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Selecting Optimum Completion Strategy ­ Examples from Saudi Arabia's Unconsolidated Unayzah Reservoir

Authors: Hasan D. Al-Anazi, Adel A. Al-Qahtani, Dr. Zillur Rahim, Mohammed A. Masrahy, Bandar H. Al-Malki and Adnan A. Al-Kanaan

ABSTRACT

The Unayzah reservoir in Tinat field is highly unconsolidated with a heterogeneous sequence of Early Permian sandstones saturated with condensate rich gas. Reservoir characterization based on the seismic, geological, and petrophysical analysis has indicated many stratigraphic units within the main Unayzah-A interval, which consists of well-developed Eolian sandstones represented by Eolian dunes, sand sheets and closely associated inter-dune and lake deposits. Due to the unconsolidated nature of reservoir rock, conventional drilling and completion pose a very high risk of producing formation sand along with gas production, which can cause immense damage to the production string, wellhead, surface flow lines, and gas plant equipment. In the early development stage of Tinat, frac-pack was used and had been successful for several years. The method consisted of creating a short fracture, packing it up with proppant, and placing a screen as part of the completion system across the reservoir section. After years of producing from wells completed with this frac-pack completion, the scale, due to fines migration buildup in the proppant within the annulus, causes high positive skin and deterioration of the screen, and consequently, needs a complete workover to regain lost potential. The workover for a frac-pack well is nothing less than drilling a new sidetrack as the completion cannot be de-completed and pulled out. After detailed study and modeling, it was decided to change this strategy and complete the wells with an expandable sand screen (ESS). The completions using the ESS were effective and successful in terms of trouble-free deployment of the equipment and sustained a long-term sand free rate. The conventional sand screen (CSS) assembly has also been used in this reservoir. The initial test of this equipment conducted in one of the high producers was successful; however, long-term sand free production as well as maintaining screen integrity cannot be ascertained. Among the disadvantages of a CSS completion is the possibility of wellbore collapse on the screens with the depletion of reservoir pressure (changing near wellbore stress environment) during production life of the well. In such a case, the screen can incur partial or total damage and can no longer resist sand production.

This article describes in detail the reservoir characteristics and presents numerous examples of different completion methods implemented in the field to prevent sand, and ensure a high, sustainable flow rate. Long-term performance data are analyzed and presented in this article to show the performance of sand screen technology.

INTRODUCTION

The Tinat field, discovered in 1982 and developed in 2003, is located southeast of the giant Ghawar field. The reservoir is in the early Permian age Unayzah A member, which underlies the pre-Khuff clastics. The reservoir is deep, highly unconsolidated, with an average porosity of 17%, but with ranges up to several Darcies. The unconsolidation of sand is presented in Fig. 1 where several core sections from two wells are depicted.

Fig. 1. TINT field samples showing severe unconsolidation of sand in the Unayzah formation.

SAUDI ARAMCO JOURNAL OF TECHNOLOGY FALL 2011

Number of Wells

Deployment

Wellbore Diameter

Frac-pack CSS

8 4

Risky Less Risky

Same Same

Forming Debris between Completion and Wellbore High Possibility High Possibility Erosion and Skin No

