Well completion issues for underground gas storage in oil and gas reservoirs in China

1. Introduction

Underground Gas Storage (UGS) is considered as a strategic method to balance the energy supply-demand chain throughout a year and shave the peak demands during the winter time and thus quite important in today's competitive natural gas transportation market place. It involves the use of appropriate facilities to produce effective gas from underground space during peak demand periods, acting as a buffer to internal and external demand or supply shocks (Escobar et al., 2011). The gas stored in an UGS is called the inventory, and is classified as either cushion gas, which is to maintain adequate pressure, or working gas, which is the maximum volume of gas available for withdrawal.

The international UGS technology in Europe has evolved with quite a long history, and the operation management technology, technical standards, and related rules and laws are quite advanced (Coffin and Lebas, 2007; Hoagie et al., 2013; Brown et al., 2003; Lawal et al., 2016). According to the 2011 Statistical Report from European Union of the Natural Gas Industry, as of January 2011, there were 124 underground storage facilities in Europe. The share of each country is listed in Table 1. Approximately 400 underground natural gas storagefacilities located strategically throughout the United States in 2011 were the key to maintaining the reliability, integrity, and capability of the Nation's natural gas transmission and distribution network. According to a report from Canadian Gas Association, in Canada the maximum working gas stored was 456 × 109 cu ft (1.29 × 1010 m3) in 2006. Alberta storage accounts for 47.5 percent of the total working gas volume. It is followed by Ontario accounting for 39.1 percent, British Columbia accounting for 7.6 percent, and Saskatchewan accounting for 5.1 percent.

Table 1. UGS distribution in Europe at 1 January 2011 (excerpt from EuroGas, 2011).

Empty Cell	Number of storage facilities	Maximum working volume, m3	Maximum withdrawal capacity, m3/day
Austria	5	4744	55
Czech Republic	8	3127	52
France	15	11900	200
Germany	46	21297	515
Hungary	5	6330	72
Italy	10	14747	153
Netherlands	3	5000	145
Romania	8	2760	28
Slovakia	1	2785	39
Spain	2	2367	13
United Kingdom	6	4350	86
EU 27	124	85990	1453

In China, the government has committed to UGS construction in the past several decades. The first UGS construction was started in a gas reservoir in Daqing oilfield in the 1970th followed by the first real commercial UGS at Dagang Dazhangtuo gas condensate field which was put into production and operation in 2000 (Ding et al., 2015). A series of UGS constructions have been built in recent years, and till now there are totally 25 UGS operations in China, as listed in Table 2. In order to prevent the gas shortage in the winter of 2009, the Chinese government has promoted the construction of gas storage facilities at Jintan, Dagang and Huabei field, which has contributed to improving the natural gas network for seasonal demand. A typical salt cavern UGS is located at Jintan, which covers an area of 8.9 km2, and the current formation pressure is 13 MPa and formation pressure coefficient is 1.1. Xiangguosi UGS, a depleted gas reservoir type, has an initial pressure of 28.73 MPa. Its average formation pressure was 2.38 MPa and the formation pressure coefficient was 0.1 during well shut-in in 2010. In addition to these, CNPC brought into operation the country's largest underground gas storage facility in Hutubi in July 2013 which has a combined storage capacity of 10.7 bcm for the West-East Gas Pipeline. It is reported that China has planned to construct over 35 underground storages in total to boost its storage capacity to 60 bcm by 2020 (Zhang et al., 2014; IEA, 2014).

Table 2. UGS distribution in China.

District	Type	Construction time	Number of storage facilities
Dagang	Oil/Gas Reservoir	1999	6
Jing58	Oil/Gas Reservoir	2007	3
Guxinzhuang	Oil/Gas Reservoir	2013	1
Suqiao	Oil/Gas Reservoir	2013	5
Jintan	Salt cavern	2012	1
Liuzhuang	Oil/Gas Reservoir	2011	1
Lamadian	Oil/Gas Reservoir	1975	1
Shuang 6	Oil/Gas Reservoir	2014	1
Xiangguosi	Oil/Gas Reservoir	2013	1
Hutubi	Oil/Gas Reservoir	2013	1
Shan224	Oil/Gas Reservoir	2012	1

Since the construction of UGS in China, pretty much research work has been conducted in the past decades. In Dagang and Jing-58 area some experiences have been obtained in terms of geological system selection, well drilling and completion, numerical simulation to optimize UGS operation and so on. However, the research of UGS operation, management and maintenance in depleted reservoirs is relatively scattered in China, and a mature technology system has not formed. The difficulties in the construction of UGS in China lie in slow UGS construction and productivity speed, low working gas volume, sharp rise of investment cost, risk identification and safe operation, and so on. Moreover, China is lacking with advanced tools and equipment with independent intellectual property rights for the construction of UGS.

