Analytical Representation of
Crossflow
and
Consequences of
Operational Details

Crossflow Following Shutdown

| From Workshop on Soft Formation, crossflow is an important issue for sand fill in injection strings.
| Field experience (e.g., Heidrun of Statoil) shows “shut downs are definitively linked with injectivity loss”.

Injection Into Multiple Layers

Injection Into Each Zone

Injection Into Multiple Layers

Skin Calculation

PEA23 Correlation

| Obtained from fracture situation.
| Based on sandstone reservoirs with permeability of 100 - 200 mD.
| Is the correlation valid for matrix injection?
| Is this correlation valid for other types of reservoirs or in other regions?

Skin From PEA23 Correlation

Spreadsheet Program

| Provide a simple platform allowing for a quick look of field data.
| Provide a quick look of plugging and thermal effects.
| Provide a convenient methodology for assessing whether or not fracturing will occur.

A Difficult Problem

| “It seems that the complete set of (crossflow) results is more complicated than what has been summarized above and is sometimes hardly understandable. It is therefore suggested to undertake an in depth study on the theme of cross flow in collaboration either with a university or a research institute.” - Statoil Report.

Analytical Approach

| An analytical approach was developed to better understand the problem.
| Boundary effects play an important role in understanding the cross flow concepts/contradictions.

Reservoir Boundary

| At early times, the well will not “feel” the present of the boundary and the solution for a well in an infinite reservoir applies.
| As injection progresses, BHP will depend on the reservoir boundary condition, for example, no-flow boundary condition versus constant pressure boundary condition.

Boundary Conditions?

Crossflow Following Shutdown

| A two-layer formation is considered here to better illustrate the important concepts.
| High Perm zone has a thickness of 50 feet, a porosity of 28.2% and a permeability of 1150 md.
| Low Perm zone has a thickness of 100 feet, porosity of 23%, perm = 338 md.
| Total injection rate = 45,000 BWPD.

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Crossflow Following Shutdown

Crossflow Following Shutdown

| What if injection time is longer?
| The analysis indicates a cross flow from low permeability zone to high permeability zone. Can the cross flow reverse direction? If yes, under what condition?

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Summary

| Even if the total injection rate is constant, the injection rate into each zone can be a function of time.
| There is nothing mysterious or unexpected in cross flow.
| Flow rate can be large enough to carry fines and sands into injection string.
| Immediately following shutdown, cross flow is from low perm zone to high perm zone. Cross flow can reverse direction.