Barium sulfate scale deposition is a serious operational problem in the Rangely Weber Sand Unit (RWSU), often resulting in lost oil production. The historical method In the RWSU to restore production was to mechanically scraps the scale from the wellbore and then hydraulically break through any scale remaining in and around perforations. An alternative mechanical method of scale removal was recently used on a trial basis in the RWSU. Water blasting was used which directs high pressure jets of water with polymer additive at the perforations and casing wall. Water blasting was used on four wells and the results indicate that while water blasting does remove some scale, it is not an attractive option for stimulating production where barium sulfate scale has caused reduced production.
The RWSU is an oil reservoir located in northwestern Colorado (see Figure 1). Production is from the aeolian, Permian-Pennsylvanian age, Weber sandstone. The Weber reservoir was discovered in 1933 with field development beginning in 1943. The field was unitized in 1957 with the implementation and expansion of a waterflood throughout the 1960's and 1970's. The implementation of the waterflood is believed to be the cause of the BaSO4 scaling problems. A miscible CO2 flood began in October problems. A miscible CO2 flood began in October 1986.
The RWSU has had a history of barium sulfate (BaSO4) scale deposition problems dating back to the early 1970's. A very active scale inhibition program is in place which mitigates such of the potential damage by BaSO4. However, BaSO4 is still deposited in and around perforations reducing production. Since BaSO4 scale is insoluble in most oilfield solvents, chemical methods have not been widely used to remove BASO4. It should be noted that several BaSO4 scale dissolvers have recently become commercially available and their effectiveness is currently being evaluated for use in the RWSU. Mechanical means have historically been the most common method for removing BASO4 scale.
The traditional method used in the RWSU to restore production where BaSO4 scale has been deposited is production where BaSO4 scale has been deposited is to first clean out the wellbore with a bit and casing scraper. Then the perforations are selectively isolated with a retrievable packer and bridge plug and "broken down" with injection system source water. The perforation breakdown is intended to hydraulically break through any near wellbore scale buildup and re-establish flow. Intervals where scale buildup is particularly severe it may be necessary to re-perforate.
As the field has matured the casing condition has deteriorated due to the corrosive environment. The historical mechanical means outlined above are often not applicable because of the poor casing condition. An alternative mechanical method to this procedure was desired which was not strongly dependent on the casing condition.
Four wells with severe BaSO4 buildup were waterblasted in a field trial to evaluate the scale removal capabilities of waterblasting. Three sources of data were used to evaluate the effectiveness of waterblasting, they were:
perforation efficiency before and after perforation efficiency before and after waterblasting,
the amount of radioactive BaSO4 scale present before and after waterblasting based on gamma ray logs, and
well production before and after waterblasting.