Optimisation of Sand Control Technique for Offshore Heavy Oil and Unconsolidated Sandstone Reservoir: Case Study from Bohai Bay
Chen, Shenghong, CNOOC Energy T&
S Ltd., Cheng, Kun, SPE, Gokdemir, Metin, Weatherford, Xing, Xuesong, CNOOC Engineering Technology Department, Cheng, Zhong, SPE, CNOOC Energy T&S Ltd Copyright 2013, International Petroleum Technology Conference This paper was prepared for presentation at the International Petroleum Technology Conference held in Beijing, China, 26–28 March 2013. This paper was selected for presentation by anIPTC Programme Committee following review of information contained in an abstract submitted by the author(s). Contents of the paper, as presented, have not been reviewed by the International Petroleum Technology Conference and are subject to correction by the author(s). The material, as presented, does not necessarily reflect any position of the International Petroleum Technology Conference, its officers, or members. Papers presented at IPTC are subject to publication review by Sponsor Society Committees of IPTC. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes without the written consent of the International Petroleum Technology Conference is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrat ions may not be copied. The abstract must contain conspicuous acknowledgment of where and by whom the paper was presented. Write Librarian, IPTC, P.O. Box 833836, Richardson, TX 75083-3836, U.S.A., fax +1-972-952-9435
Abstract As conventional hydrocarbon resources in current basins of the world continue to decline, heavy oil becomes a significant candidate to fulfill the world’s thirst for energy. In China, the potential reserves of heavy oil amount to 220 billion BOE, with more than 13% in offshore fields. For Bohai Bay, up to 80% of developed reservoirs are heavy oil and unconsolidated sandstone reservoirs. Due to the geological process and depositional environment in generating heavy oil, the common characteristics of such oils are high viscosity, low gravity, high asphaltenes, and weak homogeneity and cementation. As a result, the productivity of heavy oil is usually low in a single well, accompanied with sand production. In recent years, the sand control customization technology and single well precise design based on single formation was developed to optimize deliverability of single formation. The optimized sand control technique has become one of the major methods for increasing and stabilizing production in Bohai Bay offshore oilfield. This paper begins with an introduction of the application and limitations of traditional sand control methods in Bohai Bay offshore oilfield. Then, to meet the requirement of further enhancing oil recovery, sand management and proper sand control completion technology are introduced. After the experiment was conducted, the results of sand control were obtained from most oilfields. However, bad results of application still existed in some oilfields.
Introduction Up to 80% of developed reservoirs in Bohai Bay are heavy oil and unconsolidated sandstone reservoirs. The restrictions – such as the wide range of reservoir distribution, shallow depth, poor diagenetic grade, and sand production – become principal obstacles for reservoir development. According to official statistics, up to 90% of wells in Bohai Bay need sand control. The effect of the sand control will directly affect the enhanced oil recovery. Traditional sand control methods cannot satisfy the requirement of better enhanced oil recovery since the most of oilfields in Bohai Bay are characterized by high oil viscosity, sand production, high asphaltenes and resin content, and weak homogeneity and cementation. Since 2002, operators have applied the sand management concept and sand control completion technology to multiple oilfields with enlarged production pressure differential and expansion of well production capacity. Under the above circumstances, the sand production has been controlled within the scope of the security. It has been proven that the sand management concept and sand control completion technology have successfully controlled sand production in most oilfields. But for a few low-pressure heavy oil reservoirs, the results have been poor. In 2008, the optimized sand control technique, which can maximize every formation’ s production capacity and control sand production at the same time, became one of the key measures of oilfield production stabilization and stimulation. Reservoir Geological Characteristics Bohai oilfield belongs to fluvial facies and delta facies with features such as strong reservoir heterogeneity, unsolidated rock cementation, free sand, strong sensitivities (especially for water sensitivity and stress sensitivity), and shallow depth (within 3,600 ft to 5,600 ft). The heavy oils with 12 to 20 API are mainly produced from the Guantao and Minghuazhen formations. The oil viscosity at the reservoir ranges from dozens to thousand centipoise (cp). Therefore, most oilfields lack enough natural energy to produce with low oil mobility. Early Sand Control Technique in Offshore