Fracture gradient

Figure 2-58. Poisson s ratio vs. depth (from Eaton, Fracture Gradient Prediction and Its Application in Oilfield Operations, Journal of Petroleum Technology, October 1969).

The fracture gradient is an important parameter that must be taken into account while increasing the mud weight and during well control operations. Several equations have been proposed for calculating the fracture gradient. The best results seem to be obtained with the one suggested by Mathews and Kelly [101] ... 

Eaton, B.A. 1969. Fracture gradient - prediction and its application in oil field operations. J. Pet. Technol., October 1353. 

As seen from Fig. 14 the maximum reservoir pore-pressure at the apex of hydraulic compartments II and III lie in the order of 100 bar below the minimum fracture gradient. This pressure difference (i.e., effective horizontal stress or retention capacity, R ) decreases with depth and goes below 70 bar at approximately 3500 m. 

By associating aquifer gradients (Fig. 12) with first-order spatial pressure domains and depth of burial, aquifer pressures for individual prospects can be predicted. Retention capacity as dictated by the pressure difference between the reservoir aquifer pressure and seal pore-pressure or fracture envelope can then estimated. The critical stage in this method is the selection of the correct aquifer pressure. Of the other variables required, the crestal elevation of the prospect is usually known with a reasonable degree of confidence, and seal pore-pressures are coincident with the fracture gradient which in turn is confirmed by measured (LOT/FIT) data. Application of this method within the GEA suggests that pre-Cretaceous seals retain hydrocarbon columns within the range from 200 to over 750 m. 

Friction pressures and viscosities are two important physical characteristics of well stimulation fluids. Friction pressures, fluid hydrostatics and fracturing gradients determine the minimum surface pumping pressure necessary to maintain fracture growth. Often friction pressure is the de-... 

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.

However, some overpressured traps, such as Kristin (Figs 1 5), with a substantial column of condensate exist in Halten Vest and preservation of petroleum in such high pressure isolated fault-bounded reservoirs is fundamentally no different from a hydrostatic system, i.e. the occurrence of the accumulation simply reflects that the fracture pressure at the given depth is higher than the actual reservoir pressure (cf. general principles discussed in Holm 1996). Thus, Kristin is presently sufficiently deep to avoid the fracture gradient and has also a very thick caprock (cf Figs 4 7). No obviously visible gas chimney exists above Kristin, whilst such is indicated over the Lavrans trap. 

Still, Smorbukk, which we advocate had experienced an event of leakage, remains oil filled today throughout, apart from the Gam Formation. Furthermore, Smorbukk Sor is oil-filled despite evidence for leakage into the Cretaceous strata above. The nearby Kristin field is overpressured but is sufficiently deep as to avoid the fracture gradient, and contains a condensate as does the normally pressured Lavrans (Fig. 5), which all, accordingly fall into Scenario-1 in Fig. 7. 

Water-flood injection for pressure maintenance and additional recovery often requires filtration (to remove suspended solids). Water injection pressures typically must be maintained at pressure levels below the fracture gradient pressure of the formation. 

Models for transport distinguish between the unsaturated zone and the saturated zone, that below the water table. There the underground water moves slowly through the sod or rock according to porosity and gradient, or the extent of fractures. A retardation effect slows the motion of contaminant by large factors in the case of heavy metals. For low level waste, a variety of dose calculations are made for direct and indirect human body uptake of water. Performance assessment methodology is described in Reference 22. 

Thermal Gradients may be measured or calculated by means of heat flow formulas, etc. After they are established it is likely to be found from the formula that for most cyclic heating conditions the tolerable temperature gradient is exceeded. This means that some plastic flow will result (for a ductile alloy) or that fracture will occur. Fortunately, most engineering alloys have some ductility. However, if the cycles are repeated and flow occurs on each cycle, the ductility can become exhausted and cracking will then result. At this point it should be recognized that conventional room temperature tensile properties may have little or no relation to the properties that control behavior at the higher temperatures. 

Subsurface Rock Fracture Pressure (Fracture Pressure Gradient). The subsurface rock fracture pressure can be approximated by utilizing the known pore pressure at the same depth. The relationship between rock fracture pressure p (psi) and pore pressure p (psi) is [34]... 

The rock fracture pressure gradient at depth can be approximated by using Equation 2-174 and the variable Poisson s ratios versus depth data (Figure 2-58) and the variable total overburden stress gradients versus depth data (Figure 2-59). 

In Figure 2-57 the pore pressure gradient has been given as a function of depth for a typical Gulf Coast well. Determine the approximate fracture pressure gradient for a depth of 10,000 ft. From Figure 2-57, the pore pressure gradient at 10,000 ft is... 

This value of 0.90 psi/ft falls on the dashed line of Figure 2-57. The entire dashed line (fracture pressure gradient) in Figure 2-57 has been determined by using Equation 2-174. 

In general. Equation 2-174 can be used to approximate fracture pressure gradients. To obtain an adequate approximation for fracture pressure gradients, the pore pressure gradient must be determined from well log data. ALso, the overburden stress gradient and Poisson s ratio versus depth must be known for the region. 

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. 

Compute the fracturation pressure gradient and fracturation pressure at 8,460 ft assuming a Poisson ratio of 0.4. 

Suppose that in some area the expected formation pressure gradient is 0.65 psi/ft and formation fracture pressure gradient is 0.85 psi/ft. A gas-bearing... 

Thiercelin, M.J. and Lemanczyk, Z.R. "Stress Gradient Affects the Height of Vertical Hydraulic Fractures," SPE Prod. Eng.. July 1986, 245-254. 

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