Cost

Strength and Reliability Moderate Moderate

Long-term Productivity

High Moderate

Not Favorable due to Increased Skin Damage may Incur Failure

ESS

12

Less Risky

Expands Maximize Flow Area

High

Highly Robust and Reliable/ Borehole Support

Tested Excellent/ Zonal Isolation Possible

Table 1. Different types of completion assemblies used in TINT field

The Unayzah A member consists of well-developed detrimental to the wellbore, the surface chokes and the Eolian environment sandstones. Geological analysis has pipes, and reduces flow capacity of a well by increasing divided the reservoir into five main facies: sand dune, skin damage around the wellbore. Once sanding sand sheet, inter-dune, paleosol, and playa lakes. The happens, it only increases with time and is very difficult dune and sand sheets are the main productive reservoir to control. The remedy to sanding is to ensure that such units, while the paleosol and playa deposits have low phenomenon occurring in the reservoir never affects porosity and permeability (<1 mD) and act as flow anything inside the wellbore. barriers. Sanding can basically be controlled by implementing Sand production or sand influx is a serious problem two measures: (1) Perforating the well in a non-sanding in unconsolidated sandstone reservoirs. These interval, then fracturing it to communicate with the reservoirs usually have tendencies to produce a high developed reservoir, Fig. 2, and subsequently producing amount of sand because of fragility of rocks and their it with smaller drawdown by choking the well, or (2) ability to break into small pieces. Sand production occurs Complete the well with mechanical screens that act as UTMN_2100_0 when the well fluid under high production rates dislodges a filter between the reservoir and the well and also a portion of the formation solids leading to a continuous maintain wellbore integrity, Fig. 3. flux of formation solids. Sand production drastically Option (1) is known as the indirect fracturing increases risks by eroding casing, pipes and pumps or technique. A subset of option (1) is to perforate the plugs the well if sufficient quantities are produced. RDD_PREKHUFF.PHIE_1 FALMD_PREKHUFF.VOL_UWAT_1 Erosion creates a need for regular workovers, and it can POR 1 FAL 0 0.5 V/V WATER 0 MECPRO.YM_1 FALMD_PREKHUFF.VOL_XWAT_1 DEPTH FALMD_PREKHUFF.VOL_QUARTZ_1 force the production rate to be kept belowFEET 0 a considered QUARTZ 1 0.5 V/V MOVED OIL 0 0 PSI 1.2e+07 MECPRO_1.STRESS_1 RDD_PREKHUFF.VSH_1 RDD_PREKHUFF.PHIE_1 safe limit. In such cases, the well cannot be produced in 0 1 0.5 V/V RESID OIL 0 0.6 PSI/FT 1.2 its full potential. Sand production is also enhanced by SHALE the condensate present in the reservoir when the JAUF RESERVOIR (14018) pressure reaches below the dew point of the fluid. The Unayzah reservoir in Tinat field contains a very high 14050 condensate level. Several methods are being used in the petroleum industry to overcome this problem. Saudi Aramco had been using three types of completions in the Unayzah 14100 reservoir to avoid the sanding influx. Initially, frac-pack was applied that was subsequently changed to use sand screens. Two different types were used; the expandable sand screen (ESS) and the conventional stand-alone 14150 sand screen (CSS) assembly. The ESS was adopted as the preferred method for the further development of the field. Table 1 illustrates the characteristics of different completion types in this field and illustrates the 14200 advantages of ESS over other applications.

Fracture Conductivity

COMPLETION STRATEGIES

14250

Many of the fields producing from sandstone are susceptible to sand production. Sand production is

14300

Fig. 2. Indirect fracture treatment on a sanding well.

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Zone of interest

Fig. 3. Typical screen assembly showing filtering section.

entire interval, place a screen, and conduct a fracture treatment. This is known as the frac-pack operation. For option (2), the screen functions are to keep specified filter integrity and not allow the sand to enter the wellbore. At the same time, it must not decrease well production potential. Screen failure mode is easily measurable at the surface from the production profile and sand measurement. Two types of screens are available: the CSS, also known as stand-alone sand screens, and the ESS. When either of these types are used, fracturing is not required. For the selection of the sand screen, careful measures must be taken to select an optimal screen dimension with proper sieve analysis of the target reservoir rock under different stress regime. This process will ensure long-term screen integrity and sustained production. The following sections illustrate the pros and cons of each strategy tested in the field. Frac-Pack Completion Frac-pack technology, Fig. 4, was introduced and adopted as a completion design to minimize conductivity damage due to solid flow back control additives, reduce partial penetration effects, and eliminate proppant or solid flow back due to the presence of mechanical