Oilfields Several failed sand control wells produce large volume sand which affects normal production. Due to the weak offshore sand processing capacity, the general requirement of sand production is lower than 0.003% and most wells adopt the early sand control method. During the recent decade, Bohai Bay oilfields applied sand control treatment to more than 1,000 wells. Before 1998, wells utilized stack packing technology which had high operation risk and cost for wells with multiple sand control segments. Since year-end 1997, oilfields JZ 9-3, SZ 36-1 second section, and QHD 32-6 have been developed based on the single-trip, multi-zone (STMZ) or dual-trip, multi-zone (DTMZ). STMZ and DTMZ can minimize rig time over conventional stacked frac-pack systems by significantly reducing the number of trips in and out of the well. Formation damanage is reduced by decreasing exposure time to completion fluids with intervals perforated at once in a well. However, high operation risk and complicated tool structure present challenges. Thereforeoperators optimized the sand control tools and technique with enhanced completion operation efficiency, reduced operating costs and controlled operation risk. Consequently, STMZ and DTMZ are widely used in Bohai Bay. Prior to 2002, sand control completion for unconsolidated sandstone offshore oilfields was
preferred to gravel packing. The goal was to eliminate sand production. Gravel packing completion uses packed gravel to block formation sand and utilizes screens to block the packed gravel. The packed gravel can be used for the sand control practice, but it will create a sand ring around the screens. The sand ring reduces the permeability significantly, resulting in a high skin factor and low well productivity. Sand Managementand Proper Sand Control Completion Technology in Bohai Bay Oilfields With sand management concepts in which traditional sand control techniques are not normally applied, operators manage production through sand influx, fluid rates, and well pressures control and monitoring. North Adriatic Sea Oilfield conducted a successful pilot test that has resulted in approximately 300 wells utilizing sand management methods during production. For appropriate sand control, operators put a special sieve tube into oil reservoir sections with enlarged sand particle size. During the production process, the formation sand whose size is below the specified particle size is allowed to enter the wellbore. Artificial lift methods are adopted to lift the formation sand onto surface for further separation treatment. Final ly, the fine formation sand particles around wellbore are produced. The production formation is unloaded and could destroy the damaged areas during well completion. Then, the permeability around wellbore is optimized, which will increase conductivity and promote well productivity. Sand Management in Bohai Bay In 2002, sand management concept was introduced to Bohai Bay oilfields. Operators using sand management achieved success on the pilot well CF1 of SZ 36-1 oilfield. After increased applications and modifications in other oilfields, current sand management for offshore oilfields has been gradually established. Proper Sand Control Completion Technology in Bohai Bay CZ 36-1 oilfield is the first oilfield with over a billions tons of oil reserves, with depths ranging from 4,200 ft to 5,250 ft. The oil density is 0.968 g/cm3 and thehe oil viscosity is 380 cp. The oil reservoir permeability is around 500 to 2300 mD. The development of CZ 36-1 oilfield is divided into 2 sections. The first section has 3 platforms named A, B and J with 64 wells which were built from 1993 to 1996; the second section has 6 plateforms named C, D, E, F, G, and H with 186 wells which were built from 1999 to 2001. On Septermber 2002, the first attempt was proceeded on the pilot well CF1 of CZ 36-1 oilfield. Well CF1 is a multi-laterals horizontal well with 1 main borehole and 4 laterals, using the multi-laterals open-hole completion. The main borehole uses 7-inch metal cotton screen to control sand as shown in Fig. 1. IPTC 17036 3 图1 Fig. 1—7” Metal Cotton Screen The target layer of well CF1 is next to the well SZ 36-1-15, and its well core grain size analysis as shown in Fig. 2 can be used for the metal cotton screen effect of well CF1. The formation of well
CF1 is the fourth layer of Dongying formation’s first oil group. The oil and water interface meets at 5,227 ft. From Fig. 2, the median grain size of the fourth layer is between 0.042mm and 0.125mm. The operator decides the median grain size based on the 10% allowable sand production amount. For well CF1, the value is selected as 0.081mm. Considering that the gravel median grain size must be 6 to 8 times the formation sand particle size, the sand control ability of well CF1 is equal to 30/40 mesh gravel pack effect, which would be 40/60 mesh based on the early sand control method. After implementing sand control completion, electric submersible pump (ESP) with 100 m3/day capability was settled in downhole. 