REVIEW OF LITERATUR.....

review_9.pdf

Home >English homework help >REVIEW OF LITERATUR.....

SPE 113550

Real Time Well Placement above a Tar Mat, Leveraging Formation Pressure While Drilling and Pyrolytic Oil-Productivity Index Technologies Khalid M. Al-Salem, Said S. Al-Malki, Rabea A. Ahyed, Peter J. Jones, Peter M. Neumann/Saudi Aramco

Copyright 2008, Society of Petroleum Engineers This paper was prepared for presentation at the 2008 SPE Europec/EAGE Annual Conference and Exhibition held in Rome, Italy, 9–12 June 2008. 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 does not necessarily reflect any position of the Society of Petroleum Engineers, its officers, or members. 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 SPE copyright.

Abstract Recent development of a large oil field in the Eastern Province of Saudi Arabia achieved real time well placement above a tar mat by utilizing Formation Pressure While Drilling (FPWD) and Saudi Aramco’s Pyrolytic Oil-Productivity Index (POPI). Placement of 6 ⅛ in. horizontal power water injector wells in close proximity above an impermeable, undulating, tar layer posed a unique geosteering challenge. Additionally, a heavy oil layer of varying thickness exists above the tar. The uncertainty was to determine at what vertical depth fluid mobility stopped and heavy oil and/or tar began in the high porosity/permeability reservoir. The heavy oil/tar layer is undetectable with conventional real-time logging while drilling (LWD) measurements. Furthermore, new technology devices such as the 6 ⅛ in. NMR While Drilling tool were not available when the field was developed. A technique of combining FPWD with POPI provided a solution to identify and steer away from the immobile fluids, minimizing the risk of completing an injection well in an interval with low or no injectivity. POPI collectively refers to pyrolytic methods developed by Saudi Aramco to identify and quantify tar from residual hydrocarbon staining on drill cuttings. Pyrolytic Oil-Productivity Index provides a direct assessment of the residual hydrocarbons present on rock samples and allows an accurate determination of the volume of tar over a wide range of concentrations. The business impact is to assist with geosteering horizontal injector wells in real time. Formation pressure while drilling and POPI results were integrated to confirm favorable fluid mobility and well paths were placed to take advantage of this knowledge. Initial tests showed high injectivity in all wells, which indicates that they were placed in the desired zone. This paper includes selected case studies

demonstrating both entry into and steering away from impermeable tar layers.

Introduction The purpose of the reservoir management process is to appraise, develop and produce hydrocarbon reservoirs. The appraisal process sometimes identifies development challenges that asset teams would prefer to ignore. One of these challenges is tar and associated heavy oil above tar. The existence of a tar mat in the reservoir complicates the placement of horizontal injectors. Without tar, injectors are placed at the original oil-water contact (OOWC) in a straightforward manner. With tar, the challenge is to place the horizontal wellpath in a mobile fluid layer to ensure meeting business plan injection rate targets. The business impact of placing horizontal injectors optimally above tar is significant. Members of the multidisciplinary asset team challenged themselves to identify the lowest mobile fluid layer at each location and simultaneously place the injector path in that layer. This required the application of real time technologies. The team used two technologies (FPWD and POPI) with the vision that results from one technology had to confirm results from the other. This informational redundancy was utilized throughout the drilling of the injectors and this process is part of the discussion below. Positive results after injection startup confirm the wisdom of the approach. The NMR While Drilling tool is now a competitor to FPWD. The 6 ⅛ in. NMR tool became available after the development campaign.

2 SPE 113550

Figure 1. A 3-D image of the oil field showing a tar mat (black) around the field, a heavy oil layer (yellow) above the tar and moveable oil to the crest (green and red)1. Development Plan Developing four reservoirs in a large oil field with an underlying tar mat topped with a low mobility heavy oil (Figure 1) raises the challenge of placing water injectors in mobile fluids zone above the tar-oil contact to ensure adequate pressure support. The subject field was put on production in the early 1960s. Gravity water injectors were utilized for pressure support since 1965. Between 1981 and 1983, 18 gravity and power water injectors were drilled as part of an integrated injection development plan. These wells were typically tested using RFT, open hole logs and injectivity tests with flow meters to determine the tar-oil contact in these prospective injection wells (Figure 2). The study revealed that poor injection rates were observed in wells with too thin a perforated interval above the tar-oil contact. This poor injectivity would not give sufficient pressure support and negatively impact ultimate recovery. The study also enabled a proper mapping of the tar mat in the field. Although, the field was mothballed in 1983 due to low oil demand and the injection plan was suspended. The field remained mothballed and kept as shut-in potential until the early 1990s when it was produced for two years and then it was mothballed again. Based on the findings of the 1983 study, a new field development plan was put in place in 2006. The new plan utilizes horizontal power water injectors for pressure support. One of the key elements of the 1983 study is that the tar mat will work as a barrier preventing the oil from being lost into the aquifer. This finding was reexamined by running streamline simulation. The optimum injection height above the tar for placing the horizontal wells was selected using both the 1983 study and streamline study results, coupled with historical data. A height above tar