Three issues are quite critical for a successful UGS. One is maximizing storage capacity in the underground space, another is minimizing storage cost, and the third one is the integrity of the wellbores, including injection wells, production wells and existing old and abandoned wells. Well integrity is a prerequisite to ensure a safe and long-term containment of natural gas. The outcome of a loss of well integrity in UGS, similar to CCS, is the creation of different leakagepathways for the ascent of gas, as shown in Fig. 1. The mechanisms responsible for a loss of well integrity are subdivided into chemical loading, mechanical-thermal loading and construction defects (Reinicke and Fichter, 2010; Bai et al., 2014, 2015; 2016; Schultz et al., 2016). The key to maintaining well integrity is a successful well completion operation, which is obviously different from oil and gas production wells. The first part of this paper will introduce the UGS development status quo, especially with regard to well completion technology. Secondly, the integrity issues such as fault seal destabilization, and well integrity are reviewed. The third part will introduce the fundamental requirements for well completion, reuse and plugging of old wells in China. After that the development of well completion technology for UGS operation in China will be reviewed followed by a demonstration of a successful design of well completion string for gas injection wells with well-flushing and automatic security control functions in a UGS area in the Northeast China.

2. Well integrity issues for UGS

To achieve the purpose of large storage capacity and cost savings, it is necessary to use the existing wells as much as possible. However, the size of typical well configuration of UGS in China is small as well, which restricts the implementation of UGS and the emergency process ability. The field experience and theoretical analysis have proved that the use of Open-Hole Gravel Packs(OHGP) can not only avoid the difficulties and concerns of perforation packing, but also enlarge the capacity since the formation fluid would converge toward and through the gravel pack radically from 360°, and thus guarantees that it will be more productive. Therefore, according to a review conducted by Florian et al. (2009), by replacing Inside Casing Gravel Packs (ICGP) by Open-Hole Gravel Packs (OHGP) the existing wells can be converted to high-capacity wells whose performance could be improved significantly subsequently.

2.1. Cement integrity

To ensure well integrity during UGS, the cement sheath has to withstand cyclic mechanical and thermal loading as well as corrosion during its life time, otherwise different types of failure would occur such as interface debonding, presence of mud layer or mud channels at the contact surface, free water channel or layering in the deviated wells, and excessive thermal, hydraulic, and mechanical stresses at the wellbore. Many engineers, scientist and industries have designed fit-to-purpose cement formulations to prevent cement from such failures. For instance, the addition of latex or polypropylene fibers could improve the compressive strength (Shahvali et al., 2014). The use of lightweight slurries or engineered fiber material (EFM) in depleted and unconsolidated formations can prevent formation damage (Urraca and Balazs, 2009). An engineered expanding cement system has been introduced in China which allows the cement to remain intact under extreme downhole conditions (Wu et al., 2014; Zhang et al., 2014). Cavanagh et al. (2007) invented self-healing cements, in which an special cement formulation, based on a process called encapsulation, modifies and repairs the cracks. Ravi et al. (2007) measured the cyclic stress of self-healing cements in two successive cycles to study the mechanical properties as well as integrity of cement when put under cyclic loads. They pointed out that laboratory experiments can determine the effect of cyclic loading on the endurance limit of cement, while engineering analysis is required to set the safety factor in cement design.

