The rig, at the Castilla oil field in the wide central plains of Meta province, is one of a fleet deployed by Colombian state-run oil company Ecopetrol under a $3.5-$4 billion investment plan this year to boost production and explore for more oil to replenish dwindling reserves.

The company has boosted its investment by more than $1 billion, from $2.2 billion a year earlier, to finance drilling 620 wells in 2018. Ecopetrol is targeting output of 725,000 barrels per day (bpd) of crude and gas equivalent by the end of the year, up from about 700,000 now, and the has set a more aggressive target of 870,000 bpd by 2020.

Castilla, Ecopetrol's most profitable oil field, is pumping about 115,000 bpd but should approach 125,000 by the end of next year, said Jose Cotello, vice-president for the Orinoquia region in eastern Colombia, which includes Castilla.

The expansion would make it the country's biggest producing field. It pumps heavy oil, which typically sells at a discount to lighter crude that is easier to refine.

Right now, however, heavier crude is in demand among refineries worldwide because of a fall in output of similar oil from neighboring Venezuela, where lack of investment has pushed production to its lowest level in decades.

Ecopetrol has earmarked $1.1 billion of the investment budget for the Orinoquia region, nearly double the year before, but had to halt drilling for a month because protests closed Castilla and other nearby fields in February.

Protesters angered by Ecopetrol's alleged violations of labor agreements have blocked roads, invaded fields and burned buildings, including a control room. The disruptions led to about $100 million in losses over a month and delayed the start of operations at five wells.

"The big question is on drilling, because we were stopped for a month," Cotello said in an interview after visiting the Castilla field.

Nationwide, Ecopetrol will double the number of rigs in operation this year from last. It will operate 28 rigs in the second half of the year, and an additional 25 rigs in joint operations with partners. The number of rigs had dwindled since the oil price crash of 2014-2015.

The company is equally focused on finding more oil that it can produce in the future, Cotello said.

The reserves on the company's books have dipped to the equivalent of about 7.1 years of production, according to Ecopetrol. The country's energy ministry has a less optimistic estimate of 5.6 years. Both are well below the average of nearly 12 years for the world's top oil and gas companies.

Investors look at reserves as a gauge for the long-term sustainability of oil and gas firms.

Ecopetrol needed a big boost in spending after several years of underinvestment, said an executive working for an international oil company working in a joint venture with Ecopetrol.

FASTER SPENDING

Ecopetrol expects to quicken investment in the second half of the year after spending just $405.4 million of the total budget during the first quarter of 2018.

The country's presidential election slowed spending, because anti-corruption laws make it difficult for public companies to open bidding processes for contracts during the electoral process, Ecopetrol said.

Colombia's incoming President Ivan Duque will be inaugurated in August and has promised to help the energy industry. He has said he will invest in the state-run company's refineries and crack down on militant groups that have attacked oil pipelines. He has also promised tax cuts across the economy, including for the energy industry, although has yet to disclose detailed proposals.

Colombia has struggled to attract investment and maintain energy output as conflict, protests and attacks by criminal gangs have frequently interrupted operations.

Pumping through the Cano Limon-Covenas oil pipeline restarted last week after 180 days stoppage due to repeated attacks by Marxist ELN rebels, military and industry sources said on Tuesday.

    The 485-mile-long (780-km) pipeline has been attacked 58 times this year by the National Liberation Army (ELN), the largest active guerrilla group, according to military sources.

    Although the latest outage was one of the most extensive since the pipeline opened in the mid-1980s, activity in the Cano Limon field, operated by Occidental Petroleum Corp and located in the northern Arauca province, has not been affected.

The protests and attacks threaten to slow the momentum the company has gained since stabilizing its finances and reinvesting in production, said Jairo Lastra, investment manager at Lastra Capital Management, who previously worked at the money desk at Ecopetrol.

This year's expanded investment marks a big change for a company that cut drilling and shuttered an oil field when it lost more $1 billion in 2015, after oil prices crashed.

The rise in crude prices to 3.5-year highs has given the company the cash it needs to rebuild. Profits rose by more than 300 percent year-on-year in 2017 to more than $2 billion.

In the first quarter of this year, Ecopetrol's profit topped $900 million, executives say.

"The crisis is in the rear view mirror," Cotello said.