Despite its currently depressed price, the future of natural gas looks very bright. Over the next 30 years, global natural gas demand is expected to grow by 65%, according to projections by ExxonMobil (NYSE:XOM)

Not only is natural gas today just a fraction of the cost of crude oil on an energy-equivalent basis, but it's also generally accepted to be better for the environment. Owing to its high hydrogen content, natural gas produces about 50% less greenhouse gas emissions than coal and about 30% less than crude oil.

That's why some companies are investing heavily in natural gas and liquefied natural gas (LNG) projects right now. Two deserve particular attention: Exxon and Royal Dutch Shell (NYSE:RDS.A). Let's take a closer look.

ExxonMobil's investments in natural gas are largely shaped by the company's optimistic outlook on the fuel's future. By 2025, Exxon reckons that gas will overtake coal as the second-most widely used fuel source, pushed by Asian demand that is expected to grow by more than 50% over the next three decades.

To capitalize on these expected trends, Exxon made a huge move into natural gas by buying XTO Energy, an independent gas producer, back in 2010 for a record sum of $41 billion. It has also invested heavily in global LNG projects, such as the $52 billion Gorgon LNG complex in Australia, in an effort to continue to expand its worldwide gas portfolio. Indeed, gas now accounts for roughly half of the company's total production and reserves.

At present, Exxon boasts one of the largest global LNG positions, with liquefaction capacity of roughly 65 million tons per year. In Qatar, it recently started up four of the largest-producing LNG trains in the world, which have a combined capacity of roughly 31 million tons per year, as part of a joint venture with Qatar Petroleum.

Like Exxon, Shell is also bullish about the future of natural gas. It expects global LNG demand to double by 2025 to roughly 500 million tons a year, which would make it the fastest-growing fuel source. Not surprisingly, it has already invested more than $40 billion on LNG projects and is presently one of the largest producers in the world, commanding an approximately 7% share of the global LNG trade.

Shell presently boasts 22 million tons per year of Shell-share liquefaction capacity in Australia, Brunei, Malaysia, Nigeria, Oman, Qatar, and Russia. It also has three LNG projects under way in Australia -- Gorgon, Wheatstone, and the ambitious Prelude floating LNG project -- which should add an additional 7 million tons per year of liquefaction capacity.

Foolish takeaway
While Exxon and Shell's natural gas ventures have proved disappointing so far, considering the poor timing of Exxon's XTO purchase and Shell's writedowns of its North American shale gas assets, I think they could pay off handsomely over the long run. That's because LNG projects don't see the types of production declines that maturing oil wells do. Instead, they produce at flat levels for extremely long periods of time.

They also require little reinvestment and generate massive amounts of strong and stable free cash flow, which can be reinvested into more lucrative oil projects or be returned to shareholders through share repurchases and dividends. Over the next few years, as the share of Exxon and Shell's production from LNG projects rises, it should help offset the natural decline from both companies' existing oil wells that has resulted in stagnant production growth.