Depletion / Injection

The Depletion / Injection functionality enables evaluation of changes in in-situ stresses due to changs in pore pressure and the consequent impact on fracture stability. This is achieved with a Stress Path parameter that is derived from either models or direct measurements of changes in stress and pore pressure over time.

Scientific Background

Reservoir depletion and injection alter the effective stress acting on fractures, which directly controls their tendency to slip. The primary input parameter is Delta P +/-, representing the change in pore pressure:

  • Negative values correspond to depletion (pressure reduction).

  • Positive values correspond to injection (pressure increase).

In addition to pore pressure change, the stress change depends on the Stress Path parameter, which describes how horizontal stresses respond to pore pressure variations. Values can range from 0 to 1, with a commonly assumed value of approximately 0.6 used in the absence of direct measurements.

When Stress Path = 0, pore pressure changes do not affect total stresses, and only effective stress is modified. For non-zero values, horizontal total stresses evolve proportionally to pore pressure change:

The vertical stress remains unchanged. This coupling between pore pressure and horizontal stresses is a key mechanism controlling fracture reactivation and is widely discussed in the literature (e.g., Finkbeiner & Zoback, 1998; Engelder & Fischer, 1994; Santarelli et al., 1998; Addis, 1997; Teufel et al., 1991).

Depth Range

The Depth Range defines the interval over which depletion/injection effects are applied:

  • In Set of fractures mode, this range is fixed and automatically matches the analysis interval (TVD reference ± window), applying a constant stress gradient for all fractures within the range.

  • In Individual fractures mode, the user can define a custom depth interval where the modified stress state will be applied.

Processes (Sequential Modeling)

The Processes table provides a flexible way to model multi-stage reservoir evolution. Each row represents a distinct step in depletion or injection history, defined by:

  • Delta P +/- – pore pressure change for the step.

  • Stress Path – corresponding stress response factor.

The Active column determines how many processes are included in the calculation. All processes from the first row up to and including the selected active row are applied sequentially, meaning their effects are cumulative. This allows simulation of realistic scenarios such as:

  • Progressive depletion over time,

  • Injection following depletion,

  • Variable stress path responses during different phases.

The highlighted rows in the table indicate which processes contribute to the final stress state.

Calculation Conditions

The depletion/injection modification is applied only when at least one of the following is non-zero:

  • Delta P +/- ≠ 0, or

  • Stress Path ≠ 0

If both parameters are zero, the original stress state is preserved and no modification is introduced.

Effect on Slip Analysis

The updated stress state (modified pore pressure and horizontal stresses) is used as input to the slip analysis. This directly impacts calculated quantities such as:

  • Coulomb Failure Function,

  • Critical Pore Pressure

  • Critical Depletion / Injection

  • Critical Friction.

The resulting changes are reflected in all visual outputs, including stereoplot and Mohr diagram, allowing the user to assess how reservoir operations influence fracture reactivation potential.