Dongguo casing failures derive from many causes

Jan. 21, 2007
A program was instrumental in assessing the causes of casing failures and remediating them in wells in the Dongguo area of northeast China.

A program was instrumental in assessing the causes of casing failures and remediating them in wells in the Dongguo area of northeast China. The Dongguo area is part of Daqing oil field in Heilongjiang province.

Development of Dongguo has taken place during the last 13 years. With the drawn out exploitation program, the wells experienced problems, especially casing failures during the last 2 years.

These failures affected oil field development by complicating the proportion of fluids injected and produced, reducing oil recovery, restricting injection and production rates, and accelerating the production decline rate.

Dongguo

The Dongguo area has 136 oil producing wells and 175 water injection wells. The area produces about 276 tonnes/day of oil (2,035 bo/d), with 1,975 tonnes/day of total fluid lifted. The water injection rate is about 2,847 cu m/day (17,900 bw/d).

The produced oil is a paraffin-base crude oil with low sulfur content and a high pour point. Under formation conditions, the oil has a 0.795 g/cc density and a 6.74 cp viscosity. The reservoir had an initial 44.5 cu m/tonne GOR and a 7.85 MPa (140 psi) saturation pressure.

Under surface conditions, the oil has a 0.8535 gravity (34° API) and a 14.85 cp viscosity.

The injected water has 6.78 mg/l. of suspended solids, 4.68 mg/l. of oil content, 0.6 mg/l. of metratrophic bacteria, 60 mg/l. of sulfate reducing bacteria and 600 mg/l. of iron oxidizing bacteria.

The analysis indicated that the injected water caused little damage to the casing and attributed only about 2.5% of the damaged casings to injected water.

Casing failures

So far, the field operator has found 22 wells with damaged casing. Twelve casing failures were in oil producers and 10 were in water injectors.

The wells experienced two types of casing failures: deformation and fractures. Twelve wells had deformed casing, while 10 wells had fractured casing (Table 1). Fourteen wells had damaged casing away from the oil zone, while seven wells had damaged casing in the producing interval. One casing failure was not noted as to its location.

Click here to enlarge image

Eleven casing failures were in the Sa I, II interbed layer. Two casing failures were in the Sa 0, I interbed layer.

The number of wells with casing failures increased yearly, especially in wells that failed from deformed casing.

As seen in Table 1, 63.6% of the casing failures were in the nonoil layer. The casing failures in the nonoil layer zone were mostly in the X6-3-BW610 well district at the Sa I, II interbed layer.

There were 11 well casings in the Sa I, II interbed layer, and 5 of these wells went on production before 1998. Operation reports showed that damaged casing occurred in four wells between July and December 1998. Casing damage occurred in Well X6-3-B59 between March 2003 and April 2005. The other six wells are outlying, put on production in June 2004.

First, in terms of lithology, the grey and black shale is the primary rock in Sa I, II interbed layer in the outlying region where casing failures occurred. The top portion has an interbed layer with bentonite, which tends to break easily. A little water encroachment in the wells can therefore lead to failures.

Secondly, judging by variations of injection-production ratio, two casing failures in the X6-3-BW-610 well tract occurred after infill adjustment in 1998 and after outlying wells went on production in 2004. These two casing failures are both related to variations in the injection-production ratio.

After the first infill adjustment, the number of water-injection wells in the tract increased to 3 from the original 1 and the injection-production ratio increased to 1.19 from 0.

After the outlying wells went on production in 2004, the number of water injection wells increased to 13 from 3. One well was shut in because it did not take water and the other was shut in because of casing failure. The injection-production ratio in these wells increased to 1.22 from 0.32.

The injection-production ratio increased to 1.66 before the wells experienced casing damage and remains at 1.41 now.

The imbalance of water-injection pressure in this block also is a factor that can lead to casing failure. Fracture pressure changes from 12.69 MPa in the oil zone to 14.67 MPa in the transitional zone and then to 14.04 MPa in the outlying well zones.

