules.
- P2, the probability of drilling through the casing, which is a function of the casing program, drilling tool type, drilling parameters, formation type, offset well monitoring for early warning of contact, etc.
Since P1 and P2 are independent, the overall probability of penetration is the product of these two probabilities (P1 × P2).
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