Matrix Injection Modeling - Best Practices

Contacts

Tony Settari	ASettari@TaurusRS.com	Taurus Reservoir Solutions
Dale Walters	DWalters@TaurusRS.com	Taurus Reservoir Solutions
Karim Zaki	karim@advantekinternational.com	Advantek International

Summary

Matrix injection and radial flow are terms that are often used synonymously. This may or may not be the case in reality where there is likely in-situ permeability anisotropy. Nevertheless, the premise is that injection is at a formation face pressure that is not sufficient to open, create or propagate fractures. By default, for simplicity, most of the analytical relationships assume that there is no fracture (unless it is represented by skin) and presume that the fluid front advances equally in all directions at all times.

While most injectors are associated with propagating fractures (even if the rate of propagation is slow and sporadic), there are situations, particularly in soft formations, where matrix injection can be occurring. Data analysis and field experience have indicated a tendency for these formations to be characterized by short half-lives and the need for frequent workovers. Consequently, it is important to comprehend performance in such situations and to determine what sort of measures can be taken for mitigation and/or remediation.

Damage mechanisms incorporate in radial flow models have been evaluated. These are summarized in discussions of stimulation. Two tools have been incorporated into the PWRI Toolbox for evaluating radial flow. These are described here.

Key Issues

PWRAD

DSA

PWRAD - Produced Water Radial Damage Model:

Recommendation: Use the radial damage model PWRAD to understand progressive damage during matrix injection.

This is a model that assesses skin associated with radial matrix injection. The method used only looks at the overall skin or the equivalent impairment of permeability as an average in the flooded region. The model was developed to provide a quantitative description of damage and also to attempt to relate damage to some measurable data. The model does not solve for solids' transport and therefore it is more empirical than the WID model. , However, it contains several parameters, which can be related to laboratory data, and it also separates the effect of the completion skin. To date, it has been very successful in matching field data.

It is anticipated that the model can be used to relate the history matched damage model coefficients to reservoir properties and water quality. In addition, the model is very suitable for implementation in standard reservoir simulators.

The basic assumptions of the model are:

• Radial flow.
• Steady-state flow between wellbore radius, r_w, and fixed outer radius, r_e, at any time.
• Fixed reservoir pressure, p_e which is maintained at the outer radius, r_e.
• Vertically homogeneous with a constant thickness, h.
• Piston-like displacement of reservoir fluid by water
• A simple model of permeability reduction, relating permeability reduction to an two-parameter exponential function.

Completion skin is included as a discontinuous pressure drop at the wellbore radius.

Workovers (e.g., acid treatments) can be represented in the model by removing a part or all of the accumulated damage that existed at the time of the workover.

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