Drilling small reservoirs - unlocking offshore oil pockets
An oil drilling technology is being developed at SINTEF Petroleum Research that could unlock offshore oil reserves from small pockets that lie close to large fields on the continental shelf.
The most essential characteristic of the drilling tool is the additional rotating side cutters that give the wellbore wall optimal geometry for extraction and help improve hole cleaning and reduce friction.
Project manager, Jan David Ytrehus explains, ‘The main drill bit is not affected by the design. Instead, side-cutters are mounted into the bottom hole assembly and they extract when needed. The cutters can rotate around their own axis, powered by a positive displacement engine, in addition to the drill string rotation. ‘The improved efficiency lies primarily in the grooves, or changed geometry, that the drill tool creates rather than in the design itself.’
Ytrehus adds that smaller pockets around the main basins could enhance oil recovery rates significantly. ‘Our numbers from the Norwegian Continental Shelf (NCS) stipulate that these small hydrocarbon pockets are in the order of magnitude of 10,000. It is hard to estimate, but the overall potential that lies in these pockets is considerable. On the NCS alone [small oil pockets] could contain values of more than 20bln Norwegian Krone, given an estimated oil price of US$60 per barrel.’
The main challenge in drilling towards small oil pockets is that the cost of the well and associated technology is often too high compared to expected production and value. But Ytrehus argues that the tool will make it easier to drill branches from existing large wells out to nearby pockets of oil.
‘If we explore and drill in the traditional manner, many small reservoirs will remain unprofitable and undisturbed. But if we can drill across to some more of them from existing wells, costs will be significantly reduced, and ‘dwarf’ reservoirs close to the larger wells should become easier to exploit,’ says Ytrehus. ‘This could be an efficient way of enhancing oil recovery rates.’ Although still at design stage, the team has tested the concept on a 12-metrelong model of an oil well. If the new drilling tool meets the expectations raised by laboratory results, Ytrehus envisages drilling out to pockets up to one kilometre from current wells.
How it works
1 The pressure of the drilling fluid prevents the well from collapsing and allows gas to flow into the wellbore during drilling.
2 Drilling fluid is kept under additional pressure in order to keep it circulating so that it carries the drill cuttings up to the surface. Traditional drilling requires high pressure, causing drilling fluid to flow into depleted reservoirs when branch wells are drilled from existing wells.
3 The technology uses low pressure as the cutters create spiral grooves in the wall of the well, which makes the removal of drilling cuttings more efficient. Due to this low pressure, the loss of drilling fluid is lessened and drilling out from existing wells becomes more feasible.