that would ensure injectivity without sacrificing ultimate recovery was sought.

Figure 2. Injection profile VS vertical depth. The rates decrease as the tool gets closer to the tar zone and eventually goes to zero. Geosteering All new horizontal injectors were planned to follow a horizontal path at the specified height above the tar oil contact. All wells where geosteered to ensure the proper execution of the well plan. In addition to the standard LWD logs, the team utilized FPWD and POPI to geosteer the injectors away from tar and low mobility heavy oil.

Figure 3. The injectors’ paths were planned above the tar mat to avoid injecting in the tar mat or the heavy oil layer1. FPWD Tool Description. The FPWD tool design is similar to many probe style wireline tools, where a donut shaped rubber pad forms a seal around a metal snorkel (Fig. 4). When this probe is pressed against the wellbore, the snorkel penetrates the mud cake and contacts the formation to perform the drawdown and record the pressure measurement. Multiple pressure gauges are used in the tool; a Quartz Gauge and Strain Gauge with accompanying electronics. Software, stored in the tool’s memory controls the test sequence which is preprogrammed, but can be modified using a sequence of commands2.

X 2X 3X 4X 5X 6X 7X 8X 9X 10X X

SPE 113550 3

Figure 4. Schematic of the Probe, Equalizer Valve, and Drawdown Manifold1. FPWD Application Formation pressure while drilling has the primary function to measure formation pressure at any point in the wellbore and additionally provide an associated fluid mobility value based on the drawdown pressure and buildup profile3. The drawdown and buildup profiles of each individual test are indicative of the mobility or permeability of the zone under test. Pressure transient mobility values are calculated for each test and can be provided in real-time with the final buildup pressure values. Formation pressure sampling devices, both FPWD and wireline formation testers are fundamentally flawless when it comes to identifying “tight” and very low permeability zones. This characteristic allows one to take advantage of the normally undesirable aspect of formation pressure reading devices, namely long slow pressure buildup times in low permeability zones, no pressure build up in tight zones, or a lost seal due to the lack of mud-cake presence. For the purpose of identifying tar and low mobility oil, a fixed robust drawdown rate of 2 cc/sec and volume of 10 cc for both the initial and repeat readings makes the tight and low permeability readings stand out because the final pressure reading, before the “timed” retract, will be significantly lower than actual reservoir pressure (Fig. 5).

Figure 5. Example of a robust drawdown combined with long build up time in high porosity reservoir indicate the presence of heavy oil and require the need for a well path change to increase injectivity potential1. POPI Pyrolytic characterization methods developed by Saudi Aramco provide a direct assessment of the residual hydrocarbons present in core or cuttings samples. These methods include: assessment of the API gravity of the fluid, the POPI4, the Apparent Water Saturation Method5, the Compositional Modeling Method6, and the Volume of Organic Matter Method7. Pyrolytic Oil-Productivity Index instrumentation and methods can accurately quantify tar volumes to within ½ percent rock volume over a wide range of concentrations, and are now routinely applied to drilled cuttings in real-time to assist in geosteering horizontal development wells. A particular focus for POPI techniques has been predicting the effect of tar on reservoir injectivity for horizontal water injector wells that are drilled along the flanks of many oil fields8. Case Studies The combination of FPWD and POPI to geosteer the wells above the tar/heavy oil proved to be effective. Two case studies will be discussed to illustrate the successful application of the technologies. Well A Well A was one of the early horizontal injectors drilled in the field. Data from FPWD and POPI showed that the tar level was very well identified in the area and the well path was placed in a zone with mobile fluids. In an attempt to confirm that FPWD and POPI were working properly, a decision was made to drop angle in the last 500 ft of the well path while taking an FPWD reading every 10 ft TVD and frequently sampling for POPI analysis. FPWD showed that the mobility was plummeting with depth while the LWD showed the same rock quality with

4 SPE 113550

porosity around 20% (Fig. 6). Ultimately the tool started to lose seal, which is an indication of very low mobility.