2.2. Fault and caprock seal

Two principle factors influencing well integrity in a fault-related geological setting are fault reactivation and permeability of fractures within caprock. Numerous scientific contributions have emphasized that original fault seal capacity may be breached by reactivation resulting from fluid injection or extraction, shown in Fig. 2 (Ellsworth, 2013; Elsworth et al., 2016). However, two ways of fault activation, in fact, concerning this issue include aseismic slip and seismic slip. It is confirmed by recent studies that aseismic slip commonly occurs in clay-rich fault zones or caprock with low frictional strength at shallow depth range (<8 km) (Ikari et al., 2011; Kohli and Zoback, 2013), and these weak faults and rocks being barriers to injection fluid may only induce micro-seismic events without leading to gas leakage from the storage (Makhnenko and Vilarrasa, 2017). Yet, it should be pointed out that aseismic slip can cause severe instability for certain well trajectories (Willson et al., 1999) and borehole casing failure (Maury et al., 1992). Methods of fault reactivation analysis have been established such as “Slip Tendency” (Morris et al., 1996) and “Coulomb Failure Function” (Castillo et al., 2000), and have been proved to be quick routes to preview the fault stability by assumption of homogeneous frictional strength of fault zones. It is important to note that the low frictional strength with high reactivation risk routinely corresponding to the high sealing capacity of faults due to the reduced fault-zone permeability by increasing weak and low permeable clays (Meng et al., 2017; Vilarrasa et al., 2016), thus, fault-sealing analysis is an essential prerequisite for fault stability evaluation.

In siliciclastic sequences, caprocks are commonly clay-rich, showing a strong membrane sealing capacity. Hence, there is little possibility for amount of fluid seeping through the pore throats on production timescales. Gas seeping occurs commonly in the fracture network of caprocks, especially within the fault damage zone where the concentrated stress favors the formation and interlink of multi-orientation fractures (Bond et al., 2017). These fractures may be open if the pore fluid pressure keep rising due to fluid injection. However, the permeability may decrease with an increase of effective confining pressure due to compaction-induced self-healing of clays (Zhang and Rothfuchs, 2008). Hence, it is essential to evaluate fracture-opening risk using the method of “Retention Capacity” proposed in Gaarenstroom et al. (1993). That is, as will be seen in Fig. 3, two sections including fault stability and retention capacity of caprock are integral parts of the entire evaluation of gas storage safety to constrain maximum fluid pressure sustained by both faults and caprock avoiding occurrence of any risks of rock failure.

3. Requirements for UGS well completion in China

3.1. Newly drilled wells

1)
Mechanical analysis shall be performed based on the periodic alternating stress, temperature variation, and extreme operating conditions. Anti-corrosion materials shall be adopted based on the property of the reservoir fluid, considering both the initial and potential changes of the environment.
2)
The completion string shall be of the simplest structure, equipped with Subsurface Safety Valves (SSSV), and use reliable tools instead of expansion joints. Moreover, it shall be installed with seating joints, with appropriate drift diameter to allow for downhole monitoring or snubbing operations. Last but not least, the threads of the pipes shall be air-tight, and each pipe shall go through air-tight test under a pressure 1.1 times the maximum operation pressure of the wellhead.
3)
The well completion fluid, acidizing fluid and kill fluid shall be compatible with formation, and be of solid-free system to reduce harm to the formation.
4)
The pressure rating and anti-corrosion grade of Christmas tree shall be determined based on maximum operation pressure and properties of formation fluid. It shall use metal-to-metal seal, and go through air-tight test before mounting.

3.2. Old wells

1)
Before UGS construction, information about geological conditions, well drilling and production history must be collected and the well status must be assessed in terms of cementation quality, casing thickness, pressure test, well track and so on.
2)
In principle, the old wells can be re-used as monitoring wells, production wells and drainage wells, but not injection wells. To be reused as a monitoring or production well, three requirements have to be met. Firstly, the length of continuous cement sheath with good quality in the caprock section shall be no less than 25 m, while at least 70% of such segment in length shall be qualified. Secondly, the mechanical integrity of the string shall be checked and it has to meet the requirements of actual operation conditions. Thirdly, the production casing shall go through pressure test with fresh water under pressure 1.1 times the maximum operation pressure at the wellhead. It will be deemed qualified if its pressure drop is not higher than 0.5 MPa within 30 min.
3)
If reused as drainage well, the old well shall have its wellhead replaced to meet the operation conditions. During the operation process of UGS, once the gas-liquid ratio reaches 300 m3/m3, the well shall be plugged. In order to plug a well, cement plugs shall be placed to prevent the migration of natural gas. In addition, proper materials shall be squeezed into the reservoir section, with a pressure not higher than the formation fracture pressure.
4)
If the cement top is higher than 200 m, and the continuous cement sheath with good quality in the caprock section is longer than 25 m, the bridge plugs can be directly placed; otherwise the casing shall be milled for no shorter than 40 m and part of the boreholes shall be reamed, and injected with continuous cement plugs.
5)
The filter loss of the cement slurry shall be not more than 50 ml, the gas permeability of the set cement shall be lower than 0.05 × 10−3μm2, and the compressive strength shall be not less than 14 MPa within 24–48 h.

