When a well is drilled, drilling muds are used to circulate the...

When a well is drilled, drilling muds are used to circulate the hole to clean out the cuttings (rock chips) and also to put applied pressure on the reservoir to prevent a blowout (the uncontrolled release of oil or gas up the hole). This drilling mud induces a ‘damage’ in the near well bore area caused by partial clogging of the pores that oil and gas flow through. That damage is called ‘skin damage’. Simply put, if the pure clean rock would flow twice as much as the damaged rock then it is considered to have a skin of +2. Skin can be negative also. By fracking or acidizing the rock you can actually make it capable of transporting MORE fluid than it was capable at virgin conditions. For example, a tough, tight rock like shale is very difficult for fluid to move through. By fracking the well, you are basically placing a crack filled with ‘beach sand’ for the fluid to easily get to the wellbore rapidly.