screens. Frac-pack treatment is implemented in vertical wells where the entire reservoir is usually perforated. This provides a communication path between the entire reservoir and wellbore, even if the induced fracture does not cover all of the pay section. During the early development of Tinat field, fracpack technology was used. Consequently, frac-pack encountered many difficulties during the deployment and stimulation operation leading to the damaging of the screen and/or reservoir, and consequently lowering longterm well productivity and compromising sand influx. Even with successful operations during well completion, some wells showed declining performance and produced at a lower than expected production rate after some years of production period. For well Tinat-X completed with frac-pack, Figs. 5 and 6 show the productivity at the beginning and after five years of production period, respectively. A substantial decline in production rate of around 70% is noticed. The analysis of pressure transient data indicated high-pressure loss due to damage in the formation, Fig. 7, which led to significant reduction in well performance.

Fig. 5. Six months after frac-pack treatment (TINT-11).

FracPack Completion

Tubing

Liner

RESERVOIR

Fig. 6. Five years after frac-pack treatment (TINT-11). Fig. 4. Typical frac-pack completion showing sand screen and induced fracture.

SAUDI ARAMCO JOURNAL OF TECHNOLOGY FALL 2011

1.E+09

SKIN = 9.8 kh = 4,600 md-ft

Pressure Function

1.E+08

ModelPressure Derivative

1.E+07

Actual Pressure Derivative

1.E+06 0.001 0.01 0.1 1 10 100

Time Function

wells as well as sidetracked low performance wells were completed with ESS. The primary objective of the application of ESS was to address sand control problems while reducing well costs and simplifying operations by eliminating the other option, frac-pack gravel packing. Unlike frac-pack, ESS can also be deployed in slanted and horizontal wellbores, thereby maximizing reservoir contact. A second objective was to maximize productivity by eliminating or reducing skin damage inherently associated with gravel pack completions. Significant features of applying ESS in unconsolidated Unayzah sand reservoirs are listed as 1-3 follows . Easy-to-use application (e.g., no mud system change is needed to run the completion). The ability to selectively complete and produce from multiple intervals. The ability to isolate intervals whenever needed to avoid water production, and therefore delay the water coning effect. Substantial reduction in the inefficiency and risks associated with frac-pack completions, which require careful consideration of pumping and proppant handling issues. The considerable reduction of turbulent flow effect in near wellbore region. The elimination of the gravel-pack region around the screen in the annulus, which offers a large inflow area with low flowing friction pressure. The ability to maintain well integrity, stabilize and support the borehole in addition to effectively controlling the sand. Sand grain support and borehole stabilization, which minimizes near wellbore damage, keeps sand in place, and limits the production of fines.

Fig. 7. TINT-11 well pressure transient test showing high positive skin indicating damage.

Fig. 8. Schematic of unexpanded and expanded ESS.

Fig. 9. TINT-9 post frac-pack performance.

Expandable Sand Screens (ESS) With the advancement both in drilling and completion technology, the strategy of field development was shifted to drilling horizontal and highly deviated wells. This proved higher well performance due to increased reservoir contact and eliminated the risks associated with the deployment of the frac-pack completion system. Saudi Aramco has been implementing ESS in open hole completions in unconsolidated Unayzah sand reservoirs since 2005. Horizontal and highly slanted development

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ESS is a sand screen specifically designed to be expanded in the wellbore and is constructed of three interbedded layers; the expandable base pipe, the filter media, and the expandable outer protective shroud, Fig. 8. The base pipe and outer shroud slots open during expansion to fit the wellbore diameter, while overlapped layers of filter media slide across each other to maintain reservoir integrity. The entire length of the ESS joint is expanded, including the connectors. This allows flow along the entire ESS completion without any blank sections. Compared to frac-pack, ESS completion was installed in more than 45% of the Unayzah reservoir wells and resulted in more than a 50% reduction in well 3 completion costs per well . This is mainly due to elimination of many costly operations, such as tubing casing perforating and frac-pack sand control treatment. Figure 9 shows the pressure response from well Tinat-Y, initially completed with frac-pack. The signature of the hydraulic fracture by the presence of a half slope in the derivative is noted in Fig. 10; however, the well rate declined subsequently and it was sidetracked and completed with the ESS system.

