Wednesday, December 5, 2018

CT Logging and Perforation in Alaska

Coiled tubing logging located zones offering potential additional production that were perforated using CT-deployed guns.

 So far, ARCO's sector of the Prudhoe Bay has run 12 coiled tubing logging jobs in eight highly deviated or horizontal wells. The aims of these jobs were to obtain the production profile and verify the presence of channels using pulsed neutron logs. Where necessary, coiled tubing-deployed perforating guns were used to open up potentially productive zones. 

Well 15-07A exemplifies the job performed in Alaska. It is a virtually horizontal well completed with a 4 1/2 inch slotted liner at a total vertical depth of 8761 ft and a measured depth of 13,545 ft.

 Drilled as a sidetrack to a much older well and completed earlier this year (April 1994), the well was found to be producing lower rates of oil at a much higher gas-oil ratio (GOR) than was anticipated. Coiled tubing logging was used to determine the source of the gas production, to identify any nonproductive intervals and to tie in with previous logs using gamma ray, casing-collar locator and temperature logs.

Gas entry was located using the temperature log. Then, using pulsed neutron logs in conjuction with borax injection, the gas-oil contact was located and a possible channel behind the 7-inch liner indicated. 

Finally, CT was employed to perforate the 7-inch liner to contact potentially bypassed intervals. Some 20 ft of 2 1/8 inch guns were run in hole and detonated using a hydraulic firing head. Depth was correlated using a tubing-end locator. 

The CT perforation used a new pressure-activated firing head. It allows circulation and reverse circulation before and after firing the guns. An operating piston is attached to a sleeve that locks the firing pin in place. When sufficient differential pressure is established across this piston to sever the shear pins that hold it in place, the firing pin is driven into the detonator by the pressure.  

To establish this differential pressure, a ball is pumped down the tubing to form a pressure seal in the head. The ball diverts pressure to the underside of the operating piston, building up the pressure that severs the shear pins an detonates the guns. Up to twelve 500-psi shear pins can be incorporated into the head.

A key advantage of coiled tubing is its higher tensile strength than wireline. So, when it comes to perforation, there is no practical weight limit to the number of guns that can be run. The main constraint on gun length is the height of the lubricator. However, the downhole safety valve may be closed and successive sections of guns run into the well and connected together. 

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