Drilling Liner Technology for Depleted Reservoir (1996)

Abstract The Drilling Liner System makes use of the liner/casing as a drill string that would normally be set at the casing point. As a drilling tool, it can be set when the string becomes stuck due to formation collapse or severe differential pressure sticking. Background cases for developing this system were wellbores in the North Sumatra (Indonesia) Arun field. This field has highly pressured shales and salt water sand layers overlying a depleted, highly permeable reservoir. Conventional drilling techniques became impossible since a total loss of circulation occurred as soon as the bit drilled through the highly pressured cap rock into the severely depleted reservoir. The resulting drop of hydrostatic pressure usually causes the shale to collapse and stick the drill string. The reduced hydrostatic head also induces flow from the shallower salt water sands. With the Drilling Liner System, the liner will be set simultaneously when drilling into the low pressure formation. Although the mud losses still occur, the liner is then already in place and protects the borehole. After releasing the running tool, cementing and changing the mud, drilling operations may resume in the low pressure formation using conventional drilling techniques. Several successful runs have been performed in the Arun gas field where formation pressure drops from 8,000 psi in the overlying shale to less than 2,000 psi in the permeable Arun limestone. Prior to application of the drilling liner system, drilling through the described formation had been difficult and costly. Salt water kicks, borehole collapse and stuck pipe were common problems. The system described has allowed repair of 2 of 3 failed wells with lull productivity at a cost much less than for a new well. But the system is not fully perfected yet. In the third well, the drilling liner did not completely isolate the shale. A workover to install a scab liner will be required to allow shale-free production. Introduction Why Drilling Liner System? The Drilling Liner system sets the liner while drilling and thus prevents hole collapse which can be caused by sudden borehole pressure changes that can occur during massive lost circulation. A typical application for the Drilling Liner is therefore a wellbore featuring a formation with high pore pressure, closely followed by another layer with significantly lower pressure. Normal drilling operations usually run into trouble while drilling from the high pressure formations into the low pressure layer. Severe mud losses with simultaneous hole collapse, kick and stuck pipe are the common result (Fig. 1). These circumstances occur now in Mobil's Arun field in North Sumatra, Indonesia. This formation is a massive, permeable carbonate gas and condensate reservoir which has been extensively developed by Mobil Oil during the last 18 years. The depleted limestone has become highly prone to lost circulation. The drilling liner system has recently been used to repair 3 wells which failed due to casing shearing. Failed wells are sidetracked through 9-5/8" casing with 8-1/2" hole. Conventional directional drilling proceeds to a depth just above the reservoir with 16.8 ppg oil based mud needed to overbalance a highly pressured salt water sand and stabilize pressured shales. The 7" drilling liner is then used to drill from the high pressure environment into the depleted Arun reservoir. As the drilling liner penetrates the limestone, massive mud losses still occur, the shale collapses and the liner becomes stuck. Since the liner is already in place, it protects the borehole. Regular drilling operations may resume in the low pressure formation (Fig. 2) after the liner is cemented and the mud is changed. Drilling Liner System components Drilling Liner system components include the drilling liner Tool, the liner, the inner string and the liner hanger. Drilling Liner Tool. The drilling liner tool is run on the bottom of the entire drilling liner and serves as the bit cutting structure necessary to drill. It rotates independently of the liner. P. 127