Application of Velocity-Deviation Logs in ...

1 World Journal of Earth Sciences, 2(1): 1-12, 2012 ISSN 2077-4656

Application of Velocity-Deviation Logs in Determination of Pore Types and Permeability Trends in Sandstones of the Nubia Formation from the Rudeis-Sidri Area, Gulf of Suez, Egypt 1

Tarek F. Shazly, 2Elhamy Tarabees and 3Youssif S. Mohammed

1

Prof. Ass. In Egyptian Petroleum Research Institute Prof. Ass. In the Faculty of Science, Damanhour University 3 Lect. Ass. In the Faculty of Science, Sohag University 2

ABSTRACT A velocity-deviation log, which is calculated by combining a sonic log with a neutron-porosity or density log, provides a tool to obtain down hole information on the predominant pore types in sandstones. This information can be used to trace the down hole distribution of diagenetic processes and to estimate the causative mechanisms for trends in permeability. We calculated a velocity-deviation log for sandstones of the Nubia Sandstone Formation in the area of Rudeis-Sidri, Gulf of Suez, Egypt by converting porosity-log data to a synthetic velocity log using a time-average equation. The differences between the real sonic log and the synthetic sonic log are plotted as a velocity-deviation log. These velocity-deviation logs reflect the different rock physical signatures of different pore types: (1) positive velocity deviations mark zones where frameforming pore types dominate; (2) zero deviations show intervals where the rock lacks a rigid frame and displays dissolution porosity or microporosity and (3) negative deviations mark zones where sonic log velocities are unusually low, caused by a cavernous bore-hole wall, by fracturing, or possibly by a high content of free gas. By tracing velocity deviations continuously down hole, we are able to identify various diagenetic zones and predict permeability trends, because pore types influence the permeability of the rock. Key word: Rudeis Sidri Area, Gulf of Suez, Well logging, Velocity Deviation Log, Porosity types, Permeability. Introduction Our study deals with the velocity-porosity relationship in sandstone where these physical properties follow a more regular down hole pattern (Hamilton, 1980). Our study shows how velocity-porosity correlations can be used to improve wireline-log interpretation by allowing prediction of other parameters such as pore type, extent of diagenesis and permeability trends. A velocity-porosity log is calculated by first converting porosity-log data to a synthetic velocity log using a time-average equation. The difference between the actual sonic log and the synthetic sonic log can then be plotted as a velocity-deviation log. Because the deviations result from the variability of velocity at certain porosity, the deviation log reflects the different rock physical signatures of different pore types (Rossebo, et al. 2005). Our study area is located in the Abu Rudeis-Sidri Field, which lies on the eastern coast of the Gulf of Suez about 25 km north of Belayim Land Field and southeast of the October and Rus Budran Fields. Our study was performed on nine wells (ARM-2, ARM-3, ARS-1, ARS-3, ARS-4, ARS-5, ARS-6, R-5 and S-8) distributed across the area as shown in Fig. (1). 1. Velocity-Porosity Relationship: Pore geometry has proven to be a crucial factor in controlling acoustic properties in sedimentary rocks (Brie et al. 1985). Anselmetti & Eberli (1999) and Kenter & Invanov (1995) distinguished four categories of pore types in thin sections, and each of these pore types has a specific effect on the acoustic properties due to the geometric relationship between the pores and the solid rock phase. Four basic types of porosity occur in sandstone: intergranular, dissolution, microscopic, and fracture. All sandstones initially have intergranular porosity, which commonly is associated with good permeability, large pore aperture and hydrocarbon production. Dissolution porosity, which may consist of isolated pores, results from leaching of feldspar, carbonate, sulfate or other soluble materials. Sandstone with dissolution porosity may have adequate porosity for a reservoir, but poor permeability. Sandstone with significant clay mineral content has abundant microporosity, which is characterized by small pore aperture, low permeability, and high

Corresponding Author: Tarek F. Shazly, Prof. Ass. In Egyptian Petroleum Research Institute E-mail: [email protected]

2 World J. Earth Sci., 2(1): 1-12, 2012

irreducible water saturation. Fracture porosity, which contributes no more than a few percent of voids to storage space, will enhance the deliverability of any reservoir.

