REVIEW OF LITERATUR.....

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SPE 160891

Tar Characterization for Optimum Reservoir Management Strategy Muhammad Al-Harthi, Mohammed Al-Ali, Ronny Gunarto/ Saudi Aramco

This paper was prepared for presentation at the SPE Saudi Arabia Section Technical Symposium and Exhibition held in Al-Khobar, Saudi Arabia, 8–11 April 2012. This paper was selected for presentation by an SPE program committee following review of information contained in an abstract submitted by the author(s). Contents of the paper have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material, as presented, does not necessarily reflect any position of the Society of Petroleum Engineers, its officers, or members. Papers presented at the SPE meetings are subject to publication review by Editorial Committee of Society of Petroleum Engineers. Electronic reproduction, distribution, or storage of any part of this paper without the written consent of the Society of Petroleum Engineers is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of where and whom the paper was presented. Write Librarian, SPE, P.O. Box 833836, Richardson, TX 75083-3836, U.S.A., fax 01-972-952-9435.

Abstract Tar mats are extra-heavy bitumen that occur between aquifers and overlaying oil columns. They seal either partially or completely an oil reservoir from its aquifer and reduces aquifer support. Tar characterization includes evaluation of the tar distribution and its sealing degree. It is an important prerequisite to optimize the water injection well’s requirement and placement to maximize sweep and recovery. This paper discusses a case study and demonstrates an integrated methodology by using static and dynamic data to determine the tar distribution and its sealing degree. The study includes both early life data before the subject field was put in production, as put on production, and post-production data to refine the characterization. Additionally, the use of formation pressure while drilling and Pyrolytic Oil-Productivity Index (POPI) analysis to optimize the injector’s placement will also be highlighted. Moreover, saturation and production logging tool analysis were incorporated to determine if there is aquifer influx across the tar mat. The degree of the aquifer influx is also evaluated using material balance and reservoir simulation. Introduction Field X is one of Saudi Aramco fields that was developed with four producing reservoirs namely, 1, 2, 3 and 4 reservoirs. The producing reservoirs have

relatively good reservoir quality with average permeability of 310 md. The value is only one-third of average permeability from pressure build up which indicates an existence of fractures. The reservoir properties are relatively uniform across the reservoires and vertical variation is considered minimal with some higher permeability streak observed from the core data up to 3 darcies.

The PVT data and numerous samples indicated oil quality degrades from the crest to flank and All producing erservoirs are confirmed to have tar in the reservois boundary. The tar existence prevents an effective aquifer support to the producing area thus the water injection was chosen as part of the development plan to effectively recover the oil. Detail fractures study incorporating both static and dynamic data shows that reservoir 3 and 4 are interconnected with underlying undeveloped reservoir 5 which has reasonable aquifer size without tar existence. The reservoir 1 and 2 are also in communication but they are not expected to be connected with reservoir 3 and 4. The optimum reservoir management strategy was formulated based on the tar characterization. It is believed that not all the tar section is fully impermeable. Understanding the sealing degree of the tar is one of the key element to optimize the field develoment plan such as determine the optimum number and location of the injectors taking into account the aquifer influx. The tar is characterized with different techniques to confirm the presence and the sealing degree using static and dynamic data as follows :

Data Sources POPI Pyrolytic Oil Productivity Index is a novel quantitative diagnosis developed by Saudi Aramco and practically implemented in 2002. It utilizes residual hydrocarbon staining on core and drill cuttings to assess the reservoir quality. The method is superior since it identifies the tar presence and quantifies its percentage directly. Furthermore, it is used in real time to enhance optimum well placement. The injectors were placed slightly above the Tar Oil Contact (TOC) to ensure sufficient injectivity and provide effective pressure support and reservoir sweep towards the producer wells (figure- 1).

Figure-1: Well placement above TOC.

POPI analysis generates a plot of Hydrocarbon yield (mgHC/gRock) versus temperature (figure-2). This is generated by heating a powdered rock sample at 180o C for 3 minutes and then starting to increase the temperature up to 600o C at a rate of 25o C per minute. The end result is a plot with three peaks. The first peak –at the start temperature (180o C) - represents the amount of volatile hydrocarbons. The second peak –usually between 180o C and 400o C- represents the solvent extractable bitumen. The third peak –above 500o C- represents the amount of heavier hydrocarbons (e.g., asphaltenes, pyrobitumen). The fraction of the heavier hydrocarbon observed in real-time during wells drilling will give indication whether the interval will be effectively injected or not. Adjustment on the well trajectory during drilling will be carried out as deemed necessary based on POPI data and other information such as NMR log. POPI results confirm the tar existence around the wells and estimate the depth of the TOC which could vary between areas.

