Matrix Injection

Matrix injection is considered to be operations where the formation face pressure is low enough that fractures are not created, reopended or propagated. While you are considering matrix injection operations, consider the following.

1. Water injection for voidage replacement/sweep is a requirement for secondary recovery and pressure maintenance schemes. Usually PWRI is not strictly a disposal operation. As such, it is essential to operate to maintain injectivity and to develop the most effective vertical profile. Just as with fractured injectors, even if the well has enough capacity, it may be a failure if it does not effectively participate in recovery.
2. (Filtered) seawater or aquifer water have (are) normally been used. The rationale for this has been the observed exceptional decline in many matrix injectors.
3. Formation damage caused during drilling and completion of the injectors may impart permanent sub-par performance of the injectors regardless of the quality of the injected water. This means that, like every other aspect of reservoir management, considerations for injection start when (before) the well is spudded.
4. Depth of impairment during the construction and early operational periods of the injector is a central issue for the prediction of well performance and the evaluation of damage mitigation strategies during matrix injection of produced water.
5. Many injectors (if not the majority) may have been converted from earlier producing wells to save cost. For converted producers, there is a not insignificant possibility of causing permanent plugging in the near-well region due to fines migration, drawdown and comapction, especially in soft sediments. There usually is a good reason why the producers are no longer desirable - this often being that production has dropped to economically unsatisfactory levels that cannot be recovered by stimulation.
6. Substantial filtering is required during Produced Water Matrix Injection (PWMI).
7. Often, operators believe that they are in matrix injection flow regimes when, in fact, the wells may have been unknowingly fractured. It is essential that this possibility be investigated for proper injection well performance design and prediction.
8. There are numerous methods/models for prediction of injector's half-lives. These are still of limited validity and most require precautionary warnings and safety factors.
9. The matrix injection scenario also designates wells have the following common, desirable (or essential) completion types - openhole, perforated, gravel packing, stationary propped fracture, and non-propagating open fracture.
10. Matrix injection imposes certain economic issues that must be considered. Filtering/regular acidizing requirements increase CAPEX and OPEX. Economics is a common theme for selecting the injection scenario.
11. In addition to the technical considerations for injection below or above the fracture gradient, there is a need for an adequate mapping of the costs and economics required to rank the various injection/completions options (matrix injection vs. fractured injection vs. discharge vs....).

For More Information

• Matrix or Fractured? Your first task, in almost any aspect of produced water injection management is to determine whether or not you are injecting under matrix or fracturing conditions. Guidelines are provided for helping you to determine which scenario is the case. Once you have made this evaluation, you can proceed to evaluating mitigation and/or stimulation protocols.
• Damage Mechanisms: The next step in determining any sort of mitigation/stimulation is to be certain what the cause of the reduction in injectivity is. What are the main physical or chemical mechanisms by which produced water injectors are damaged. These include plugging, sand production, relative permeability degradation, corrosion, scaling and bacteria.
• Click here for a high level discussion of mitigation and stimulation considerations for matrix injectors.
• Various models have been developed in the last two decades or so for forecasting matrix injection performance. To effectively determine the need for stimulation, at least a qualitative understanding of future injector performance is required. This link provides an overview of models available, the pitfalls, and suggests logical procedures for using available matrix injection models.
• Water Quality is an especially important consideration for matrix injection. Some guidelines are available for determining the water quality parameters that you may need.
• Methods of analysis are available for appraising and using available matrix injection information. This field example demonstrates the philosophy of evaluating matrix injectors.
• What are the Best Practices for designing and performing matrix strimulation treatments?
• What are the Tools that have been developed for designing and performing matrix strimulation treatments?
• There are field examples that demonstrate important considerations for matrix stimulation.

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