Sudan on Sunday prepared to stop the oil flow from South Sudan on the orders of President Omar al-Bashir but an expert said the process could take weeks.
Bashir said petroleum companies working in South Sudan will be informed about "shutting down the pipeline" from Sunday, the official SUNA news agency reported.
The order came after Sudan's leader warned the South over backing rebels, who analysts say humiliated the authorities with recent attacks.
South Sudan's government in Juba denies supporting insurgents in the north.
"I think if you do it properly it would take 45 days," to stop the oil without causing damage, said the independent expert who asked not to be further identified.
"It's not like opening and closing a water tap."
At a press conference scheduled for 1100 GMT Sudan's Information Minister Ahmed Bilal Osman was expected to comment further on relations with South Sudan.
This will be the second closure of South Sudan's oil wells and the Sudanese pipeline system in about 18 months.
Production had only resumed in early April after the two countries agreed on detailed timetables to normalise relations, after intermittent border clashes, by implementing the oil deal and eight other security and economic pacts.
In early 2012 the South stopped its crude production after accusing Khartoum of theft in a dispute over export fees.
The previous shutdown "went very well", the expert said, adding Sudan's oil ministry has enough experience to safely close the system and its pipeline running 1500 kilometres (930 miles) to the Port Sudan terminal.
Thousands of wells on the South Sudanese side will need to be shut one by one and the pipeline flushed, he said.
"You need to evacuate the oil somewhere," the expert said.
"If they do not do that properly the oil will gel. It's not easy to reverse it to liquid again."
The expert was not sure what point the oil had reached in the pipeline but said "it should be close to Port Sudan".
Bashir warned on 27 May that he would block the oil if the South's government provides assistance to rebels fighting in South Kordofan and Blue Nile states, or in the Darfur region.
Khartoum has long accused South Sudan of supporting rebels in the north, a complaint which for months held up implementation of the oil and security pacts.
Bashir's late-May threat came at a ceremony following the army's recapture of Abu Kershola in the far north of South Kordofan.
Rebels held Abu Kershola and its garrison for a month after seizing it during a coordinated attack on several areas including the strategic and previously peaceful town of Umm Rawaba in North Kordofan.
Analysts called the initial attack a humiliation for the authorities.
More recently there were very strong rumours that the "liberation" of Abu Kershola only resulted from a withdrawal by rebels of the Sudan Revolutionary Front coalition, one Sudan analyst told AFP.
"The problem is that nobody has seen any evidence" of continued South Sudanese support to the insurgents, the analyst said.
"They (Sudan) have their own internal difficulties and they want to use South Sudan as a scapegoat," South Sudanese Information Minister Barnaba Marial Benjamin told AFP .
South Sudan split from Sudan in July 2011 in the wake of a referendum vote for independence under a peace deal that ended a 22-year civil war.
Independence left key issues unresolved, including how much the landlocked South should pay for shipping its oil through Sudan's export infrastructure.
In a March report the Small Arms Survey, a Swiss-based independent research project, said it found no evidence of weapons supplies from Juba to the Sudan People's Liberation Movement-North (SPLM-N) after the South's independence.
"There are, however, some reports that both SPLM-N and JEM are benefiting from other kinds of assistance," including logistics, fuel and food, the report said.
SPLM-N has been fighting for two years in South Kordofan and Blue Nile states.
Analysts say they have been assisted by JEM, the Justice and Equality Movement of Darfur.