Leak-off

The MOSEET has three regions of operation. The cutoff region occurs for V g < Up. In this region, the drain-to-source current is the reverse saturation current of the back-to-back source and drain junctions. This leakage current is small but nonzero and allows charge to leak off capacitors which are isolated by cutoff MOSFETs. Because this is how bits are stored in dynamic memory (DRAM) ceUs, DRAMs must be regularly refreshed to retain their memory. 

There is a field operation method by which the fracture pressure gradient can be experimentally verified. Such tests are known as leak-off tests. The leak-off test will be discussed in Chapter 4. 

Hesitation continues until no pressure leak-off is observed. A further test of about 500 psi over the final injection pressure will indicate the end of the injection process. Usually, well-cementing perforations will tolerate pressures above the formation fracture pressure, but the risk of fracturing is increased. 

A fluid loss additive is described [1849] that helps achieve a desired fracture geometry by lowering the spurt loss and leak-off rate of the fracturing fluid into the surrounding formation by rapidly forming a filter-cake with low permeability. The fluid loss additive is readily degraded after the completion... 

Water-soluble organic polymers, sueh as hydroxethylcellulose, have been used to slow the leak-off rate of clear brines into permeable formations. Fluid loss or leak-off, however, can be effectively controlled only by bridging the pore openings with rigid or semirigid particles of sufficient size and number. 

Water-based polymers Thickener, to transport proppant reduces leak-off in formation... 

Fluid-loss additives Form filter-cake reduce leak-off in formation if thickener is not sufficient... 

The fluid leak-off during hydraulic fracturing can be modeled, calculated, and measured experimentally. Procedures for converting laboratory data to an estimate of the leak-off under field conditions have been given in the literature [1426]. 

Fluid loss additives are used widely as additives for drilling fluids. When fracturing zones have high permeability, concern exists about damage to the matrix from deeply penetrating fluid leak-off along the fracture or caused by the materials in the fluid that minimize the amount of leak-off. 

Static leak-off experiments with borate-crosslinked and zirconate-cross-Unked hydroxypropylguar fluids showed practically the same leak-off coefficients [1883]. An investigation of the stress-sensitive properties showed that zirconate filter-cakes have viscoelastic properties, but borate filter-cakes are merely elastic. Noncrosslinked fluids show no filter-cake-type behavior for a large range of core permeabilities, but rather a viscous flow dependent on porous medium characteristics. 

Penny, G.S., Conway, M.W., and Lee, W.S. "The Control and Modelling of Fluid Leak-Off During Hydraulic Fracturing," SPE paper 12486, 1984 Formation Damage Symposium, Bakersfield, January. 

Thus the amount of back discharge cannot be readily calculated. It would seem unlikely, however, that the potential difference would drop below the contact potential as the bodies are separated. This would mean that the actual charge leaking off would be only a small fraction of that originally present. 

A188 Balance / leak-off size B LEAK SI c 10 The decimal numeric value Is followed by "mm" or "in". Example 1.5 in or 38 mm... 

Graphite-filled PVC provides resilient coatings which can leak off dangerous electrostatic charges from explosives propellants (Ref 3) Remote control sites for handling explosives capable of withstanding detonations of up to 10 lbs TNT are described (Ref 4)... 

Here, P s and /<. s are the cesium permeability in the ith transport experiment and in the first run, respectively, and R = (Vfeed+ Vslnp)/VSLM, Vfeed, Vstrip, and VSLM are the volumes of feed and stripping aqueous phases and the volume of organic phase in the membrane, respectively. Kp is the apparent partition constant of the carrier between the SLM and both aqueous and stripping solutions. Values of this apparent partition constant can be deduced via linear regression of log PCs versus (i-1). They are equal to 128,100, 29,100, 106,700, and 295,000, respectively, for decyl-benzo-21-crown-7, calix[4]arene-bis(crown-6), calix[4]arene-bis(benzo-crown-6), and calix[4]arene-bis(naphtho-crown-6). Calix[4]arene-bis(crown-6) rapidly leaked off the membrane... 

By placing a small vinyl tube over the end of the sniffer probe (see Fig. 7.64), it is possible to press the tip of the probe over the area of the possible leak. This method closes the leak off from ambient air, which both increases the sensitivity at this location and helps to precisely pinpoint the leak site. If you hold the vinyl tube too long near the helium, it will become saturated, and your sniffer will be reading the leak from the plastic on the nose of the probe. If this saturation occurs, have another plastic tube (that has been kept away from any helium) ready to slip on. A saturated tube will lose its helium in several days. 

Leak-off or loss of acid through the walls of worm holes often results in worm holes being too short to provide significant productivity increase. Therefore, effective stimulation often requires retardation of the mineral dissolution rate. The use of microemulsions is one method to accomplish this retardation. The hydrochloric acid is injected as an water-in-oil microemulsion. The diffusion rate of the dispersed aqueous acid to the rock surface is slower than molecular diffusion of acid from a totally aqueous system. Thus the rate of limestone dissolution is retarded with the microemulsion system. 

Fig. 6. Retain capacity, (a) Leak off pressures and repeat formation test data from the North Sea Central Graben. The curve is the minimum in situ horizontal stress trend, determined from the lower bound of leak off test data, (b) Retain capacity in the North Sea Central Graben. Retain capacity is the difference between the minimum horizontal in situ stress and the fluid pressure at any point. At low retain capacity (1000 psi or lower) the likelihood of trap failure is increased (Gaarenstroom et al., 1993).

Fracture pressures are estimated from leak-off tests (LOT), where mud is pumped into the formation until the first evidence of fractures is detected. Leak-off pressure is the pressure at which the formation develops very thin fractures prior to rock failure. A great advantage of this kind of test is that it is an in situ test, thus we do not have to deal with relaxation or unloading problems. Fracture pressures estimated from LOT is in the range of 0.6-0.8 times the litho-static pressure, which is within the fracture pressure domain (0.7-0.9 lithostatic pressure) given by DuRouchet (1981). The overpressures do not reach fracture pressure (LOT) in any of the wells investigated. However, the pore pressure reaches 80% of... 

In most extensional basins it can be assumed that minimum total stress is horizontal, and maximum total stress (S,) is equivalent to vertical lithostatic load. From Eq. (3) it can also be predicted that minimum total stress plus tensile rock strength is equivalent to the maximum formation fracture pressures that are measured from leak-off tests performed after drilling out casing shoes (LOT), or more approximately from formation interval tests (FIT). As suggested by Gaarenstroom et al. (1993), the lower bound envelope of LOT values may correspond to the... 

Therefore, the photoconductor must not only be free of thermal carriers but it must be protected against the dark injection of external charge, as well. Injection from the base electrode may be avoided by a thin blocking dielectric—typically 50 A.U. of a dense oxide or polymer--thin enough to allow any accumulated residual charge to leak off between cycles. 

---