Actual injection pressure changes from 12.6 MPa in the oil zone to 14.67 MPa in the transitional zone and then to 14.04 MPa in the outlying well zone.

The number of wells with damaged casing increased because of the strata sliding, which was caused by the pressure difference among different zones.

Another factor for the casing damage was that the number of outlying water injection wells in the Dongguo region was greater while the number of producing wells was lower.

Thirdly, if the well cement quality was poor, the injected water would enter the shale, causing the pressure to rise in the shale interval and lead to casing failure. Because of the low quality of cement at top of the perforated interval, injected water at the top of perforation was directly injected into Sa I-II interbed layer, so that the shale swelled and caused casing failure.

Click here to enlarge image

Figs. 1 and 2 show the first and second cement-bond log runs. The first log shows good well cement in August 2004 when the well was completed, but the second run in April 2006 shows poor cement quality.

Click here to enlarge image

Of the wells analyzed, seven had casing damaged in the oil layer. Reasons for the damage were attributed to the injection and production imbalance causing pressure buildup that led to small-scale fractures that allowed formation water to enter and swell the clay and shale layer, thus damaging the casing.

Remedial treatments

The operator has repaired six wells, one oil and five injectors. Water injection resumed in the five injectors. To keep the pressure balanced in the casing failure zone, the operator started water injection at a moderate rate and increased pressure smoothly and slowly. The five wells have a designed injection rate of 370 cu m/day and actual injection rate of 209 cu m/day.

Production from the one producing well repaired is 3 tonnes/day fluid of which 1 tonne/day is oil.

To prevent more casing failures, the operator is controlling water-injection rates. After adjustment of the injection in three wells, the designed injection rate decreased to 20 cu m/day from 90 cu m/day with the actual injection decreasing to 9 cu m/day from 72 cu m/day.

At the same time, two production wells were remediated. One of the wells was hydraulically fractured and the other one had a larger, higher efficiency pump installed.

Liquid production from the wells increased to 45 tonnes/day from 24 tonnes/day, with oil production increasing to 12 tonnes/day from 3 tonnes/day.

After 1 year of remedial treatments and training of the staff in preventive measures, the operator has prevented additional casing failures.

Acknowledgment

This work described in this article was supported by the National Natural Science Foundation of China under Grant No. 50634020.

References

  1. Wang Zhonmao, et al., Mechanism and Prevention and Control of Casing Damage in Oil and Water Wells, Beijing, Petroleum Industry Press, 1994
  2. Han Xiuting, et al., Further Investigation and Prevention and Remedial Techniques for Casing Damage in Daqing Oil Field, Report, 2001.
  3. LI Haiqing, et al., “Discussion of damage cause and repair for casing,” Oil Field Equipment, March 2005.
  4. Wang Xuezhong, “Mechanism of Casing Pipe Damage in Loose Sandstone Reservoirs,” West China Petroleum Geosciences, April 2006.
  5. Liu Dongsheng, et al., “Matching technique of comprehensive harness of casing damage wells in later stage of high water cut,” Petroleum Geology & Oilfield Development in Daqing, May 2006.
  6. Li Lianping, et al., “New knowledge of casing damage genetic mechanism and integrated control technique,” Petroleum Geology & Oilfield Development in Daqing, January 2007.

The authors

Click here to enlarge image

Zhang Ji-Cheng (zhangjc2006 @tom.com) is associate professor in the petroleum engineering department, Daqing Petroleum Institute, Daqing, China. His main focus is in research for oil and gas field developments.

Click here to enlarge image

Song Kao-Ping is professor in the petroleum engineering department, Daqing Petroleum Institute, Daqing, China. His main focus is in research for oil and gas field developments.

Click here to enlarge image

Du Ruo-Xia is an engineer with the 4th Oil Production Co., Daqing Oilfield Corp. Ltd. She is in charge of research and management of field operation and workover.