Permeability layered reservoirs

Bioturbation, due to the burrowing action of organisms, may connect sand layers otherwise separated by clay laminae, thus enhancing vertical permeability. On the other hand, bioturbation may homogenise a layered reservoir resulting in an unproducible sandy shale. 

When a layered reservoir has high permeabihty contrast in vertically different layers, polymer can be injected through separate layers to control the injection profile, as mentioned previously. Another method is alternate injection of polymers with different molecular weights (MW). As discussed in Section 5.4.4, high MW polymer can be used in a high-permeability reservoir, and low MW polymer must be used in a low-permeability reservoir. For the alternate injection, the layers are grouped into different permeability layers high, intermediate, and low. 

As an example, some of the adsorption levels in Figure IS will be applied to the layered reservoir in Figure 8. The anionic DPES—AOS adsorbs at 0.11 mg/g on sandstone from 2.1% TDS reservoir brine. At this adsorption level, the surfactant can propagate 369 m in the upper, high-permeability layer. The betaine is comparable to the anionic surfactant in terms of gas mobility reduction. However, it adsorbs at 1.3 mg/g on sandstone from the same brine and would only propagate 109 m in the high-permeability layer. In a limestone, on the other hand, the betaine would travel 223 m compared to 20S m for the anionic surfactant. 

PERMEABILITY IN LAYERED RESERVOIRS FIELD EXAMPLES AND MODELS ON THE EFFECTS OF HYDROFRACTURE PROPAGATION... 

Brenner, S.L. Gudmundsson, A. 2001. Permeability development during hydrofracture propagation in layered reservoirs. Norges Ge-ologiske Unders0kelse Bulletin 439 pp. 71-77. 

FIG 5 DEPENDENCE OF POLYMER CROSSFLOW ON VERTICAL PERMEABILITY OF 2-LAYER RESERVOIR... 

CROSSFLOW OF OIL IN 2-LAYER RESERVOIRS OF DIFFERENT VERTICAL PERMEABILITY... 

This issue is discussed and illustrated using simulation results from a very simple two-layer reservoir with high- and low-permeability regions. This allows the illustration of the effects of a number of parameters on the basic fluid cross-flow mechanisms which occur. The dependence of cross-flow on features such as reservoir permeability contrast, vertical/horizontal permeability ratio (kjkx) and the relative thickness of the layers is examined. Although the cross-section is sufficiently simple to allow the performance of a wide range of sensitivities, it does capture the basic flow mechanisms very satisfactorily. This is shown later in results from much more realistic models which have been taken from real field systems. 

Figure 8.20. Dependence of polymer cross-flow on vertical permeability of a two-layer reservoir (Clifford and Sorbie, 1984).

Figure 8.30. Horizontal (x) and vertical (z) permeability with depth in the eight-layer reservoir cross-section.

In nearly all oil or gas reservoirs there are layers which do not contain, or will not produce reservoir fluids. These layers may have no porosity or limited permeability and are generally defined as non reservoir intervals. The thickness of productive (net) reservoir rock within the total (gross) reservoir thickness is termed the net-to-gross or N/G ratio. 

The macroscopic sweep efficiency s the fraction of the total reservoir which is swept by water (or by gas in the case of gas cap drive). This will depend upon the reservoir quality and continuity, and the rate at which the displacement takes place. At higher rates, displacement will take place even more preferentially in the high permeability layers, and the macroscopic displacement efficiency will be reduced. 

M. S. Determination of the effect of lipophilidty on the in vitro permeability and tissue reservoir charaderistics of topically applied solutes in human skin layers. J. Invest. Dermatol. 2003, 120, 759-764. 

The fifteen layers of constant permeability and porosity were taken as the reservoir zones for which these parameters would be estimated. The reservoir pressure is a state variable and hence in this case the relationship between the output vector (observed variables) and the state variables is of the form y(t,)=Cx(t,). 

In this case the observed data consisted of the water-oil ratios, gas-oil ratios, flowing bottom hole pressure measurements and the reservoir pressures at two locations of the well (layers 7 and 8). In the first run, the horizontal permeabilities of layers 6 to 9 were estimated by using the value of 200 md as the initial guess. 

It was also attempted to estimate permeability values for eight zones but it was not successful. It was concluded that in order to extent the reservoir that can be identified from measurements one needs observation data over a longer history. Finally, in another run, it was shown that the porosities of layers 5 to 10 could be readily estimated within 10 iterations. However, it was not possible to estimate the porosity values for eight layers due to the same reason as the permeabilities. 

In the first attempt to characterize the reservoir, all 15 layers were treated as separate parameter zones. An initial guess of300 md was used for the permeability of each zone. A diagonal weighting matrix Q, with elements y j (t j) 2 was used. 

Figure 18.26 Observed and calculated bottom-hole pressure and reservoir pressures at layers 7 and 8 for the 2" SPE problem using 7 permeability zones [reprinted from the Journal of the Canadian Petroleum Technology with permission].

Volumetric sweep efficiency is determined by the permeability and wettability distribution in the reservoir and by the properties of injected fluids. Waterflooding characteristically exhibits poor volumetric sweep efficiency. The more expensive the injection fluid, the more important it is to have a high volumetric sweep efficiency so that the injected fluid contacts and thus mobilizes a larger volume of oil. High permeability streaks or layers (thief zones) and natural or induced rock fractures can channel the injected fluid through a small portion of the reservoir resulting in a low volumetric sweep efficiency. 

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