33.166

33.150 28.865

N

28.865

ARM-2

ARS-4 ARS-1

ARM-3

R-5 ARS-3 ARS-5

ARS-6

28.855 0

28.855

1m.

33.150

S-8

33.166

Fig. 1: Location and distribution of wells in this study of the Abu Rudeis-Sidri Field. The different pore types form groups with characteristic clusters on a porosity-velocity diagram. Thus, most of the velocity scattering at equal porosities could be attributed to the occurrence of these dominant pore types. A crossplot of porosity vs. Vp (Figs. 2−10) displays a distinct inverse trend, where velocity decreases with increasing porosity. Measured velocities can be compared with those calculated from the time-average equation of Wyllie et al. (1956), shown as follow:

 1  1     ….    V V matrix   V fluid   rock 

(1)

This empirical equation states that the travel time of an acoustic signal through a rock is the sum of the travel time through the volumetric percentage of a pore phase filled by pore fluid plus the travel time through the volumetric percentage of the solid phase represented by a matrix velocity. Despite the inverse trend, the scattering of velocities at equal porosities can be higher. In log studies, this pattern of a strong velocity scatter is typically attributed to secondary porosity (Schlumberger, 1972 and 1974, Asquith, 1985; Doveton, 1994), which is measured by the neutron-porosity tool, but is avoided by the sonic log reading because the acoustic signal circumvents any secondary porosity (Merkel,1979). 2. Velocity – Deviation Log: To quantify the scattering of velocities at equal porosity values, the term "velocity-deviation" is defined as the departure of the sonic velocity from the velocity predicted by the time-average equation for the same porosity value and a given lithology (Morshedi, et al. 2010). Thus, a sample with positive velocity-deviation has a velocity that is higher than the velocity derived from the Wyllie equation, whereas a negative deviation has a velocity that is lower than the Wyllie equation would suggest for the porosity of the rock.

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Fig. 2: Cross plot of Vp (compressional velocity) and porosity of Nubia Sandstone Formation in ARM-2 well

Fig. 3: Cross plot of Vp (compressional velocity) and porosity of Nubia Sandstone Formation in ARM-3 well A velocity-deviation log can be calculated for a continuous down-hole record by comparing porosity and velocity information from wireline-log data. Velocity data are obtained from the sonic log, which measures transit times through a short vertical interval of rock (Eskandari, et al. 2007). Porosity values are obtained from the neutron-porosity log or from the density log. In the following section, the methods for calculating the velocity-deviation log will be explained.

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Fig. 4: Cross plot of Vp (compressional velocity) and porosity of Nubia Sandstone Formation in ARS-1 well

Fig. 5: Cross plot of Vp (compressional velocity) and vorosity of Nubia Sandstone Formation in ARS-3 well 2.1Calculation from Neutron Porosity and Sonic Logs: A standardized synthetic velocity log is calculated from the porosity values determined from the neutronporosity log. The porosity values are converted to velocity by applying the time-average equation (Wyllie et al., 1956). These velocities represent values that are expected from the measured porosity values. The synthetic velocity log can be compared with the actual velocities determined from the sonic log. The velocity-deviation is then obtained by subtracting the velocity values of the synthetic velocity log calculated from neutron-porosity log values from the actual velocity values calculated from the sonic log. Velocity-deviation = VP

(sonic)

− VP

(neutron)

….

(2)

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Fig. 6: Cross plot of Vp (compressional velocity) and porosity of Nubia Sandstone Formation in ARS-4 well

Fig. 7: Cross plot of Vp (compressional velocity) and porosity of Nubia Sandstone Formation in ARS-5 well These velocity-deviations can then be plotted as normal log data in plots of deviation vs. depth. The best graphical displays are made by drawing the deviation with respect to a zero-deviation line. 2.2Calculation from Density and Sonic Logs: In cases where a neutron-porosity log is not available, a density log can be used to create a synthetic velocity log. Once the conversion from density to porosity is performed, the procedure is the same as that previously described. Porosity values are converted to expected velocities, and the difference between that and the actual velocity log represents the velocity-deviation, as shown below: Velocity-deviation = VP

(sonic)

−VP

(density)

….

