At a time when gas demand seems to be outstripping supply, small gas pools need to be developed (see other story in this issue) and “left-behind” gas in mature pools must be accessed as well, contend energy analysts.
They point out that an unbelievable 50 percent of gas in a productive reservoir can fail to make it to the surface when the pressure in the reservoir drops below that at the wellhead.
In the case of the country’s biggest gas producer, bpTT, the amount of that “stranded gas” has been estimated at 25 to 40 percent.
The gas in the scores of accumulations mapped by the bpTT comes up to about “1 to 2 trillion cubic feet (tcf),” declares Yatindranath Keith Bally, the company’s vice president for reservoir development.
For a company that adheres to a “no molecule left behind” policy, this situation is clearly unsatisfactory, no matter how many new gas fields are discovered in the meantime.
As long ago as 2008, bpTT began investigating the financial and technological feasibility of a low-pressure reserves access (LPRA) project, designed to recover as much stranded gas as possible, which was due to be given the green light by late 2009.
Somehow, the programme fell off the front burner and other projects took precedence, but Bally insists that “LPRA has not been abandoned and we still have it in our development plan. We are still looking at various ways of how best to do it.”
The concept behind LPRA is fairly simple. As Bally explains: “If you reduce the pressure at the Cassia hub, for example, all the fields that come into the hub will see a lower pressure back on their systems, which translates to a lower pressure at the sand face and allows you to produce more gas. But you have to work it out — if you reduce the pressure, how much are you going to get?”
That’s the key, because the whole idea is to retrieve as much of the stranded gas as possible.
But it’s a matter of timing, Bally insists, which probably explains why LPRA has not yet been put into effect.
“You don’t want to pull the pressure down on these facilities if you believe there is new stuff that is going to come through, because the new stuff will overwhelm that. So you want to make sure you do it properly.”
bpTT undoubtedly also wants to make sure it can keep costs down. A price tag of US$1 billion was originally put on LPRA in 2008.
The final cost will be influenced by the development concept chosen. LPRA will entail modifications to existing infrastructure offshore, including pipelines and platforms and, of course, the addition of compressors.
Fluid-handling systems and control system modifications are also almost always part of a compression project. Onshore infrastructure improvements are also likely.
“We are looking at various options on how we do the compression,” Bally points out. “Will it be strictly all offshore? What type of compression will we need? This is where technology plays a role and we are looking at the most cost-effective approach.”
While Bally stresses that LPRA is still on the cards, he also says the country should not expect that stranded gas to make it to the market soon.
Right now, bpTT is busy with reservoirs that can produce under existing primary pressure, such as the Juniper field development, which will have an output of around 590 million cubic feet per day (mmcfd) from the Coralita and Lantana gas discoveries made many years ago.
Water depth in the area is about 360 feet and the reserves are being tapped through subsea wells, a first for bpTT in this country.