reservoir · volumetrics
Well Spacing (Drainage Radius)
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Inputs
acres
Description
Distinct from Land & Ownership's spacing-unit acreage (a legal/PLSS pooling-unit size, not a distance) and from Drilling's planned wellbore-separation calculator (directional-survey-based, wellbore-to-wellbore 3D distance) — this is the reservoir-engineering drainage-radius spacing concept: the drainage radius implied by a target per-well spacing area, and the minimum distance between offset wellbores so their circular drainage areas do not overlap and interfere with each other's depletion.
Variables
| Symbol | Unit | Description |
|---|---|---|
| re | ft | Drainage Radius |
| Spacing | ft | Minimum Well Spacing |
| A | acres | Target drainage area per well (e.g. 40, 80, 160, or 640 acres). 259 ha (≈640 acres / 1 section) is a common single-well drainage assumption for unconventional spacing. |
Assumptions
- The reservoir is areally homogeneous enough around each well that a circular drainage-area idealization is reasonable
- Wells are developed on a uniform, symmetric spacing pattern with equal per-well drainage areas
- No strong permeability anisotropy, faulting, or induced fracture network materially distorts the drainage shape away from circular
Limitations
- Assumes idealized circular drainage — actual drainage shape depends on permeability anisotropy, faulting, and (for unconventional horizontal wells) induced fracture geometry, which is often elongated rather than circular
- Gives a geometric minimum spacing only — it does not account for pressure-interference timing or degree, which requires a dynamic (simulation or interference-test) analysis
- Not the same concept as Land & Ownership's spacing_unit_acres (legal pooling-unit acreage) or Drilling's planned wellbore-separation calculator (directional-survey 3D distance) — do not substitute one for another
Use Cases
- → Development spacing pattern design: Convert a target per-well drainage area (e.g. 40, 80, or 160 acres) into a physical minimum spacing distance for well-placement planning.
- → Infill/refrac candidate screening: Check whether a proposed infill well location falls within an offset well's drainage radius before committing to a location.
- → Volumetric drainage-area assumption support: Cross-check the drainage area assumed in an OOIP/GIIP volumetric calculation against the spacing pattern actually being drilled.
Related Calculations
Region Notes
Permian Basin
Unconventional development spacing units (DSUs) of 1,280 acres are common, but per-well drainage within a DSU is typically modeled at 40-160 acres depending on lateral count and stacked-pay spacing.
Bakken
Historical 1,280-acre spacing units with 2 wells per unit imply ≈640-acre per-well drainage; more recent downspacing programs test 320- or 160-acre per-well assumptions.
Gulf of Mexico
Conventional reservoirs are typically spaced by structural/seismic drainage-area mapping rather than a fixed acreage rule, so this calculator's uniform circular idealization is a rougher approximation than in onshore unconventional plays.
Global
Always state clearly whether a reported "well spacing" figure is this drainage-radius concept, a legal pooling-unit acreage, or a directional-survey wellbore separation — the three are frequently confused in casual usage.
References
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