Porosity vuggy

Figure 4. Pyrite filling vuggy porosity in core S-35 in the U.S. midcontinent. See Fig. 3 for the location of this core. Pyrite (bright phase) occurs as both intergrowths and intergrowths with hydrothermal dolomite. Both are growing into a void in the rock.

A DOUBLE-POROSITY POROELASTIC MODEL TO RELATE P-WAVE ATTENUATION TO FLUID FLOW IN VUGGY CARBONATE ROCK... 

When compressional, or P, waves excited by a seismic source propagate in an aquifer that is characterized by heterogeneities of different dimensions, such as variations in lithology and vuggy porosity, different regions respond with different fluid pressures. The associated fluid-pressure diffusion attenuates the wave energy. Three types of waves can be identified - a fast and a slow P wave and a shear wave. For the fast P wave, the pore fluid and porous matrix are compressed simultaneously for the slow P wave, the porous matrix relaxes when the pore fluid is compressed. 

Class II deposits in reservoirs made up of consolidated terrigenous sediments sands not running reservoirs lying at depths from 800 to 1500 m Class III deposits in reservoirs made up of carbonate rocks with complex porosities of fracture, cavern, and and vuggy types. 

The crystal carbonate fiicies is a secondary dolomitic texture characterized by the dominance of well-developed dolomite crystals. The most cxnnmon porosity types in the dolomitized parts are i) moldic and vuggy porosity, ii) intercrystalline matrix porosity iii) iniraciystalline porosity due to the selective dissolution of crystalline dolomite iv) karst and firacture porosity. The pore structures vary fiom monomodal to trimodal with an incr e of porosity and permrability according to the development of vugs and karsts. In this fiides, the reservoir properties are mainly controlled by the crystal size. 

Figure 2.21 shows permeability versus porosity for non-vuggy carbonates. The average particle size is used as the controlling parameter. Straight lines separate data collectives with respect to particle size. The figure confirms two tendencies as learned from siliciclastic sediments ... 

Following the rock-fabric classification, Jennings and Lucia (2001) developed a systematic plot presentation for non-vuggy carbonates and subdivided limestone and dolomite into three classes with rock-fabric numbers. The generalized carbonate permeability model provides a relationship between permeability, interparticle porosity, and rock-fabric number ... 

FIGURE 2.21 Permeability (air) versus porosity for various particle-size groups in non-vuggy carbonate rocks. Lucia (1983, 1999, 2007). 

Rock-fabric numbers range from 0.5 to 4 and are defined from porosity-permeabUity plots for non-vuggy limestone and dolostone ... 

In carbonate rocks, the time-average equatimi works for most intergranular, interparticle porosities this pore distribution is somewhat similar to sandstone. In vuggy carbonates, the slowness is dominated by primary intergranular, interparticle porosity, and the resulting porosity tends, in some cases, to be underestimated by the secondary porosity (Asquith and Krygowski, 2004). 

FIGURE 6.13 Interval transit time (slowness) and pOTosity for non-vuggy limestones in two wells and for oomoldic limestones from three wells compared with the Wyllie time-average curve and separate vug porosity values. Jennings and Lucia (2001). 

Focke and Munn (1987) found that low porosity carbonates with mostly interparticle (intercrystalline) porosity are associated with m values below 2. A higher percentage of moldic or vuggy porosity raises m values from 1.8 at low porosities to over 5 at high porosities. 

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. 

For (separate) vuggy porosity, Lucia (2007) describes that the exponent m is a function of the vug porosity ratio VPR defined as ... 

MilUer-Huber, E., 2013. New concepts for understanding the effects of complex pore structures on petrophysical properties of carbonate rocks. Dr. thesis, Montanuniversitat Leoben. MilUer-Huber, E., Schdn, J., 2013a. A modified archie relationship—a novel approach to evaluate carbonate reservoirs with regard to fracture and isolated vuggy porosity for Sw = 1. In Paper IPTC 16695 Presented at the 6th International Petroleum Technology Conference, Beijing, China, 26-28 March. http //dx.doi.org/10.2523/16695-MS. 

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