Figure-2: Typical POPI plot. Here is an example of one of the POPI analysis in one of the flank wells. The apparent API gravity data show an average gravity of 13.2° for the reservoir (figure-3) with a range from 19.0° to 5.8°. The apparent API gravity is lower than that reported for produced oils in the field and it shows a clear transition zone at x954.1’. Therefore, the reservoir at the well area appears to contain medium gravity oil and tar that is not producible.

Figure-3: Depth plot of apparent API gravity as determined from pyrolysis using two different formulae and total pyrolytic yield for core samples on of the wells located in the flank area. For the sake of this study, POPI is used to confirm the existence of tar around the area of interest and evaluate the possible different TOC and tar thickness across the reservoir MDT MDT is a common tool used to evaluate the pressure profile across the reservoir which can indicate the formation pressure, mobile phase, the contacts and the possible pressure depletion difference if it is run post production period. It has been observed that the

MDT pressure points to be supercharged whenever it is taken across the tarry interval. The fact that the tar has extreme low viscosity, the mobility (k/m) in the survey point will be very low and it would act as a tight section with super charge behavior. Pressure Performance Pressure performance is the important dynamic data which can indicate the tar sealing degree. Whenever the field was shut-in in 1980s and 1990s, it was clearly observed that the reservoir pressure built up in each reservoir, indicating connectivity with the aquifer. A material balance and simulation models are used to estimate the degree of aquifer influx and its connectivity to the producing area based on the rate of reservoir pressure increase observed. A more complex situation exists in reservoir 3 and 4 where these reservoirs are inter-connected through fractures system with underlying reservoir 5 which does not have tar mat. Most of the external energy came from reservoir 5 (not the aquifer itself) eventhough further assessment shows that not all tar mats in reservoir 3 and 4 are fully sealing. The availability of aquifer observation wells in specific locations aids the interpretation of tar sealing degree. Should the tar be fully sealing, the pressure in aquifer will not be reduced at all whenever the field is produced. Logs Another approach used to evaluate the tar sealing degree by analyzing open-hole and saturation logs. Saturation logs are run periodically in the field to monitor the flood front progression and confirm the sweep efficiency. Carbon-Oxygen log is utilized as saturation log as it has ability to determine water saturation independent of salinity. In addition to the C/O ratio, the log also works in the sigma mode which can be used to identify high salinity aquifer water encroachment. The relatively high salinity aquifer water will be observed clearly as high sigma reading if it is present within vicinity of the surveyed wells. The open hole log of flank wells drilled post production are also used to identify any possible aquifer water encroachment. Should the wells penetrate the impermeable tar, the open hole or saturation logs show unchanged saturation compared to the typical pre-production saturation profile. PLT is another log that can be used to confirm tar sealing degree. In a case of sealing tar penetrated by a well, the PLT shows no injection or production contribution across the tar section.

Geochemical Analysis Additionally, geochemical analysis of produced water has been utilized to determine the source of water production in the producers. The salinity of aquifer formation water is varying between 200,000 ppm to 240,000 ppm which is significantly higher than the injected water salinity of 25,000 ppm. Several wells at the flanks showed formation water salinity which indicates aquifer influx and non- sealing tar mat. Injection Fall Off (IFO) Test In IFO test, an injector well is shut in for a specific amount of time. Meanwhile, a pressure gauge is utilized to measure the pressure response. The pressure is falling off when an injector is shut in. This period is called the fall off period (see figure-4). The falling trend is used to characterize the reservoir in terms of both rock and fluid properties. For analysis, different plots containing mainly calculated variables from the pressure response versus time in different scales. In this study, the pressure derivative will be used to characterize tar. Basically, when fluid flow disturbance propagation caused by shutting an injector hits a sealing boundary (e.g. sealing fault), the flow regime changes from an infinite acting radial flow regime to linear flow regime. This is reflected on the derivative as a change from zero slope state to a half slope state (see figure-5). If tar is sealing, then it will work as a sealing barrier and derivative will follow the trend explained earlier. If otherwise the tar is not sealing, the aquifer effect will be felt as constant pressure boundary and the derivative will be declining. The distance to the tar (no-flow boundary) interpreted from the test is cross checked with the estimated distance from geological map to confirm the test result reliability. The interpretation is more challenging in the case of horizontal injectors as the linier flow will be observed due to the flow to the horizontal section. The slope change as result of boundary may not be detected clearly. All the tests done in the study were carried out using real time data to optimize the testing period and ensure the required information has been collected from the test.