图2 Fig. 2—Median Fromation Grain Size of Well SZ 36-1-15 Since production on September 17th, 2002, before changing ESP on November 15th, 2002, the daily liquid production had been around 80 m3, and the production pressure differential was around 0.9 Mpa without sand production. On November 15th, 2002, with the attempt to produce sand and increase pressure differential, ESP changed to 200 m3/day capability. After starting a new pump, the maximum liquid production rate was180 m3/day with sand production blocking the platform processing screens twice. Then the liquid production became stable without sand production. The liquid rate was 165 m3/day. The water cut was approximately 1%. The production pressure differential was around 1.5 to 2 Mpa. Consequently, daily production rate of Well CF1 is more than its designed production rate of 100 m3/day. The designed production pressure differential is 3 Mpa. Two followup multi-laterals horizontal wells with proper sand control are C25hf and C26hf on platform C. Their practical production rates are both more than their designed production rate, which can be seen in Fig. 3. 图3 Fig. 3—The Comparison between Normal Wells Group and Wells Group With Proper Sand Control Completion Technology From the production data in Fig. 4, it can be concluded that the liquid production rate of Well CF1 had been above 100 m3/day until July 2011. The proper sand control technology proved successful on the first attempt. 图4 Fig. 4—The Production Curve of Well CF1 of SZ 36-1 Oilfield Promotion of Proper Sand Control Completion Technology in Bohai Bay Oilfields After 2002, the proper sand control completion technology was optimized and promoted on adjustment wells and new oilfiled development. The adjustment wells are the second section of SZ 36-1 oilfiled and QHD 32-6 oilfield.The new developments include the first section of PL 19-3 oilfield (2003), BZ 25-1 and Lvda oilfileds group (2004-2005), and NB 35-2 oilfield (2005-2006). When making sand control completion decision, operators predicted the sand production quality, analyzed the well logging interval transit time curve, tested core lab works, and investigated the
sand production situation of adjacent wells and records of oil and sand production from exploratory wells’ drill stem tests to decide the sand control completion type and sand filtration precision. Proper Sand Control Completion Technology on Directional Wells For the well-sorted and cemented reservoir conditions with large formation particle size and shale content that is less than 10%, it is recommended that the mash screen be settled after the perforation. For the opposite situation, it is suggested that gravel pack with wire wrapped screen be utilized in regular directional wells or that mash screens be used for highly deviated wells. Proper Sand Control Completion Technology on Horizontal Wells Open-hole completion is adopted for horizontal wells. For the well-sorted and cemented reservoir conditions with large formation particle size and shale content which is less than 10%, it is recommended to settle the mash screen. For the opposite situation, it is suggested to use gravel pack with mash screen. Application Examples of Proper Sand Control Completion Technology One group of oilfields located in Liaodong Bay of Bohai Bay has achieved success with the application of proper sand control completion technology. This group is combined with three heavy oil and unconsolidated sandstone reservoirs. For example, oilfield A has 41 wells. The target layer is Dongying formation’s second oil group. The depth is around 4,265 to 5,580 ft. The average porosity is 33%. The permeability is around 100 to 1000 md. The oil density is 0.97 g/cm viscosity at Dongying formation’s upper second oil group is 210 cp. For the lower second oil group, it is around 36.1 to 75.9 cp. From the formation sand grain analysis, operators have gathered a set of data, such as median grain size is 330 μ m for the upper second oil group and 140 μ m for lower second oil group. The uniformity coefficient is less than 5 and fine sand content with a grain size below 44 μ m is less than 5%. Among the 39 producing wells, only one is a horizontal well. The others are directional wells. For the horizontal well, open-hole completion is adopted with mash screens to perform sand control. For the directional wells using tubing perforation completion, only 4 wells adopt gravel packing, the others have mash screens. The development started on July 2005. From April 2007 to now, all wells are in production status. According to the statistical data, all the directional wells have good production performance with average single well production rate of the 70 m The Application of Customized Sand Control Completion Technology The proper sand control completion technology is implemented successfully in most of oilfields. However, in some low pressure heavy oil reservoirs, it cannot perform optimally. Based on proper sand control completion technology, in 2008, operators of Bohai Bay proposed the customized sand control completion technology for shallow buried and unconsolidated reservoirs with major sand production problem.
