Previous Injection History

Previous injection may have caused substantial changes in the near-well/fracture saturations and pressures, impacting fluid loss. Previous injection will reduce fluid loss and can also cause changes in the rate at which the fracture propagates. Settari and Warren considered this in detail. Beyond changes in formation pressure, it is certain that injection rates will vary and there will be injection plant downtime. In addition, many injectors are converted producers that were hydraulically fractured. These issues place additional demands on hydraulic fracturing simulators - including tracking formation pressure during shut-ins and multiple injection cycles. In addition, representing conductivity associated with previous fractures is an issue that needs to be addressed. To account for reopening and residual conductivity before and during reopening, Settari and Warren modified the permeability of the opened fracture using empirical information to account for roughness, tortuosity, turbulence and closure stress. This issue is becoming more and more important as finite conductivity below "closure stress" is acknowledged (Abou-Sayed, 1999). At the opposite end of the spectrum, rather than reopening, a fracture can recede if a high fluid loss regime is encountered.

In reopening (either for a newly converted, fractured producer or for an injector being brought back on line) any pre-existing cake will impact new cake deposition. Wennberg, 1998, provided a good analytical analog for this for radial injection. Suppose that there is an initial skin, s0. The injectivity during startup or reinjection is:

    (6)

where:

I0 is the initial injectivity,
I is the current injectivity,
q is the injection rate,
pwf is the injection pressure,
pR is the average reservoir pressure,
k is the permeability,
h is the thickness,
reis the external radius,
rw is the wellbore radius,
s0is the initial skin, and,
s is the current skin.

As can be seen, the higher the initial skin, the less the impact of cake that is deposited later. However, if there is initial skin, the injection skin, s, already has a head start and will develop more quickly.

<Fluid Loss Leakoff   PWRI is dominated by fluid loss.
 
 
 
 
Differences From Stimulation Models

An Introduction to PWRI Models

PWRI Models
  Conformance and Sweep> Conformance   PWRI is not just a disposal operation. Conformance and sweep are essential economic considerations, for ensuring adequate injectivity and for recovering bypassed hydrocarbons.
Differences From Stimulation Models

Types of PWRI Models

Audit of PWRI Models

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