Novel completions increase horizontal well production

April 6, 2015
Two novel completion techniques that solve the problems of acid fracturing in complex carbonate horizontal wells have been successfully implemented in China. Using them has enhanced production in the Tarim, Sichuan, Bohai Gulf, and Ordos basins.

Guo Jian-chun
Gou Bo
Yu Ting

Southwest Petroleum University
Chengdu City, China

Two novel completion techniques that solve the problems of acid fracturing in complex carbonate horizontal wells have been successfully implemented in China. Using them has enhanced production in the Tarim, Sichuan, Bohai Gulf, and Ordos basins (Fig. 1).

The carbonate reservoirs of these basins share several attributes. They are heterogeneous, deep, high-temperature, and possess complex formation fluids. They also feature discontinuous fracture cavities that need to be connected through multi-stage acid fracturing to achieve commercial production.

This article looks at these reservoirs, the difficulties faced during acid fracturing wells in them, and how two techniques-diverting acid fracturing and segregated completion-have been widely applied to increase productivity.

Complex carbonate reservoirs

China's carbonate reservoirs are widely distributed and hold much of the country's total oil and gas resources. New hydrocarbon-rich fields have been discovered in marine carbonate reservoirs including Yuanba and Moxi-Anyue gas fields in the Sichuan basin, Tazhong gas field in the Tarim basin, and Daliudi gas field in the Ordos basin.

As shown in the accompanying table, the reservoir characteristics of these fields are complex.

• They have a low porosity and ultralow permeability, which causes a loss of storage capacity for the reservoir matrix.

• They are highly heterogeneous because of the random distribution and sizes of fractures, cavities, and dissolved pores.

• The target strata, Triassic to Cambrian, are at more than 5,000 m deep for most wells with the deepest well in Yuanba gas field reaching nearly 7,000 m total vertical depth.

• Reservoir temperatures can reach 160° C.. in the deep formations, such as Tazhong, Yuanba, Puguang, and Moxi-Anyue gas fields. Formation pressure is as high as 78 MPa.

• Fluid composition is diverse, including oil, gas, water, hydrogen sulphide, carbon dioxide, and nitrogen gas.

Enhancing single-well production in these reservoirs requires drilling horizontal wells perpendicular to the natural fractures to hit as many hydrocarbon storage spaces (cavities, dissolved pores, and fractures) as possible.

Multi-stage acid fracturing is necessary to create hydraulic fractures with conductivity. These connect the hydrocarbon percolation path and storage spaces. Acid treatment, however, is difficult in China's complex carbonate reservoirs for three reasons:

1. Conventional acid fracturing is not effective in heterogeneous reservoirs with discontinuous pore volumes. Although the horizontal well is segmented by the downhole tool, the heterogeneity is still a concern in the interior of each stage because of the uncertain distribution of hydrocarbon pore volumes.

The acid-etching double-wing fracture created by conventional acid fracturing only connects a few hydrocarbon storage spaces due to the acid-fracture propagation controlled by ground stress. (Fig 2a).

2. Gelled acids are usually used in low-permeability carbonate reservoirs. Their viscosity, however, declines to 10 MPa•sec in China's high formation temperatures.

This condition creates a quick acid-rock reaction rate and a large volume of acid filtration, which shortens the acid-etching fracture length, reducing the chances of connecting remote hydrocarbon storage spaces and decreasing well performance after stimulation.

3. There are obstacles to selecting the best segregated completion method. Downhole tools must meet the high-temperature and pressure demands of China's carbonate formations. The high content of acidic gas, including hydrogen sulfide and carbon dioxide, can cause tool degradation and safety concerns. And the water zone in some stages can be breached by acid-etching fractures, causing natural water flooding, which is damaging to a horizontal well.

Two new technologies have been introduced in recent years to overcome these difficulties.

Diverting acid fracturing

Diverting acid fracturing, which combines fracture reorientation technology (FRT) and fluid diversion technology (FDT), can connect many more hydrocarbon storage spaces than the conventional method.

When hydraulic fractures are initiated and extended in an unfavorable direction, FRT, using degradation fiber as a temporary plugging material (TPM), can be applied to block the fracture and reorient it in an advantageous direction.

Meanwhile, FDT with temperature-control viscosity acid (TCA) or a self-diverting acid (SDA) system can achieve deep penetration and efficient acid placement.

This process can form an acid-etching fracture network, which increases connections between hydrocarbon storage spaces (Fig. 2b).

Lab simulations show that this method can compel hydraulic fractures to reorient and form a new fracture by using a high-strength TPM. Some popular TPMs, however, such as naphthalene and oil-soluble resin, aren't suitable for China gas wells due to a narrow range of applicable temperatures.

A new TPM had to be designed that could form into a mud cake with a certain thickness and strength to reorient the hydraulic fracture in FRT. Fiber with a high plug intensity and degradation rate was developed.1 2 It was effective because it did not pollute the formation and could be applied to reservoirs with a wide range of temperatures while meeting their acid stimulation requirements.