Since 2008, the customized sand control completion technology has been used on hundreds of wells with conclusive results. Most of these wells produce more than the production allocation amount. Several wells produced more than 1000 mRapidly declining production is eliminated using proper sand control completion technology. Using multi-lateral Well A25m as an example, operators implemented the customized sand control completion technology. The unified drilling and completion concept was adopted. During the drilling process, the ESP depth was considered to be the steady inclined section. To stop blocking sieve tube in the completion process, 210 mesh shaker cloths were used to clear solids in mud. To secure the sieve tube settlement, wellbore trajectory was optimized. Considering the risk of reservoir bottom water, a staged sieve tube was settled to delay water coning. Finally, with customized sand control completion technology, Well A25m achieved the goal of producing 800 m3/day. Conclusions The optimisation of sand control technique for offshore heavy oil and unconsolidated sandstone reservoir changes the traditional thinking which we relaid on the gravel pack completion technique. The introduction of different sand control completion technologies at different periods can help conclude that sand control technique aid improvement in Bohai Bay oilfields. Meanwhile, the optimization of sand control technique should be joined with other technologies, such as unified drilling and completion mode. Currently, the optimization of the sand control technique is widely used for offshore heavy oil and unconsolidated sandstone reservoirs in Bohai Bay. The goal of enhancing well productivity and lowering the development cost is achieved. References Cheng, K., Cheng, Z., and Wu, W. 2007. A Correction Method for Density Log in Thin Bed. Well Logging Technology (6): 31. Cheng, K., Ye, Z., Wu, W., and Cheng, Z. 2008. A Thin-Layer Density Log Calibration Method and Application Of Software. Petroleum Geology & Oilfield Development in Daqing (3): 27. Cheng, K., Wei, Y., Wu, W., and Holditch, S.A. 2010. A Novel Optimization Model for Analyzing Production Data. Paper SPE 132545 presented at SPE Western Regional Meeting, Anaheim, California, USA, 27-29 May 2010. Cheng, K., Cheng, Z., Wei, Y., Wu, W, and Holditch, S.A. 2011. China Offshore Anti-collision Risk Management Standard and Remedial Measures. Paper SPE 141089 presented at SPE Americas E&P Health, Safety, Security, and Environmental Conference, Houston, Texas, USA, 21-23 March 2011. doi: 10.2118/141089-MS. Cheng, K., Gokdemir, M. O., Cheng, Z. 2013. China Offshore HSE Management for Securing Platform Drilling Operations. Paper SPE 163745 presented at SPE Americas E&P Health, Safety, Security, and Environmental Conference, Galveston, Texas, USA, 18-20 March 2013. Chen, S., Deng, J., Fan, B., Li, B., and Xu, R. 2010. Lessons Learned from a Well’s Sand Control Failure in Bohai Bay. Paper SPE 136734 presented at International Oil and Gas Conference and Exhibition in China, Beijing, China, 8-10 June 2010.
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