The role of fines during acidizing treatments

Drilling, workover, injection and production operations can all cause physical impairment of the sandface and formation immediately around a wellbore. Matrix acidizing treatments are aimed at removing this type of damage. To obtain maximum damage removal most operators use in general a hydrochloric acid preflush followed by a mud acid main flush. During most acidising treatments large amounts of acids, usually of the order of one hundred gallons per foot of formation, are pumped into the reservoir, where various interactions occur with the matrix constituents, for instance:

• 1.• Cementing material is dissolved and clays and fines are released from their setting.
• 2.• Clays and fines are dissolved and gradually reduced in size.
• 3.• Acid-released clays and fines can be dislodged and plug pore throats.
• 4.• The pH of the spending acid increases and secondary precipitates can be formed.

The relative importance of the above processes varies for each treatment and depends on factors such as the spatial distribution of cement and minerals inside the porous network, the types and concentrations of the acids used, the temperatures at which the reactions take place and the rates at which the acids are displaced into the formation. These effects have been investigated at KSEPL in a series of core flooding experiments, carried out under representative conditions (pressure/rate/temperature), supported with modern petrographic techniques (quantitative SEM analyses).