China's petroleum engineers also looked at diverting acid fracturing to reduce the acid-rock reaction rate and acid leak-off volume. By looking at both TCA and SDA, they were able to create a low frictional resistance and a combination of stimulation and formation protection in deep, high-temperature, complex carbonate reservoirs.3 4

TCA has a lower viscosity on the ground and during pumping, which is favorable for large displacement due to its low frictional resistance. Underground, however, the spent acid and fresh acid of TCA have a high viscosity (more than 220 MPa•sec), which causes acid diversion and produces longer etched fractures. TCA viscosity will decrease after stimulation due to continuous high temperatures, which helps the spent acid flowback (Fig. 3).

TCA viscosity increases with high reservoir temperature, overcoming the problems faced when conventional acid viscosity declines at high temperature. TCA has been applied to reservoirs with temperatures of 100-160° C.

SDA is another novel diverting acid. A special viscoelastic surfactant controls acid-system viscosity through the physicochemical actions of acid-rock reaction products.

Most viscoelastic surfactants exist in the fresh acid of SDA in the form of single-molecules during pumping. SDA therefore has a lower viscosity and is easier to pump.

SDA's viscosity increases to more than 350 MPa•sec after entering the formation. This is due to the decrease in acid concentration and increased concentration of calcium and magnesium ions during the acid-rock reaction. A network structure of viscoelastic surfactants is formed.

After stimulation, the network structure is broken by hydrocarbons, reducing SDA viscosity and easing flowback.

Segregated completions

The segregated completion method, which employs special downhole tools for complex horizontal wells, is used to meet the requirements of acid-fracturing in China's carbonate reservoirs.

These have long horizontal sections (more than 1,500 m in some wells) and strong heterogeneity, making sufficient stimulation difficult even when employing diverting acid fracturing. The sections must be broken up into stages and acid-treated individually.

Openhole completion is often applied to horizontal sections and there are two novel and midstream open-hole segregated completion methods being used in China: a combination of sliding-sleeve and openhole packers (OHP), and a combination of screen pipes and OHPs.

The former method is the most popular. The completion strings are composed of a holddown packer, oil tube, OHP, sliding sleeve, and ballseat (Fig. 4a). The holddown packer is hung under the casing pipe to prevent it from being corroded by hydrogen sulfide and carbon dioxide. The oil tube is made of a nickel-based alloy to protect against acid-gas corrosion. The OHP is the primary segregation tool and its rubber sleeve must endure high temperatures and corrosive elements. A modified hydrogenated butyronitrile material is the best choice for the rubber sleeve, which is controlled by pressure differential.

After the first stage is acid-fractured, the sealed ball is sent to the sliding sleeve position to seal the first stage and open the sliding sleeve. The completion string can be used to a reservoir depth of 7,000 m, at temperatures up to 170° C., and at a pressure differential of 70 MPa.

Employing this method, long horizontal wells can be separated to 15 stages by the completion string.

With the second method, the combination of screen pipes and OHP, the completion strings consist of a hold-down packer, oil tube, OHP, screen pipe. and ballseat (Fig 4b).

Valve-opening pressure increases from the first screen pipe in the first stage to the last pipe in the last stage. Screen pipes with low valve-opening pressure will open first when the tubing pressure is up to the opening pressure of the value during acid treatment.

After the first-stage treatment, 50 seal balls are sent to the first stage to seal the first screen pipe. The second stage is then stimulated. This completion method does not restrict the number of stages used.

These new technologies have been applied in 677 carbonate horizontal wells in China's Tarim, Sichuan, Bohai Gulf, and Ordos basins with cumulative production increases. Engineers recreated this success in other carbonate oil fields around the world with similar results (Fig. 5).

Acknowledgments

The authors acknowledge the support of the "Study on acid filtration mechanism and treatment design optimization during acid fracturing in complex medium (project ," sponsored by the National Key Projects Fund.

References

1. Fujian Zhou, Yuzhang Liu, Xianyou Yang, et al., "Case Study: YM204 Obtained High Petroleum Production by Acid Fracture Treatment Combining Fluid Diversion and Fracture Reorientation," presented to the 8th European Formation Damage Conference, May 27-29, 2013, Scheveningen, the Netherlands.

2. Xianyou Yang, Xiongfei Liu, Fujian Zhou, et al., "Laboratory Study and Field Application of Fiber-Based Fracture Reorientation Technology," presented to the International Petroleum Technology Conference, Mar. 26-28, 2013, Beijing.

3. Fujian Zhou, Xianyou Yang, Fuxiang Zhang, et al., "A Novel Diverting Acid Stimulation Treatment Technique for Carbonate Reservoirs in China," presented to the Asia Pacific Oil and Gas Conference & Exhibition, Aug. 4-6, 2013, Jakarta.

4. Fujian Zhou, "Application and Study of Acid Fracture Tecnhique Using Noval Temperature Control Viscosity Acid in Carbonate Reservoir, TARIM," presented to the International Oil & Gas Conference and Exhibition in China, Dec. 5-7, 2013, Beijing.

The authors
Guo Jian-Chun ([email protected]) is a professor and the executive president of the Oil & Natural Gas Engineering department at Southwest Petroleum University, China. He holds a PhD in engineering from the same university and is a member of the Society of Petroleum Engineers (SPE).
Gou Bo ([email protected]) is PhD candidate in oil and gas well stimulation in the State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation at Southwest Petroleum University. He holds an ME from the same university and is a member of SPE.
Yu Ting ([email protected]) is teaching assistant at Southwest Petroleum University's school of science. She holds an MS in science from the same university.