Figure-4: Fall of period.

Figure-5: Half slope state. Material Balance One of the indications of the tar is not fully sealing around the reservoir is the increasing reservoir pressure during field shut-in. Material balance model was constructed to investigate the degree of aquifer influx. To match actual pressure history, an aquifer has to be connected to the reservoir in addition to the gravity water injection as energy source. The aquifer contributed about 25% of total energy to produce the reservoir. Further tar characterization to determine the connectivity to the aquifer in each area is performed using integrated static and dynamic data (see figure-6).

Figure-6: Material balance model for Reservoir-1 Integration of Data Sources Sealing Tar Case # 1 Reservoir 3 is developed using peripheral water injection strategy. In traditional peripheral water injection strategy, injectors are placed in flanks directly below the oil water contact. However, in this reservoir, injectors are placed directly above the tar oil contact to ensure that injection effect is transferred to the producing area in the crest.

Figure-7: Reservoir-3 structure map.

Well L is a vertical power water injector located and completed in the east flank of Reservoir 3 as shown in figure-7. The MDT results showed super charged point at the lower part of the reservoir despite of relatively uniform porosity, indicating tar existence (see figure-8). The standard open hole log suite will not be able to differentiate tar from light oil.

Figure-8: FAL log and MDT data for well-L. A carbon-oxygen log was run across well L after two years of injection (see figure-9). The dark blue color in the 3rd track from the left shows the portion of the rock filled by water when the well was drilled whereas the light blue indicates the portion of water based on CO-log. The calculated water saturation is presented in the middle track between red (original) and blue (current based on CO-log). The log showed that oil in the upper section of the well was flushed away leaving residual oil saturation around 25%. On the other hand, the water saturation in the bottom section did not change indicating that the tar around the well is sealing. The injected water did not displace this section. This was also confirmed by the flow meter log shown in figure-10.

Figure-9: C/O log for well-L (2009).

Figure-10: PLT log for well-L. In order to further confirm the results, an IFO test was conducted on the subject well, as shown in figure -11. The derivative plot shows a half slope indicating a no flow boundary around the well. It is believed to be the response from sealing tar. The increase in derivative plot happens immediately after short radial flow which suggests a very close location of no-flow boundary. The behavior is consistent as the well did penetrate the tar in the bottom section of the reservoir.

Figure-11: IFO test of well-L. The sealing tar condition is also inferred from the historical pressure in the offset aquifer observation well. Well F is an aquifer observation well located downdip of well L. It shows that pressure in well F was not affected by production since 1960s (see figure-12). All of this information confirms that tar is sealing around this well.

Figure-12: Reservoir-3 historical pressure data. Sealing Tar Case # 2 Some areas had been evaluated with the integration of more limited data. The tar sealing degree in eastern part of Reservoir 3 has been evaluated with C/O log of well-B. Similar to well-L in the previous case, C/O of well-B confirms that there is no aquifer influx at that area. Moreover, the top oil leg is almost swept by injection water coming from Injector-W1 and W2. Also, the MDT data of well-B confirms the tar oil contact at that area. (Figure-13, 14 and 15)

Figure-13: Reservoir-3 structure map.

Figure-14: C/O log for well-B (2010).

Figure-15: FAL log and MDT data for well-B. Sealing Tar Case # 3 Reservoir 4 is developed using similar strategy to that used in reservoir 3. Well A is a vertical power water injector that is located and completed in the east flank

of Reservoir 4 .The well is placed in a close proximity to tar area without cutting any tar section. This case study has very limited data to investigate about tar. However, with proper utilization of the data, the tar sealing degree can be concluded.The following figure-16 showed an injection fall off test result. From the pressure derivative (shown in white) a no-flow boundary was detected which is attributed to tar effect. The estimated distance to the no-flow boundary is 1,100 ft which matches the spacing between the well and the tar-oil contact. This proves that this area has a sealing tar.

