Porosity, fracture

N2 injection rapidly increases the methane production rate. The timing and magnitude depends on the distance between injection and production wells, on the natural fracture porosity and permeability, and on the sorption properties. N2 breakthrough at the production well occurs at about half the time required to reach the maximum methane production rate in this ideal case. The N2 content of the produced gas continues to increase until it becomes excessive, i.e., 50% or greater. 

Preferred fluid migration pathways are influenced by porosity and permeability, sedimentary sequences, facies architecture, and fractures. Porosity is a measure of pore space per unit volume of rock or sediment and can be divided into two types absolute porosity and effective porosity. Absolute porosity (n) is the total void space per unit volume and is defined as the percentage of the bulk volume that is not solid material. The equation for basic porosity is listed below ... 

Figure 12 Quartz precipitation that overlaps the timing of fracture opening results in dramatic crack-seal fabrics. Here, a pale blue-luminescing quartz grain has been repeatedly fractured to yield rafted grain slivers encased in blue- and red-luminescing authigenic quartz. Quartz precipitation ultimately did not keep pace with fracture opening as considerable fracture porosity survives (green). Cozzette Sandstone, lies Formation (upper Cretaceous), Eastern Piceance Basin, Colorado. Photomicrograph by Rob Reed.

Narr, W. and Currie, J.B. 1982. Origin of fracture porosity - example from the Altamont Field, Utah. Am. Assoc. Pet. Geol. Bull., 66 1231-1247. 

DC, dissolved cement DFG, dissolved framework grains DRC, dissolved replacive cements F, fracture porosity PP, primary porosity SP, secondary porosity ToT.P, total (thin-section) porosity. 

Porosity. The fraction of total (bulk) volume occupied by the voids is defined as the porosity of the porous medium. A porous medium can be classified according to the type of porosity involved. In sandstone and unconsolidated sand, the voids are between sand grains, and this type of porosity is known as intergranular. Carbonate rocks are more complex and may contain more than one type of porosity. The small voids between the crystals of calcite or dolomite constitute intercrystalline porosity (47). Often carbonate rocks are naturally fractured. The void volume formed by fractures constitutes the fracture porosity. Carbonate rocks sometimes contain vugs, and these carbonate rocks constitute the vugular porosity. Still some carbonate formations may contain very large channels and cavities, which constitute the cavernous porosity. 

Nemcok, M., Gayer, R., Miliorozos, M. 1995. Structural analysis of the inverted Bristol Channel Basin implications for the geometry and timing of fracture porosity. In Buchanan, J.G. Buchanan, P.G. (eds.). Basin Inversion. Geological Society Special Publication 88 pp. 355-392. 

These additional studies carried out by the Krasnodar Scientific Research and Project Development Institute have thus disproved the earlier interpretation of the oil trap as a reservoir of fracture porosity type. 

Five-millimeter wide strips of foil, 100 microns in thickness, were inserted between the inner walls of the tube and the glass strips. In this manner, the model formed a system of parallel strips, SO cm in length. The width of the fractures varied depending on their position along the cross section of the tube. The openings within these cracks were 100 microns wide, corresponding to the presumed size of fracture opening in the real reservoir of the fracture type. Fracture porosity of such models calculated by the method of material balance equation was 13.5%. 

First, a discrete block is selected in an oil field which is made up mostly of reservoir sections of microporosity type and of some sections of macroporosity (fracture porosity) types. Next, wells are drilled into this block at close spacing of not more than 100 m either on a checkered grid or in rows. The cyclic steaming of the reservoir block follows according to a pre-set plan. With the arrangement of wells in rows, steam is injected into the second line of wells. The row of production wells is in the middle. 

Reduced heat loss represents one of the advanta s of the cyclic-steaming-by-block. In this connection, special studies should be carried out to develop methods for determining heat losses in oil-bearing formations with reservoirs of fracture porosity type. Such methods are not available at the present time so that it is difficult to make even any approximate calculations. 

Characterization of fractures is difficult. A volumetric description by fracture porosity in most cases cannot explain the effects. Additional parameters describing geometry and orientation are necessary (for example aperture, aspect ratio, crack density parameter). Therefore, imaging technologies (acoustic, resistivity) in logging techniques are a very important component for evaluation and detection. 

The calculated tensor elements allow studies of anisotropy as well as of the velocity ratio Vp/Vs- Figure 6.36 gives an example of forward calculation for the shear wave anisotropy (Thomsen parameter y) as a function of fracture porosity and aspect ratio. Clearly expressed is the strong influence of aspect ratio on anisotropy. Such studies can help to interpret and understand shear wave splitting effects. 

Rasmus (1983) states that the ratio of fracture porosity to total porosity is critical for the amount of resistivity decrease in low porosity carlxMiates. For fracture porosity, m values are less than the common matrix value of 2.0 in a low porosity limestone and can approach values as low as 1.0. If a secondary vuggy porosity is present without any fractures, a resultant m value of at least 2.0 is recommended. Depending on the amount of non-connected vug porosity, Rasmus (1983) recommends values exceeding 2.0. 

Figure 8.15 shows the plot with an equation assuming m = 2. The upper part with m >2 describes the effect of isolated vugs, the lower part with m <2 the effect of fractures and connected vugs. This figure is identical the plot isolated and fracture porosity (For-la) in the Schlumberger Log Interpretation Charts (2000). 

FIGURE 8.15 Model calculation of m for a three-porosity model with matrix porosity, partial isolated vug porosity (phi-sv), and fracture porosity (phi-frac). 

The Hanai-Bmggeman equation (11.7) can be applied on carbonate rocks with a matrix porosity and fracture porosity. If fractures have a preferred orientation, conductivity is a tensor. The tensor simplifies if the axes of the... 

FIGURE 8.19 Fracture praosity effect in a material with matrix porosity of < , =0.10. Input CnM =0.01 Cwi aspect ratio a=0.01 (red curve) and a=0.10 (black curve), (a) Ratio of rock and water conductivity and versus fracture porosity (b) Ratio of rock and water... 

Figure 8.19a is plotted = and versus fracture porosity and Fig. 8.19b is plotted the ratio of rock resistivity and water resistivity (=formation factor of the dual-porosity system) versus fracture porosity Both plots demonstrate... 

In fractured or cracked rocks, thermal conductivity is additionally influenced by the properties of crack-filling materials, fracture porosity, geometry, and distribution. 

FIGURE 9.16 Thermal conductivity versus fracture porosity calculated with inclusion model (a) thermal conductivity of matrix material (quartz) l = l.5 W m thermal conductivity of pore fluid (water) lfi = 0.6 W m (b) Thermal conductivity of matrix material (carbonate) lnja = 4.5 W m and thermal conductivity of pore fluid (water) lg = 0.6 W m. ... 

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