(3)

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Fig. 8: Cross plot of Vp (compressional velocity) and porosity of Nubia Sandstone Formation in ARS-6 well

Fig. 9: Cross plot of Vp (compressional velocity) and porosity of Nubia Sandstone Formation in R-5 well In the studied wells, we applied the second method to obtain the velocity-deviation log in all wells except in ARM-3 well, where we were able to use the neutron-porosity log method (Figs. 11−19). 3. General Interpretation: Based on the pore type dependence of velocity-deviations, the following predictions can be made by interpreting a down hole velocity-deviation log showing zones with various characteristic patterns. 3.1Zones with Positive Velocity-Deviation Values: Positive velocity-deviation values indicate relatively high velocities with respect to porosity and are caused mainly by porosity, such as intergranular porosity, that imparts a frame-like fabric to the rock. These fabrics typically have both high porosities and high velocities and are displayed on the deviation log as positive values.

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Fig. 10: Cross plot of Vp (compressional velocity) and porosity of Nubia Sandstone Formation in S-8 well

Fig. 11: Velocity-Deviation log of ARM-2 well.

Fig. 12: Velocity-Deviation log of ARM-3 well.

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Fig. 13: Velocity-Deviation log of ARS-1 well.

Fig. 14: Velocity-Deviation log of ARS-3 well.

Fig. 15: Velocity-Deviation log of ARS-4 well.

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Fig. 16: Velocity-Deviation log of ARS-5 well.

Fig. 17: Velocity-Deviation log of ARS-6 well.

Fig. 18: Velocity-Deviation log of R-5 well.

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Fig. 19: Velocity-Deviation log of S-8 well. In general, the pores of this type are commonly connected, so a positive velocity-deviation may indicate good permeability. The velocity-deviation log, consequently, can be used not only to detect the rock frameintegrated porosity, but also to suggest their associated diagenetic processes. 3.2Zones with ± Zero Velocity-Deviation Values: Zones with small velocity-deviation values (± 500 m/s or less), represent sections that have values predicted by the time-average equation. These zones are dominated either by dissolution porosity or by high microporosity. All of these pore type are particularly predominant in a sediment just after deposition. Most of the zones with low velocity-deviation values thus indicate zones with high diagenetic alterations. These small velocity-deviation zones contrast sharply with the zones of low diagenetic alteration, which, as described above, typically have large positive velocity-deviations. 3.3Zones with Negative Velocity-Deviation Values: Some of our velocity-deviation logs show zones where velocity-deviation values are consistently negative, indicating that factors other than lithology control the velocity-deviations. There are three possible explanations: 1- Caving or irregularities of the borehole well. 2- Despite the fact that other workers have typically included fracture porosity with other types of secondary porosity, which is an equivalent of too high velocity or positive deviations (Schlumberger, 1974), several studies have shown that fracturing decreases velocities on both in a small scale and large scale (Gardner et al. 1974, and Anselmetti and Eberli, 1993). 3- Negative deviations also could be caused by a high content of free gas. Free gas would have a strong negative effect on the deviation log because gas drastically reduces the VP and results in a reduced neutron porosity reading due to the lower content of hydrogen in the fluid phase (Hilchie, 1982). Both of these effects could theoretically cause a strong negative signal in a velocity-deviation log. 4. Velocity-Deviations and Permeability Trends: Permeability in sandstone rocks is affected more by pore type and pore connectivity than by the total amount of porosity (Lucia, 1987). The wide variety of pore types results in highly variable permeability values. Because pore type has a major control on both velocity-deviation and permeability, it seems appropriate to investigate whether the velocity-deviation log can be used to trace down-hole permeability trends. It is shown that, most of the positive velocity-deviations are a result of intergranular porosity in which most of these pores are connected. Consequently, rocks with high positive velocity-deviations show generally high permeability values, whereas small or negative deviation may indicate low permeability resulting from isolated pores formed either by leaching or by the presence of clays

