Drilling Schedule and Multi-Well Forecasting

Overview

Real field development involves drilling wells over months or years. The drilling schedule determines when each well starts producing and directly shapes the field production profile. Multi-well aggregation combines individual well forecasts into a composite field forecast accounting for staggered start dates.

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Schedule-Based Aggregation

Concept

Each well in the field has:

• A start date (determined by drilling schedule)
• A type curve (production profile from first production)
• Individual parameters (peak rate, decline rate, b-factor)

The field rate at any time is the sum of all active wells:

$$Q_{field}(t) = \sum_{i=1}^{N} q_i(t - t_{start,i}) \cdot H(t - t_{start,i})$$

Where:

• $q_i$ = individual well production function
• $t_{start,i}$ = start date of well $i$
• $H$ = Heaviside step function (0 before start, 1 after)

Example

    ▲ Rate
    │
    │          Well 1    Well 2    Well 3
    │         ╱╲        ╱╲        ╱╲
    │        ╱  ╲      ╱  ╲      ╱  ╲
    │       ╱    ╲    ╱    ╲    ╱    ╲
    │      ╱      ╲──╱──────╲──╱──────╲────
    │     ╱                                 ╲
    │    ╱    Aggregate Field Profile          ╲
    │   ╱                                       ╲
    └───────────────────────────────────────────────▶ Time
        t1      t2      t3

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Drilling Schedule Parameters

Key Inputs

Buildup Rate

The buildup rate depends on how quickly wells are added:

$$\frac{dQ}{dt}\bigg|_{buildup} \approx \frac{q_{peak}}{t_{cycle}}$$

Where $t_{cycle}$ is the average time between new well starts.

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Cumulative Production

Field Cumulative

$$N_p(t) = \sum_{i=1}^{N} \int_{t_{start,i}}^{t} q_i(\tau - t_{start,i}) d\tau$$

For practical calculations, this is evaluated using the cumulative function of each well's decline model (Arps, modified hyperbolic, etc.).

EUR Estimation

Total field EUR:

$$EUR_{field} = \sum_{i=1}^{N} EUR_i$$

Where each well's EUR is calculated from its individual decline parameters.

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Type Curve Approach

Single Type Curve

The simplest approach assumes all wells follow the same type curve:

$$Q_{field}(t) = \sum_{i=1}^{N} q_{type}(t - t_{start,i})$$

This is valid when:

• Wells target the same zone
• Completion designs are similar
• Reservoir quality is relatively uniform

Multiple Type Curves

For heterogeneous fields, wells are grouped by:

Each group gets its own type curve, and the field profile is the sum of all groups.

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Practical Considerations

Plateau Constraint

When the aggregate well capacity exceeds facility capacity:

$$Q_{field}(t) = \min\left(\sum_{i=1}^{N} q_i(t - t_{start,i}), \quad Q_{plateau}\right)$$

During plateau:

• Some wells may be choked back
• New wells replace declining ones
• Plateau length depends on total deliverability vs. constraint

Production Efficiency

Actual production is less than theoretical due to downtime:

$$Q_{actual}(t) = Q_{theoretical}(t) \times E_{prod}$$

Typical production efficiency: 90-97% for well-managed fields.

Infill Drilling

Later infill wells often show:

• Lower peak rates (pressure depletion)
• Steeper decline (smaller drainage area)
• Interference with existing wells

These effects should be captured in the type curves for later drilling phases.

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Related Topics

• FPP Overview — Buildup-plateau-decline framework
• DCA Overview — Individual well decline models
• DCA Arps — Type curve equations for individual wells

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References

1. Arps, J.J. (1945). "Analysis of Decline Curves." Transactions of AIME, 160, 228-247.

2. Towler, B.F. (2002). Fundamental Principles of Reservoir Engineering. SPE Textbook Series Vol. 8.

3. Dake, L.P. (2001). The Practice of Reservoir Engineering, Revised Edition. Elsevier.

4. Mishra, S. (2012). "A New Approach to Reserves Estimation Using Field Production Profile Analysis." SPE-159646-MS, SPE Annual Technical Conference and Exhibition, San Antonio, Texas.

5. Doublet, L.E. and Blasingame, T.A. (1996). "Decline Curve Analysis Using Type Curves: Analysis of Oil Well Production Data Using Material Balance Time." SPE-35731-MS.