Leak-off tests

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. 

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... 

Fig. 4. Whilst the Smerbukk and Smerbukk Ser fields (Fig. la) today have pressures on the oil gradient just above the hydrostatic gradient, pressures in structures in Halten Vest are close to the estimated fracture pressures (from leak-off tests). Most wells in the overpressured region are dry, unless traps are sufficiently deep to avoid the fracture gradient, e.g. Kristin (Figs la 5), and this region coincides roughly with the area where the Spekk Formation is in the very last part of the oil window, or already over-mature.

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. 

Investigations to determine the leak-off control mechanisms of foam have shown (26—29) that the effective permeability of a porous medium is greatly reduced in the presence of foam. Some basic assumptions were used during the testing to determine the leak-off control mechanisms of foamed fracturing fluids. The first assumption was that the liquid or continuous phase moves freely, and permeability reduction is a function of the liquid saturation. The other assumption was that the gas or discontinuous phase flows only by rupture and reformation of the foam film. The resistance of foam to flow through porous media is a function of the stability of the foam. 

Laboratory tests comparing conventional and foamed add leak-off were completed in reference 26. 

The primary lid is made of forged carbon steel (TStE 355) and is bolted with 28 bolts to the cask body. The primary lid is sealed off with a metal and an elastomer gasket. In the primary lid, one penetration is located for performing the necessary leak-tightness tests. This orifice is closed with a small lid (flange) and a metal gasket. 

Valved off pressure versus time ( leak-up tests)... 

After catalyst charging and the flow vs. RPM measurement is done, the reactor should be closed and flushed out with nitrogen while the impeller runs, until O2 drops below a few tenths of a percent. Then a static pressure and leak test should be made by turning off the forward pressure controller and the flow controller. If an observable drop of pressure occurs within 15 minutes, all joints and connections should be checked for leaks and fixed before progressing any fijither. 

The example demonstrates that operating the compressor off the built-in pressure ratio means operating at a lower efficiency. This could be anticipated from Figure 4-3. The optimum port configuration for the various types of screw compressors was determined from a series of prototype tests. The change in volumetric efficiency is not a result of the built-in volume ratio, but is due to the increased slip (internal leak " n... 

A similar incident occurred on a solids drier. Before maintenance started, the end cover was removed, and the inlet line was disconnected. When maintenance was complete, the end cover was replaced, and at the same time the inlet pipe was reconnected. The final job was to cut off the guide pins on the cover with a cutting disc. The atmosphere outside (but not inside) the drier was tested, and no flammable gas was detected. While cutting was in progress, an explosion occurred in the drier. Some solvent had leaked into the inlet pipe and then drained into the drier [19]. 

In addition, they are usually constructed without isolation valves on the fuel supply lines. As a result the final connection in the pipework cannot be leak-tested. In practice, it is tested as far as possible at the manufacturer s works but often not leak-tested on-site. Reference 32 reviews the fuel leaks that have occurred, including a major explosion at a CCGT plant in England in 1996 due to the explosion of a leak of naphtha from a pipe joint. One man was seriously injured, and a 600-m chamber was lifted off its foundations. The reference also reviews the precautions that should be taken. They include. selecting a site where noise reduction is not required or can be achieved w ithout enclosure. If enclosure is essential, then a high ventilation rate is needed it is often designed to keep the turbine cool and is far too low to disperse gas leaks. Care must be taken to avoid stagnant pockets. 

When a plant was being leak-tested with nitrogen after a shutdown, a leak was found on a manhole joint on the side of a vessel. The pressure was blown off, and a fitter was asked to remake the... 

The period between such tests will depend on the maintenance schedule for the plant, which should also include the in-situ soundness testing of non-return valves and safety shut-off valves by bubble leak detectors, etc. 

Off-the-shelf catalogue sales of micro reactors have just started [15]. With an increasing number of commercial products, quality control will become more important. Brandner et al. describe quality control for micro heat exchangers/reactors at the Forschungszentrum Karlsruhe [23]. All manufacturing steps are accompanied by quality control and documentation. Leak rates (down to 10 mbar 1 s for He) and overpressure resistance (up to 1000 bar at ambient temperature) are measured. Under standardized conditions, the mean hydraulic diameter is determined. Dynamic tests supplement this quality control. 

When conducting laboratory hydraulic conductivity tests, two criteria should be met before testing is terminated. First, the rate of inflow should be within 10% of the rate of outflow. Measuring both the rate of inflow and the rate of outflow is necessary to detect problems such as a leak in the system or evaporation from one of the reservoirs. Second, a plot of hydraulic conductivity versus time or pore volume of flow should essentially level off, indicating that hydraulic conductivity is steady. 

System test—turn off all modules and power-up to perform startup diagnostics. Perform static leak test of the pump and compression test of the autosampler as suggested by the manufacturer. 

As explained on p. 22, thin glass phials cannot be tested for leaks directly with a Tesla coil, and must therefore be tested by the other method (Section 2.3.2) if they are suspected of being leaky. If a phial is leaky, it must be fused off and discarded. Because of the deleterious effect of a narrow tube on the pumping speed, the apparatus for filling phials should be joined to the vacuum line as close as possible to the pumps. 

Perform the vacuum leak test with an empty freeze drier and condenser. After reaching the maximum available vacuum, the vacuum pump is switched off after a delay time and the valves are closed. Monitor the vacuum decrease for a period of 24 h. Let the condenser cool to avoid water evaporation and vacuum decrease. 

Fix the leak rate and thus p . Allow the ribbon to come to a steady state at any value of 7 c, i.e., 7 c..id. Determine 9 by the flash-off method. Call this 6, or steady-state 9. Again let 0 increase to 0 at T = 7. Now reduce the leak rate to a very low value and wait until the pressure in the system reaches a new low value of p . This will be determined by the rate of gas evolution from the glass walls and the pump speed. Now suddenly raise the ribbon temperature to 7 i, at which the evaporation rate E is to be determined. From a preliminary test this T, should be so chosen that p will rise to about 100p . Record p vs. time t. E at any time t can then be calculated from the following equation ... 

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