Many carbonate formations cannot be successfully fracture acidized at pressures above the formation fracture pressure using standard fracture acidizing techniques. This situation exists because standard techniques rely on acid etched, uneven removal of the fracture face to produce fracture flow channels or fracture conductivity. Many times the etched fracture face is either too smooth, softens with acid, or the formation strength itself is insufficient to prevent closure due to overburden pressure, as in chalk formations.
The "closed" fracture acidizing technique is designed to allow acid to flow through existing "closed" fractures below fracturing pressure in a channelling manner. Wide grooves or channels in the fracture face are then formed as acid is pumped at low rates and allowed to dissolve large flow channels. These grooves tend to remain open with good flow capacities under severe closure conditions and also allow fines or emulsions to be more easily produced. The initial fractured system can be natural fractures, previously created fractures, or fractures created and etched just prior to the "closed" treatment.
Laboratory test results on formation cores and field results verify this technique.