Reservoir geology and mineralogy

In damage assessment, it is necessary first to find information on the reservoir geology, as well as the mineralogy. An understanding of the rock type (sandstone or carbonate) and other features, including permeability and porosity, is critical. This includes the nature of porosity (matrix versus naturally fractured) and how permeability is distributed (i.e., how it varies) across the producing interval—or the injection interval, as the case may be. 

I was once asked to design a matrix acidizing treatment for an exploratory well in the Middle East. The acid treatment was expected to be required in order to induce production and continue with the exploration program. The first question I asked was, What type of formation is it The answer was, Either a carbonate or a sandstone. I then contacted the service company representative assigned to this treatment to ask for more details. 

When I told the representative that my understanding was that the formation was either a carbonate or a sandstone, he replied that that was more than he had been told. Sometimes there is not enough information to proceed with treatment design. That did not stop us, though, as we had no choice. The point of this anecdote is that whatever information is known about the reservoir must be gathered and applied. 

The mineralogical characteristics are also important. This includes knowledge of the bulk mineralogy, which defines the types of minerals present. Mineralogy also takes into account the location of mineral phases in and around the rock pore spaces and the natural fracture network, if present. 

Photomicrograph of sandstone formation pore space showing pore-iining smectite ciay covering iarge quartz grain surfaces (SEM) (Courtesy BJ Services) 

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