1.E+09

Xf = 120 ft kh = 1,020 md-ft

Pressure Function

1.E+08

Model ressure Derivative

1.E+07

Actual Pressure Derivative

1.E+06 0.001 0.01 0.1 1 10 100 1000

Time Function

Fig. 10. TINT-9 post frac-pack pressure transient analysis.

portions of the screen are exposed to a higher rate of influx, thereby creating weak areas. These weak areas, also known as "hot spots," can eventually give up and break causing partial or total failure of the screen. The long-term screen integrity depicted in Fig. 12 therefore shows the risk of sanding through the completion when the "hot spots" are created due to formation collapse and preferential gas flow; nevertheless, a few wells were still selected to test the concept of CSS. The Tinat-AA well was the first well completed with the CSS assembly. The open hole logs across the Unayzah-A reservoir showed excellent reservoir development with 16% average porosity. Figure 13 presents the wellbore trajectory and the open hole log for this well. After the successful deployment of CSS, the well was put to production when it tested sand free production rate as illustrated in Fig. 14. Production history with about four months of production is provided in Fig. 15. Examples of two more wells are given in Figs. 16 and 17 where the initial tests were conducted after CSS deployment.

Conventional Screen

Fig. 11. TINT-9 post ESS performance.

Expandable Screen

Figure 11 illustrates well performance after ESS completion showing distinct improvement in pressure response over frac-pack. Several examples of wells completed with ESS and flowed back at high production rates show successful sand control. Currently, those wells are flowing at high rates and with a very stable performance. Sufficient well test/production data from the wells prove the effectiveness of the ESS in terms of improvement of well productivity. Conventional (Stand-alone) Sand Screens (CSS) A third method tried to complete wells in the unconsolidated Unayzah sand was the CSS assembly. The CSS assembly is deployed and requires no special tool to set it in position. As the screen is not expanded to the formation, there remains a small annulus between the formation and the screen after deployment. With continuous production and depletion of the reservoir, sand particles are generated, particularly in high condensate wells. This is because the presence of condensate further reduces rock strength and some rocks actually disintegrate into debris and particles, which eventually accumulate inside the annulus. When this happens and the well is on production, the rate is no longer evenly distributed along the screen, rather certain

Fig. 12. Sand control in CSS and ESS assemblies.

Fig. 13. TINT-27 open hole log and wellbore.

Fig. 14. TINT-27 initial flow back results.

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Fig. 15. Production history for TINT-27.

discussed and presented the results of numerous field examples completed by frac-pack, ESS and CSS technologies. Deployment of the frac-pack system and fracturing the reservoir are not risk-free. Due to the nature of gravel packing, these wells gradually show high near well skin due to increased non-Darcy flow effects. Although CSS has shown short-term success in wells where this system was applied, the long-term well integrity is questionable with reservoir pressure depletion and increased condensate drop-out in the near well area that generally reduces rock strength. Reduction in rock strength causes rocks to partially disintegrate and the debris created by such disintegration can fall and accumulate in the screen/formation annulus thereby creating "hot spots." These hot spots, where preferential flows from the formation to the well occur, cause the screen to partially or totally fail. With ESS, hot spots are not created, and thereby screen failure risks are greatly reduced. When ESS is deployed and expanded, it provides solid and firm contact with the wellbore, thereby adding strength to the total completion system. Improvement in production rates have been observed from wells completed with ESS as compared to other completion systems. Screen performance is of course highly affected by its construction, distribution of particle size of the reservoir rock, and migrating fines. Therefore, based on reservoir properties and sand analysis, appropriate selections must be made.

Fig. 16. Test rate after CSS (TINT-33).

ACKNOWLEDGMENTS

The authors would like to thank the management of Saudi Aramco and Schlumberger for their support and permission to publish this article. Thanks are also due to the crew who struggled hard to implement these strategies in the field to ensure high rates and optimal reservoir depletion. This article was presented at the SPE Saudi Arabia Section Technical Symposium and Exhibition, al-Khobar, Saudi Arabia, May 15-18, 2011.

