By Heri Retnowati

SIDOARJO, Indonesia, March 16 (Reuters) - Efforts to use concrete balls to brake a torrent of mud that has swamped villages on Indonesia's Java island have yielded positive results, a scientist said on Friday.

The mud has displaced about 15,000 people following an oil-drilling accident in May in Sidoarjo, an industrial suburb near Surabaya city in the east of Java island, and destroyed toll roads, railways tracks and submerged factories.

The flow has decreased by about 20 percent after 374 clusters of concrete balls linked to steel cables were dropped into the mouth of the eruption, said Endar Bagus, one of three scientists who initiated the experiment.

"There have been positive signs. We're seeing a decrease in the mud debit but our observation is relative, not absolute," Bagus told Reuters.

The mud currently flows at a rate of about 150,000 cubic metres a day.

Bagus said a more thorough evaluation involving the measurement of the sludge's chemical contents, temperatures and the eruption's pressure was being carried out and the results could be announced within days.

The concrete ball experiment is believed to have never been tried before and some experts expressed doubt about its chance of success.

A further 500 chains of concrete balls will be fed into the source of the mud flow starting on Saturday, said Rudi Novrianto, spokesman for a government team tasked with handling the disaster.

POSITIVE TREND

"Our preliminary evaluation indicates a positive trend following the insertions. However the mud has become thicker now," he said.

Each chain consists of four 20-40 cm (8-16 in) diameter balls.

Anger has been mounting in the area over the mud, with displaced residents frequently holding street rallies demanding cash compensation instead of resettlement.

PT Lapindo Brantas, the operator of the well where the mud has erupted, had been told by the government to pay 3.8 trillion rupiah ($412.5 million) to victims and for efforts to plug the mud flow.

But a team set up by the government said the cost would reach around 7.6 trillion rupiah.

Vice President Jusuf Kalla said on Friday the victims' refusal to be resettled meant Lapindo must compensate them with cash.

"If the people refuse and they want more, that means someone has to buy that area. That means the company has to buy it," he said.

Lapindo and PT Energi Mega Persada Tbk <ENRG.JK>, which indirectly controls it, dispute whether the mud flow was caused by the drilling and also whether Lapindo alone should shoulder the cost.

Energi is owned by the Bakrie Group, controlled by the family of Indonesia's chief social welfare minister, Aburizal Bakrie.

Lapindo holds a 50-percent stake in the Brantas block from where the mud is gushing. Energi International Tbk holds 32 percent and Australia-based Santos Ltd the remaining 18 percent.

($1=9,177 rupiah)

(Additional reporting by Ahmad Pathoni and Muhamad Al Azhari)

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