Figure-16: IFO test of well-A. Non-Sealing Tar Case # 1 The evaluation of the north east flank of reservoir 1 has been by the integration of the historical pressure performance of Well M, geochemical analysis of the well M and C/O log of well-I. The trend of the pressure behavior of well-M, which is located in the tarry area, is consistent with the pressure behavior in well-J, H, E and well-Y (see figure-19). That is a clear sign of reservoir aquifer connectivity on well-M area. Also, this connectivity is confirmed by geochemical analysis of well-M which is estimated around 200,000 ppm. Aquifer water salinity is varying between 200,000 ppm to 240,000 ppm. Another source confirms the sealing degree of the tar on the same area is C/O log of well-I which was run in 2010(figure-18). There is a significant increase in the sigma at the top layer of the reservoir. This sigma increase represents the high salinity water encroachment or aquifer influx form non sealing tar at that area.

Figure-17: Reservoir-1 structure map.

Figure-18: C/O log for well-I (2010)

Figure-19: Reservoir-1 historical pressure performance. Non-Sealing Tar Case # 2 In the south west flank in Arab-1 reservoir has, the tar sealing degree on that area has been evaluated based on C/O log of well-S, geochemical analysis of the same well and by results of IFO test of the horizontal injector Well-X (figure-20). C/O log run in 2010 of well-S indicated obvious aquifer influx represented by both dark blue color (open hole log in 1983) and light blue color from CO-log result (figure-21). The high sigma reading confirms the source of encroached water. Aquifer influx is

confirmed also by the geochemical analysis of well-S that had been taken before drilling injector well-II. The estimated water salinity of well-S was estimated by 196,769 ppm. Based on the IFO test of well-X, the derivative plot shows radial flow infinite acting which means there is no flow boundary in that area at reasonable distance from the well based on Radius of Investigation of the IFO test (figure-22). In other words, the tar is not completely sealing for the flank on that area.

Figure-20: Reservoir-1 structure map.

Figure-21: C/O log for well-S (2010).

Figure-22: IFO test of well-X

Optimum Reservoir Development Having determined the location of the non-sealing tar in each flank area, the simulation model is updated with this information to provide more representative model taking into account different and directional aquifer support. The stream line simulation model was further run to evaluate the injectors’ efficiency and injectors’ allocation. The model is also used to optimize the future injectors location and requirement. In the area of non-sealing tar with more significant aquifer influx, it is likely possible to leave the area without injectors as the aquifer will provide pressure support and sufficient sweep. On contrary, the area with full sealing tar may need additional injector(s) to improve the area sweep efficiency. Depends on the geological heterogeneity, the sweep pattern may have been already sufficient without additional injectors. Conclusion Tar characterization is an important part to optimize the reservoir development plan with water injection. Integration both static and dynamic data will allow a better tar characterization which covers both the tar distribution and sealing degree. It is highly recommended to optimize the location and number of injectors required to ensure good areal sweep and fully utilize the natural support from the aquifer if the tar is not completely sealing. References 1. Khalid M. Al-Salem, Said S. Al-Malki, Rabea A.

Ahyed, Peter J. Jones, Peter M. Neumann, Saudi Aramco. ‘’Real Time Well Placement above a Tar Mat, Leveraging Formation Pressure While Drilling and Pyrolytic Oil-Productivity Index Technologies’’.SPE-113550, SPE Europec/EAGE Annual Conference and Exhibition held in Rome, Italy, 9–12 June 2008.

2. Neumann, P. M, Salem, K. M., Tobert, G. P., Seifert, D. J., Dossary, S. M., Khaldi, N. A., Saudi Aramco, Shokeir, R. M., Halliburton. ‘’ Formation Pressure While drilling Utilized for Geosteering”. SPE-110940, SPE Saudi Arabia Technical Symposium held in Dhahran, Saudi Arabia, 7–8 May 2007.

3. M.H Tobey, H.I. Halpern, G.A. Cole,J.D. Lynn,

J.M. Al-Dubaisi, and P.C. Sese, Saudi Aramco. ‘’ Geochemical Study of Tar in the Uthmaniyah

Reservoir’’. SPE 25609, SPE Middle East Oil Technical Conference and Exhibition, held in Bahrain, 3-8 April 1993.

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Al-Arfaj and A.H. Al-Huasaini, KFUPM. ‘’ Characterization of Tar From a Carbonate Reservoir in Saudi Arabia: Part I- Chemical Aspect’’. SPE-21004, SPE International Symposium on Oilfield Chemistry held in Anaheim, California, February 20-122, 1991.

Acknowledgements Special thanks go to Hassan Al-Mubarak and Udeh Pius for their help in publishing this paper.