Skin value

Its characteristics are summarized in Table II. As common for exploratory wells, whose purpose is not economical, its perforation density is low. Only 163 perforations had been opened, the perforated intervals being regularly spaced all over the entire formation height. Most of the initial skin value originates from this low density. 

Onset of the diversion stage (27.4 m3), with the skin now increasing due to the closure of some perforations by ball sealers (the rest of the curve would have shown that the relative skin value reached -2.5 at the end of the diversion stage). 

These authors calculated the SC permeabihty coefficient from the differences in resistances presented by whole skin and tape-stripped (no SC) skin. The permeability coefficients in the database were taken from the SC permeabihty values reported in Table 4. SC permeabihty values were not calculated for three chemicals (2-nitro-phenol, 2,4-dimethylphenol, and 2,4,6-trichlorophenol) because the permeabihty coefficient from tape-stripped skin was not available. The permeabihty coefficient values for these three chemicals were taken from the whole skin values in Table 1. 

Used muscle value, used skin value. ... 

The deeper the stimulation is, the lower will be the value of s. However, skin values less than approximately -5 are rare. Such negative skins exist only in wells with deep, conductive hydrauHc fractures (propped). Slightly negative skins may arise in frac-and-pack completions and in naturally fractured formations. If a well is neither damaged nor stimulated, then r = 0 (undamaged). 

Wells that show high skin values as measured by well testing (upward of 30, 50, 100, or even 300 or more) may be expected to have skin from other, nondamage effects. In such cases, stimulation can still be substantial, as long as is significant. 

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. 

Poststimulation well testing is the truest indicator of success or failure. Well testing should be conducted before and after stimulation to identify pretreatment and posttreatment skin values (preferably identifying skin damage [yj). If this is accompHshed, a genuine before-and-after comparison can be made, in conjunction with prestimulation and poststimulation production rates. 

In the simplest case, for a pressure drawdown survey, the radial inflow equation indicates that the bottom hole flowing pressure is proportional to the logarithm of time. From the straight line plot ot pressure against the log (time), the reservoir permeability can be determined, and subsequently the total skin of the well. For a build-up survey, a similar plot (the so-called Horner plot) may be used to determine the same parameters, whose values act as an independent quality check on those derived from the drawdown survey. 

From downhole pressure drawdown and build-up surveys the reservoir permeability, the well productivity index and completion skin can be measured. Any deviation from previous measurements or from the theoretically calculated values should be investigated to determine whether the cause should be treated. 

At sufficiently high frequency, the electromagnetic skin depth is several times smaller than a typical defect and induced currents flow in a thin skin at the conductor surface and the crack faces. It is profitable to develop a theoretical model dedicated to this regime. Making certain assumptions, a boundary value problem can be defined and solved relatively simply leading to rapid numerical calculation of eddy-current probe impedance changes due to a variety of surface cracks. 

The xylenes are mildly toxic. They ate mild skin irritants, and skin protection and the cannister-type masks are recommended. The oral LD q value for rats is 4300 ppm. The STEL for humans is 150 ppm. Xylenes show only mild toxicity to fish, and the threshold limit for crop damage is 800—2400 ppm. Biodegradation with activated seed is slow, and sewage digestion is impaired by 0.1% concentrations. In the event of a spih, oil-skimming equipment, adsorbent foam, and charcoal maybe used for cleanup. 

These salts are corrosive and are to be considered toxic because of the presence of Ag+ ions. The American Conference of Government Industrial Hygienists (ACGIH) (1992—1993) has adopted TWA values of 0.01 mg/m for silver metal and 0.01 mg/m for soluble silver salts. TWA for fluorides as F ions is 2.5 mg/m. The MSDS should be consulted prior to use. Skin contact and inhalation should be avoided. 

Although there is Httle toxicity information pubHshed on hydrides, a threshold limit value (TLV) for lithium hydride in air of 25 fig/has been established (52). More extensive data are available (53) for sodium borohydride in the powder and solution forms. The acute oral LD q of NaBH is 50-100 mg/kg for NaBH and 50-1000 mg/kg for the solution. The acute dermal LD q (on dry skin) is 4-8 g/kg for NaBH and 100-500 mg/kg for the solution. The reaction or decomposition by-product sodium metaborate is slightly toxic orally (LD q is 2000-4000 mg/kg) and nontoxic dermally. 

The threshold limit value (TLV) for cyclohexane is 300 ppm (1050 mg/m ). With prolonged exposure at 300 ppm and greater, cyclohexane may cause irritation to eyes, mucous membranes, and skin. At high concentrations, it is an anesthetic and narcosis may occur. Because of its relatively low chemical reactivity, toxicological research has not been concentrated on cyclohexane. 

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