Acid-removable skin

Sandstone acidizing is, for practical purposes, a method for removing acid-removable damage s) only. Except in rare cases, the production rate from an imdamaged well producing from a sandstone formation could be increased up to perhaps twice the original rate, at best. Acid-removable skin, discussed in detail in chapter 6, may manifest itself in the wellbore, in the perforations, or within the formation. 

Ascertain whether acid-removable skin damage is present. 

A well producing from a sandstone formation is a reliable candidate for acidizing only if acid-removable skin is present. With respect to add treatment design, it is important to determine not only whether damage (positive skin) is present but also, if it is, whether the damage can be removed with an acid treatment. 

Generally speaking, acid-removable skin is a reduction in permeability, caused by plugging or constriction in pore throats, that can be removed by acid. Non-acid-removable skin includes changes to the pore structure resulting in increased resistance to flow, such as wettability effects. Resistance to flow also may result from plugging by acid-insoluble materials, such as certain scales, paraffin (wax), and asphaltenes. 

Extremely damaged sandstones (very high acid-removable skin)... 

Preliminary Remarks. The material to be hydrolyzed should be in a milled or pulverized condition or reduced to minute pieces. Wood and pulp samples should be extracted with ethanol-benzene and ethanol according to TAPPI-Standard T6m-59. Each method described below can be applied to 2-50 mg of material. The evaporator for removing the TFA should be kept in a hood. Contact of the acid with skin and eyes must be avoided. 

Figure 16 CH2 asymmetric stretching frequencies, VajCH2 (originating from the SC lipids), as a function of SC weight removed, following treatment with ethanol alone (O) or 5% (v/v) [ H] oleic acid in ethanol ( ). Mean SD, n = 7 or 8. (From Ref. 153. Reprinted from Journal of Controlled Release, 37, Naik et al. Mechanism of oleic acid-induced skin penetration enhancement in vivo in humans, pp. 299-306,1995, with kind permission from Elsevier Science, NL, Sara Burgerhartstraat 25, 1055 KV, Amsterdam, The Netherlands.)...

Caution. Avoid skin contact with formic acid. Serious skin damage might occur. Use rubber gloves. If contact has occurred remove contaminated clothes and rinse with plenty of water ... 

Beilstein Handbook Reference) BRN 1792857 EINECS 211-466-1 Emerest 2324 HSDB 2177 Isobutyl stearate Kemester 5415 Kessco IBS 2-Methylpropyl octadecanoate Octadecanoic acid, 2-methylpropyl ester Stearic acid, 2-methylpropyl ester Stearic acid, isobutyl ester Uniflex IBYS. Emollient, lubricant for textiles, metalworking compounds slip aid, wetting agent for pigmented lipsticks, bath oils, nail polish and removers, skin cleaners, creams, lotions. Waxy solid mp = 28.9° bpis = 223° d = 0.8498 very soluble in Et20. CK Witco (Europe) S.A. CK Witco Chem. Cap Henkel/Emery Henkel Stepan Union Camp. 

Micellar acidizing solvents [99] are sometime used in removing skin damage and near-wellbore plugging problems. These micellar systems are... 

In a new well (or in a recompleted or reperforated well), if perforations are incomplete, then a positive skin value will be present. However, it is present as i pseudo-skin, not as skin due to acid-removable formation damage. The total skin effect may be written as equation (3.3), which refers to the large number of pseudo-skin factors that may exist, including phase- and rate-dependent effects partial completion and slant and incomplete perforations." In equation (3.3), s is the only skin that acidizing can address. 

If a well is not damaged or if it is but the damage is not acid removable, then there should be little to no expectation from an addizing treatment. Such a well should not be acidized. This is because addizing does not decrease skin much below zero, as can be shown from the radial-flow mathematics. However, an undamaged well may respond to hydraulic fracturing, or short skin bypass fracturing, if feasible from both a mechanical and a reservoir standpoint. 

It is the aim of sandstone acidizing treatments to reduce that portion of the total skin s) that is due to damage s). Skin damage must be present, but it must be acid removable, as manifested in the wellbore, in the perforations, and/or within the formation. In evaluating a well producing from a sandstone reservoir as a stimulation candidate, skin must be measured—or at the least, assessed as best as possible— to select the proper course of treatment (or nontreatment). 

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