REFERENCES

1. Metcalfe, P. and Whitelaw, C.: "The Development of the First Expandable Sand Screen," OTC paper 11032, presented at the Offshore Technology Conference, Houston, Texas, May 3-6, 1999. 2. Ginest, N., Al-Sagr, A., Al-Malki, B.H., Al-Khawajah, M., Jones, C., Morgan, Q., et al.: "A Successful Expandable Sand Screen Case History in a Deep, Corrosive Gas Well Application," SPE paper 122847, th presented at the 8 SPE European Formation Damage Conference, Scheveningen, the Netherlands, May 27-29, 2009. 3. Kabir, M.R., Wai, F.K., Ali, A.R., Omar, N. and Moran, P.M.: "The use of Expandable Sand Screens (ESS) to Control Sand in Unconsolidated Multi-Zone

Fig. 17. Test rate after CSS (TINT-34).

CONCLUSIONS AND RECOMMENDATIONS

With the evolution of sand control technology, mechanical sand screens have proven the ability and effectiveness to reduce/eliminate completion damage and sand production, yet maintain target gas production rates. Saudi Aramco had undergone three major completion strategies for the unconsolidated Unayzah gas reservoir and now has selected the most optimal technique that addresses and ensures long-term wellbore integrity and production rate. This article

SAUDI ARAMCO JOURNAL OF TECHNOLOGY FALL 2011

Completions in the Baram and Alab Fields Offshore Malaysia -- A Case Study," OTC paper 15154, presented at the Offshore Technology Conference, Houston, Texas, May 5-8, 2003. 4. Rahim, Z., Al-Malki, B.H. and Al-Kanaan, A.A.: "Selection of Completion Strategy for Sand Control and Optimal Production Rate ­ Field Examples from Saudi Arabia's `Unayzah Sandstone Reservoir," SPE paper 131078, presented at the SPE Asia Pacific Oil and Gas Conference and Exhibition, Brisbane, Queensland, Australia, October 18-20, 2010. 5. Rahim, Z., Al-Qahtani, M.Y., Bartko, K.M., Goodman, H., Hilarides, W.K. and Norman, W.D.: "The Role of Geomechanical Earth Modeling in the Unconsolidated Pre-Khuff Field Completion Design for Saudi Arabian Gas Wells," SPE paper 84258, presented at the SPE Annual Technical Conference and Exhibition, Denver, Colorado, October 5-8, 2003. 6. Rahim, Z. and Bartko, K.M.: "On the Use of Acid and Proppant Fracturing Treatments to Develop Carbonate and Sandstone Reservoirs ­ Field Examples," SPE paper 106328, presented at the SPE Technical Symposium of Saudi Arabia Section, Dhahran, Saudi Arabia, May 14-16, 2005.

Adel A. Al-Qahtani is a Petroleum Engineer working for the Gas Reservoir Management Department of Saudi Aramco. He has been working for Saudi Aramco for the last 8 years. In 2002, Adel received his B.S. degree in Chemical Engineering from King Fahd University of Petroleum and Minerals (KFUPM), Dhahran, Saudi Arabia, and in 2009, he received his M.S. degree in Petroleum Engineering from the University of Calgary, Calgary, Canada. He is a member of the Society of Petroleum Engineers (SPE) and the Arabian Society for Human Resources Management (ASHRM).

BIOGRAPHIES

Hassan D. Al-Anazi is a Petroleum Engineer working for the Gas Reservoir Management Department of Saudi Aramco. He has been working for Saudi Aramco for over 10 years. In 2001, Hassan received his B. S. degree in Petroleum Engineering from King Saud University (KSU), Riyadh, Saudi Arabia. He is a member of the Society of Petroleum Engineers (SPE).

