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DCA vs RTA

DCA vs RTA
DCA vs RTA

 

Decline Curve Analysis (DCA)

DCA is an empirical technique that uses production rate data and time to estimate reserves.

Equation Used:

Arps Equations: An empirical equation that depends on:

  • Initial rate (qi)
  • Decline rate (Di)
  • Curvature of the decline (b)

Formula:

q= qi / ( 1 + bDit)^1/b

Main Features:

  • Requires a decline in production rate; not suitable for strong aquifer support or pressure maintenance methods
  • Assumes Boundary Dominated Flow (BDF)
  • Operates under constant conditions:
    • Constant choke size or wellhead pressure
    • Unaltered producing mechanism

Deliverables:

  • Production forecasting
  • Reserve estimation

Cons of DCA:

  • Non-unique results, especially for tight gas ( transient flow )
  • May underestimate reserves if production is temporarily increased by lowering drawdown (e.g., increasing Pwf or Pwh)
  • May overestimate reserves if minimum Pwf is not considered

Rate Transient Analysis (RTA)

RTA is a technique that uses production data, such as rate, pressure, and time, to estimate reserves and other parameters based on physical principles.

Equation Used:

Flowing Material Balance (FMB) Equation: Combines compressibility and Darcy's equations:

It is a combine of compressibility equation and Darcy one.

Formula:

(Pi - Pwf )/q= mpss * (Np /q)+ bpss

where:

mpss = 1/(ct * N)

bpss = 1 / Productivity index

Main Features:

  • Utilizes both rate and pressure data
  • Analyzes both transient and BDF phases
  • Based on physical principles, not empirical methods

Deliverables:

  • Production forecasting
  • Reserve estimation
  • Permeability (K)
  • Skin factor (S)
  • Original Oil in Place (OOIP) or Original Gas in Place (OGIP)
  • Drainage area and shape

Cons of RTA:

  • Best suited for single-phase flow; analysis can be misleading for two-phase flow
  • Requires production rate and flowing bottom hole pressure versus time, which may not always be available

Overall, RTA is a valuable tool for reservoir engineers seeking a physics-based understanding of reservoir behavior and a more robust estimation of recoverable reserves.

 

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