Damaged zone refers to that region around the wellbore which has been negatively affected by the initiation, propagation and interaction between the drilling and/or completion fluids and the reservoir (and/or reservoir fluids). Another name for the damaged zone is the “flushed zone”.
Permeable formations are more susceptible to damage and the more permeable a formation is, the thicker the damaged zone will be. Damage zone thickness is a measure of how deep the foreign material (drilling or completion fluid) has penetrated into the reservoir. The goal during drilling and completion phase is always to ensure that the damaged zone is as thin as possible.
Damaged Zone Thickness
The damaged zone is not always a perfect circle around the wellbore. Some zones may experience greater damage than others. Within the same reservoir, zones with the highest permeability will have more fluid invasion and a thicker damaged zone than lower permeability zones.
Drilling practices may also influence how thick the damaged zone will be. Turbulence created by the drilling fluid or cement slurry will force fluids deeper into the producing formation. Damage from drilling or cementing fluids could originate from two sources: it could be from solid invasion or filtrate invasion.
For the first kind, fine solids from the drilling or cementing fluid can migrate into the pore spaces of the reservoir thereby plugging the pathway through which reservoir fluids would have flowed through. Damage can also be from fluid filtrate invasion into the producing formation. The drilling or cementing fluid filtrate could invade and react with the formation fluids to yield emulsions and precipitate paraffins or asphaltenes.
Emulsions and precipitates also block fluid pathways and serve as formation damage. The damaged zone thickness represents how deep the damage has penetrated into the reservoir.
Minimizing and By-passing the Damaged Zone
The drilling fluid plays a crucial role in limiting how thick the damaged zone will be through the plastering of a low permeability filter cake on the wall of the borehole. One role of a filter cake is to limit the extent of drilling fluid invasion into the reservoir.
The damaged zone gives an indication of how much the well’s productivity has been negatively impacted. Restoring that zone to its original undamaged state may be difficult, if not impossible. This is why, most times, it is better to adhere to practices that either prevent or minimize the occurrence of formation damage.
If, however, the formation is damaged then the solution might be to bypass this damaged zone by creating channels through which reservoir fluids can flow from the reservoir into the wellbore, as if the damaged zone did not exist. This is achieved through hydraulic fracturing or fracture acidizing.
Hydraulic fracturing or fracture acidizing creates fractures that penetrates into the reservoir beyond the damaged zone connecting the undamaged zone to the wellbore. Thus, when fluids flow from deeper undamaged portions of the reservoir, they simply flow through the channels created by hydraulic fracturing or fracture acidizing and by-pass the damaged zone.