Dr. Zillur Rahim is a Petroleum Engineering Consultant with Saudi Aramco's Gas Reservoir Management Division. His expertise includes well stimulation design, analysis and optimization, pressure transient test analysis, gas field development, planning and reservoir management. Prior to joining Saudi Aramco, Rahim worked as a Senior Reservoir Engineer with Holditch & Associates, Inc., and later with Schlumberger Reservoir Technologies in College Station, TX. He has taught petroleum engineering industry courses and has developed analytical and numerical models to history match and forecast production and well testing data, and to simulate 3D hydraulic fracture propagation, proppant transport, and acid reaction and penetration. Rahim has authored 50 Society of Petroleum Engineers (SPE) papers and numerous in-house technical documents. He is a member of SPE and a technical editor for the Journal of Petroleum Science and Engineering (JPSE). Rahim is a registered Professional Engineer in the State of Texas and a mentor for Saudi Aramco's Technologist Development Program (TDP). He is an instructor for the Reservoir Stimulation and Hydraulic Fracturing course for the Upstream Professional Development Center (UPDC) of Saudi Aramco. Rahim received his B.S. degree from the Institut Algerien du Petrole, Boumerdes, Algeria, and his M.S. and Ph.D. degrees from Texas A&M University, College Station, TX, all in Petroleum Engineering.

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Mohammad A. Masrahy previously worked as a high school geology teacher for 1 year, then as an administrator of scientific activities in the Directorate of Education for another year. He joined Saudi Aramco in November 2001, and worked with the Area Exploration Department (well site geology) as part of his PDP rotational development. In September 2004, Mohammad joined the Gas Field Characterization Division (GFCD) and worked as an Operation Geologist for Hawiyah and Haradh (Khuff and pre-Khuff), and Unayzah reservoirs. In October 2009, he moved to the special studies gas team within the GFCD. Mohammed has contributed significantly in the training of geoscientists within the exploration organization concerning pre-Khuff clastic reservoirs. Currently, he is working for the Reservoir Characterization Integration Team. Mohammad received his B.S. degree in Petroleum Geology in 2000 from King AbdulAziz University (KAAU), Jiddah, Saudi Arabia, and in 2011 he received his M.S. degree in Geology from King Fahd University of Petroleum and Minerals (KFUPM), Dhahran, Saudi Arabia. Mohammad is an active SPE member specializing in development geology.

Adnan A. Al-Kanaan is the General Supervisor for the Gas Reservoir Management Division, where he heads a team of more than 30 petroleum engineering professionals working to meet the Kingdom's increasing gas demand for its internal consumption. He started his career at the Saudi Shell Petrochemical Company as a Senior Process Engineer. Adnan then joined Saudi Aramco in 1997 and was an integral part of the technical team responsible for the on-time initiation of the two major Hawiyah and Haradh Gas Plants that currently process 6 billion cubic feet (BCF) of gas per day. He also manages Karan and Wasit, the two giant offshore gas increment projects, with expected total production capacity of 4.3 BCF of gas per day. Adnan has 13 years of diversified experience in reservoir management, field development, reserves assessment, gas production engineering and mentoring young professionals. His areas of interest include reservoir engineering, well test analysis, reservoir characterization and reservoir development planning. Adnan received his B.S. degree in Chemical Engineering from King Fahd University of Petroleum and Minerals (KFUPM), Dhahran, Saudi Arabia. He is a member of the Society of Petroleum Engineers (SPE).

Bandar H. Al-Malki joined Saudi Aramco in 1998 as a Production Engineer, working in the gas fields. He is currently the Supervisor of the HRDH Unit in the Gas Reservoir Management Division. This role requires him to monitor the production capacity of the plants, while optimizing the productivity of the wells and preventing wasted time and resources. Bandar received his B.S. degree in Petroleum Engineering from King Fahd University of Petroleum and Minerals (KFUPM), Dhahran, Saudi Arabia. In 2004, he earned his M.S. degree in Petroleum Engineering from the Imperial College, London, U.K., focusing on gas condensate reservoirs.

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