4. Development of well completion technology for UGS wells

If existing wells cannot guarantee well integrity, new wells shall be drilled. The drilling and completion technology for UGS has evolved significantly in the past 10 years in China (Li et al., 2014; Chen et al., 2003, 2008). The key technology includes lost circulation control, directional well drilling, completion and cementation of large wellbores, anti-corrosion and so on. To ensure wellbore integrity, the technical and regulatory considerations for injection wells need to focus on the mechanical and chemical aspects, and their coupling effects (Dharmananda et al., 2004; Syed and Cutler, 2010). Due to the corrosive injected gas, the corrosion problems of downhole equipment require more attentions during the construction and operational phase (Yuan et al., 2008; Zhang et al., 2001; Li et al., 2014). Especially the part of casing below the packer is prone to acid and therefore warrants corrosion resistant alloy. A series of measures can be adopted against the corrosion problems such as oxygen scavenger, coating protection, and cathodic protection etc (Yuan et al., 2008). Moreover, corrosion resistant alloys such as duplex stainless steel or lined material can also be used (Kermani and Smith, 1997; Bai et al., 2015). It is found that tubing made of seamless L-80 material with Hydfil CS premium connections can prevent Sulfide Stress Cracking (SSC) and lowering of the tubing tensile stress. Composite lined material like Glass Reinforced Epoxy (GRE) linings, Internal Plastic Coatings (IPC), thermoplastic coatings are more commonly used as corrosion barriers for injection tubing (Kenneth, 2010; Bai et al., 2015; Zhang et al., 2011). UGS in depleted gas reservoirs in China is generally deeper than 2500 m, with multiple vertical pressure systems which makes it imperative to use suitable drilling and completion technology. Some cutting-edge technologies have not been applied in China, e.g., welded casing technology.

Similarly to oil and gas wells, well completion method for UGS includes perforation completion, open-hole completion, gravel packing and so on. A difference lies in that the bare area of boreholes should be as large as possible for UGS so that the gas filtrational resistance can be reduced and the individual well productivity can be improved. Well completion technologies vary for different types of UGS, as listed in Table 3. A salt cavern UGS such as Jintan allows for a rapid gas production speed, and therefore a large-diameter completion string with big internal pressure strength and air tightness is needed. Different from salt caverns, gas in depleted hydrocarbon reservoirs and aquifers such as Xiangguosi is stored in rock matrix with large flow resistance; therefore, the string diameter is relatively small. In Jintan Φ177.8 mm tubing is used, while for conventional gas reservoirs, tubing with a diameter of 152.4 mm is generally satisfactory.

Table 3. Well completion comparison for different types of UGS in China.

UGS Type	Injection/production rate	Storage medium	Diameter of string	Anti-corrosion requirement	Well cementing
Salt cavern	High	Cavity	Large	Extremely high	Expansive cement
Oil/Gas reservoir	Low	Matrix	Normal	High	Expansive cement
Aquifer	Low	Matrix	Normal	High	Expansive cement

The completion string for the first UGS wells in China comprises basically wellhead and tubing, flow coupling, safety valve, XD sliding sleeve, expansion joint, anchored seal assembly, permanent packer, landing nipple, mechanical gun release and perforating gun, as shown in Fig. 4. The Dagang UGS wells adopted such design. All the well completion methods thereafter are optimized on this basis. Generally, the completion string in a gas reservoir consists of SSSV, sliding sleeve, retrievable packer and landing nipple, etc. A SSSV can prevent gas from leaking along the tubing, a retrievable packer can seal the annulus between injection tubing and production casing, and the subsurface safety condition may be monitored in real time through the pressure and temperature on the landing nipple.