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5. Interpretation: Figure (11) of ARM-2 well shows that the well is characterized by negative velocity-deviation values to a depth of 3520 m. This indicates that the porosity may be due to caving or irregularities of the pore holes. From 3520 m, to the bottom of the well, the velocity-deviation is approximately ±500, indicating the presence of dissolution porosity with abundant diagenetic alteration. These pore types are typically unconnected, and rocks typically have low permeability. The velocity-deviation log of ARM-3 well (Fig. 12) shows that, the value is around ± 500 m /s from the top to the bottom of the well, which indicates the presence of dissolution porosity with major diagenetic alteration. As mentioned before, pores are typically unconnected, resulting in low permeability. Some parts of the log display negative velocity-deviation values, though, which may be due to the presence of gases, fractures or caves. The velocity-deviation log of ARS-1 well is represented in Fig. (13), and shows low values from the top to the bottom of the well, indicating the presence of dissolution porosity with major diagenetic alteration and low permeability. The ARS-3 well (Figure 14) is characterized by velocity-deviation values around ± 500 m/s, suggesting dissolution porosity. This pattern may reflect isolated pores resulting from leaching of feldspars, carbonate, sulfate or other soluble materials. This porosity type typically has low permeability due to the unconnected pores. A negative velocity-deviation value is observed in the upper part of the well. The ARS-4 well (Fig. 15) is characterized by negative velocity-deviation values in the upper part and different intervals at the lower part, this observation indicates the presence of caves or gases while, the rest of this formation is characterized by the presence of isolated pores. The ARS-5 well (Fig. 16) shows that dissolution porosity is dominant throughout the well and that the well is characterized by low permeability. The ARS-6 well (Fig. 17) displays no positive velocity-deviation values. The velocity-deviation is close to zero in the lower part, which indicates the presence of dissolution porosity with only minor diagenetic alteration. The occurrence of this porosity suggests that we might expect an interval with low permeability at the bottom of the well. A large negative velocity-deviation throughout the rest of the well suggests caving or fracturing except toward the bottom of the well. The velocity-deviation log of R-5 (Fig. 18) shows a positive velocity-deviation, indicating the presence of intergranular porosity with high permeability and minor diagenetic alteration. Dissolution porosity is observed in the S-8 well (Fig. 19), suggesting major diagenetic alteration and low permeability. Summary and Conclusions: Acoustic velocity in sandstone is a complex function of porosity and pore types, which are controlled by the combined effect of lithology and diagenetic alteration. The presence of different pore types in sandstone rocks causes significant scattering in the velocity-porosity diagram, and most samples show deviations from an average velocity-porosity trend. The term ‘velocity deviation’ is defined as the difference between a measured acoustic velocity and the velocity calculated from a measured porosity value by applying the commonly used empirical time-average equation. A velocity–deviation log provides a tool for inferring down hole information on the predominant pore type in sandstone. This type of log can also be used to trace the down hole distribution of diagenetic processes and to estimate permeability trends. Positive deviations result from higher velocity values than expected for a given porosity value and are typical of pore types that are integrated in a rock frame and that yield high elastic properties of a rock. This type of porosity is characterized by high permeability. A zero velocity deviation is typical of dissolution porosity resulting from leaching of feldspar; this type of porosity has low permeability. We determined velocity–deviation logs for nine wells in the Nubia sandstone Formation distributed in the study area. The area under investigation is Abu Rudeis-Sidri Field located on the eastern coast of the Gulf of Suez, about 25 km. north of Belayim land Field, to the southeast of October and Rus Budran Fields. This application of velocity deviation logs shows that the major pore type for the Nubia Formation in the nine wells studied is dissolution porosity, which means that the sandstone rock of this formation is affected by leaching processes that typically lead to the presence of the unconnected pores, suggesting that this formation can not be counted on as a good reservoir rock.

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