Figure 6. Well A LWD log showing FPWD readings with tight/lost seal readings at the end. Pyrolytic Oil-Productivity Index was showing a tar amount of 1%-2% of rock volume from Target Entry (TE) up to 300 ft from Total Depth (TD), where the tar volume started to increase rapidly. At TD, the tar volume reached 5% of the rock volume and the zone was reported as a non-injectable zone (Fig. 7). The initial short-term injectivity test, which was conducted after the well’s completion, showed a high injection rate that exceeded the planned rate for the well. These results proved that the tar was well defined in the area and the combination of FPWD and POPI was a good practice to identify tar and low mobility heavy oil zones.

Figure 7. Well A POPI analysis showing the increase of tar volume at the end of the well path. Well B The FPWD tool showed that Well B was placed in a zone with low mobility fluids (heavy oil) from TE (Fig. 8). The fluid was still mobile and injection was possible, but meeting the target rate for the well was doubtful if continued in the same zone. Pyrolytic Oil-Productivity Index, on the other hand, showed a tar level that ranged from 2%-4% of rock volume in the target zone (Fig. 9), which confirmed the results of FPWD that the zone had low mobility fluids. To avoid the possibility of a low injection rate, the well path was steered updip away from the low mobility zone. The well path was leveled in the zone when the FPWD showed good mobility tests and POPI indicated a tar level that is less than 2% of rock volume (Figs. 8 and 9). The initial short-term injectivity test showed that the well had a high injection rate and meeting the target rate was achievable.

SPE 113550 5

Figure 8. Well B LWD log showing FPWD readings with low mobility readings at the beginning of well path.

Figure 9. Well B POPI analysis showing high tar volume with low API at the beginning of the well path.

Conclusion The combination of FPWD and POPI is an excellent practice that works well in identifying tar and zones with low mobility fluids. This combination was utilized to successfully geosteer horizontal injectors above a tar/ heavy oil zone in a large Saudi Arabian field. The initial field results confirmed the success of this practice, where it showed high injectivity in all injectors. References

1. Neumann, P.M., Salem, K.M., Tobert, G.P., et al.: “Formation Pressure While Drilling Utilized for Geosteering,” SPE paper 110940, SPE Saudi Arabia Technical Symposium, May 7-8, 2007.

2. Seifert, D.J., Dossari, S.M., Burinda, B.J. and

Kellett, S.: “Application of Formation Testing While Drilling in the Middle East,” Paper presented at the 14th SPE Middle East Oil & Gas Show and Conference held in Bahrain International Exhibition Center, Bahrain, March 12-15, 2005.

3. Proett, M.A., Walker, M., Welshans, D. and

Gray, C.: “Formation Testing While Drilling, a New Era in Formation Testing,” SPE paper 84087, SPE Annual Technical Conference and Exhibition held in Denver, Colorado, USA, October 5-8, 2003.

4. Jones, P.J. and Tobey, M.H.: “Pyrolytic Oil-

Productivity Index Method for Characterizing Reservoir Rock,” 1999, U.S. Patent Number 5,866,814.

5. Jones, P.J., Al-Shafei, E.N., Halpern, H.I., Al-

Dubaisi, J.M., Ballay, R.E. and Funk, J.J.: “Pyrolytic Oil-Productivity Index Method for Predicting Reservoir Rock and Oil Characteristics,” 2004, U.S. Patent 6,823,298.

6. Jones, P.J. and Halpern, H.I.: “Compositional

Modeling and Pyrolysis Data Analysis Methods,” 2003, Patent Pending, U.S. Patent and Trademark Office.

7. Jones, P.J. and Halpern, H.I.: “Method for

Determining Volume of Organic Matter in Reservoir Rocks,” 2007, Patent Pending, U.S. Patent and Trademark Office.

8. Jones, P.J., Halpern, H.I., Dahan, M., et al.:

“Implementation of Geochemical Technology for “Real-Time” Tar Assessment and Geosteering: Saudi Arabia,” Offshore

6 SPE 113550

Mediterranean Conference and Exhibition in Ravenna, Italy, March 28-30, 2007.

Acknowledgements The authors would like to thank the other members of the project team for their contribution to the success of the project. Special thanks go to Nasser Al-Khaldi, Isidore Bellaci and Gordon Tobert for their help in publishing this paper.