Water Injectivity Decline

Hi All,

I am working on water injectivity decline project in a sandstone reservoir. Injection wells needed to be subjected to periodic acid jobs. Produced Water Used was may or may not exposed to air. Water used was commingled water of nearby fields. Reservoir Permeability and porosity are as under

[B][U]Reservoir Permeability[/U][/B]:
Min = 1.1768
Max = 2950.7
Range = 2949.6
Mean = 116.28
Median = 35.177
[B][U]Porosity:[/U][/B]
Min = 0.05442
Max = 0.14983
Range = 0.09541
Mean = 0.09366
Median = 0.09304

To study the decline which things i need to focus or investigate? What experiments I must perform to identify the reasons behind the decline in the injectivity.
Pls suggest from your experiences and expertise

Firstly - massive perm range. Would suspect you have significant vertical heterogeneity with associated conformance issues. If the water injection is to drive sweep efficiencies, this may cause you problems that you should investigate

With injection declines that respond to acidizing, it is highly likely that you are introducing damage through poor water treatment. Typical areas to focus on are;
1. Biocide to redcuce bacteria
2. Filters to reduce fines
3. Better management of lines -> Focus on reducing/eliminating rust (dont forget your tubing in the well)

I suggest you first determine whether injectivity index decline is due to 1. Increasing reservoir pressure around the injector(s) well(s) or 2. Due to skin damage. This can be done by running a Fall-Off-Test (FOT) and comparing the interpretation with previous FOT.

If it's actually a matter of skin damage, from my experience, it's probable that the reason for the observed reduction in the injectivity index is water injection quality issues (mainly these two parameters: solids/fines concentration, and oil in water concentration), however there are also some other reasons that should be analyzed (mechanical-integrity related problems, scale deposition anywhere in the well or surface lines, bacteria, etc).

Try to make as many analysis with your current information, for example:

1. Check for existing FOT data.
2. Make Hall plots and correlate them with water injection quality measurements (injectivity is very sensitive to solids concentration, particle size vs pore throat size, oil in water concetration). If your problem is related with water injection quality you'll see a nice correlation between these plots.
3. Check for historical ILTs (injection logging tools, such as spinner logs): determine which units/intervals are critical.
4. Simple mass-balance calculations (such as VRR) will provide insights into whether your declining injectiviry is due to represurization around injectors or not.
5. Be familiar with the information of the actual filtering procedures applied in your field and compared with your reservoir pore throat sizes. Since your reservoir has a wide range of permeabilities, the skin damage due to fines (solids in the water injection stream) varies throughout the reservoir.