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Thu 3 Dec 2009 04:00 AM

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Tight times

With trillions of cubic feet of gas tied up in low permeability reservoirs, tight gas production has become the hot topic. Schlumberger’s Lee Ramsey talks to O&G ME.

Tight times
Lee Ramsey is manager of the Schlumberger Tight Gas Center of Excellence in Saudi Arabia.
Tight times
Seismic field work being carried out by Saudi Aramco.
Tight times
The drilling of multiple stage exploration wells is yielding encouraging results.

With trillions of cubic feet of gas tied up in low permeability reservoirs, tight gas production has become the hot topic. Schlumberger’s Lee Ramsey talks to O&G ME.

Around the world there are trillions of cubic feet of gas that could be used to supplement the world’s energy supplies. However, much of this abundant resource is difficult to produce because the gas is held in tight reservoirs where the permeability of the reservoir rocks is extremely low.

Producing tight gas is challenging, yet considering the quantities available and the long-term producability of this resource, such non-conventional gas is now being regarded as a significant energy resource for the future. Tight gas could help to address the predicted deficit between energy supplies and demand in the coming decades.

To understand the economic viability of the complex, unconventional gas developments of tomorrow, Middle East operators must focus on new solutions today.

A concerted technological effort to better understand tight gas resource characteristics and to develop solid engineering approaches is necessary to deliver significant production increases.

Schlumberger has risen to the challenge, and through real collaborative efforts with
Saudi Aramco

and its international oil company partnerships, its Tight Gas Center of Excellence in Dhahran, is delivering bona fide leaps in reservoir understanding.

Lee Ramsey, manager of the Tight Gas Center of Excellence (TGCoE), tells Oil & Gas Middle East that the exciting developments are important steps to a real game changing approach to the tight gas conundrum.

The basics

The permeability of rock formations is essentially what governs how easily gas held within will flow. High permeability carbonate formations have higher porosity and often have natural fractures through which gas will easily flow, and are typically prolific producers without the need for technically sophisticated approaches.

“In the Middle East people are accustomed to working with carbonates which have a very high permeability that will produce naturally or, if the wellbore is damaged, a matrix stimulation with a little bit of acid,” explains Ramsey. “However, tight gas fields (today Schlumberger has a working definition of 0.1 millidarcy and below as tight gas), will usually not flow with a simple perforation. It requires a much deeper understanding of the reservoir and accurately tailored solutions that fit, as well as specialist treatment fluids and techniques,” he adds.

“What we are advocating is that the decision of whether to use a well for production should not be made exclusively on initial flow appraisals. With improved understanding of the reservoir characterisation, better informed decisions could be made.”

Indeed, obtaining commercial flow rates is a crucial factor when drilling and completing tight gas wells. In the Middle East, tight gas wells are typically deeper and hotter than other gas wells, making them more expensive to complete.

“On the surface of things it would seem simple to transfer the technology and knowledge from experiences in North America. However, because we can be working at depths of 18 000ft – 20 000ft and temperatures approaching 350 degrees Farenheit in Saudi Arabia, the wells in North America do not share the same high temperature and high pressure characteristics, as they are typically between 6000ft and 12 000ft so we find that the toolbox is much smaller.”

This hostile environment becomes additionally challenging when the highly specialist equipment needed is rarely the sort that sits in inventory. “These pieces of kit tend to have long lead times; a good example would be 15K packers, sliding sleeves, upper and lower downhole completion equipment, which are essentially made to order,” says Ramsey.

An additional benefit which has come from the concentrated learning environment is that the research is highlighting crucial gaps in the technology – and the feedback from Dhahran to research centres throughout the Schlumberger regional and global network is helping to address these. To add to the already significant challenges, the fields in question are often remote from existing facilities and operating bases.

Access point

To achieve optimum flow rates, options to drill vertical, horizontal, or multilateral wells must be considered alongside the optimum number of zones, or stages within a zone.

In tight gas wells, production from a single zone is often less than expected. Especially when you have been used to working on prolific high permeability wells. To increase tight gas production, multiple zones in vertical wells or multiple stage fracture treatments within a zone on a horizontal well are required. In striving for maximum productivity from a tight gas reservoir, whether in new or existing wells, attaining that production level crucially depends on an appropriate open hole or cased hole strategy.

“Generally when we talk about perforating in tight gas fields we mean taking a shaped charge and going through the well casing. Tight gas wells aren’t going to produce without a simple fracturing treatment. This is quite the opposite of typical Middle Eastern field experiences, where matrix acidizing yields good results.” To fully exploit tight gas operators need to break- through that well bore damage and get a fracture of significant length out into the reservoir. “When we are dealing with exploration wells, this fracturing is really a part of the reservoir characterisation. If you want to understand what you really have there, you going to have to confirm that by fracturing and testing extensively,” explains Ramsey.

Marginal economics

For real results multiple zones and multiple stages are needed, as well as extremely detailed petrophysical and geomechanical analysis to make these profitable. That requires a real, genuine paradigm shift in thinking, says Ramsey. “Not only in terms of the clients, but also with the relevant ministries in terms of the rules and regulations – multiple zone projects require quite intricate approvals.”

Most of what the TGCoE is engaged in is as much research, meaning gaining a much better understanding about what is there through exploration wells, as it is about production. “Reservoir engineers can get very stressed out on the economics, because we are working with a lot of exploration wells, which is expensive.”

But, for the time being, the upper management levels are willing to fund this research and gain that hugely valuable knowledge. “There is the chance they could pull the plug and wait for technology to move along five years, but the message we are hearing is that this is about really learning and understanding what our technology gaps are – so the gas price has not altered that desire,” he says.

According to Ramsey, IOC partners in Saudi Arabia have been working with production rates two or three times below economical expectations. “If a field is targeting 10 Million scfd a day, and production is falling short of 3 million scfd a day, then it’s time to look very closely at the problem, but that’s where we can help,” beams Ramsey.

The economic imperative is there, and it is encouraging to see that today’s bottom line isn’t holding back this essential research. “The amount of reserves recoverable will be huge once we understand where the sweet spots are. There are still a lot of challenges out there in terms of geophysics and geology and petrophysics and other domains, but we have some extremely encouraging case studies from both North America and Saudi Arabia where production increases are not only encouraging, but profitable operations.”

Right location

A great deal of the learning taking place through the TGCoE will be applicable to many markets, both regionally and internationally, but what was really exceptional, and made Saudi Arabia a great fit was a combination of the potential, the activity levels in the country, and particularly the level of collaboration between the companies involved, explains Ramsey.

“The importance of gas to regional economies has changed – so that is perhaps a game changer. Having been through the prolific gas wells, and then the deep tough stuff, I think the evolution in the Gulf region will follow the same pattern that the North American market has been through.”

The extent of the reserves that exist in Saudi Arabia’s tight gas fields simply can’t be ignored. And despite a bumpy 2009 for gas prices (though largely confined to a sickly North American market) the desire to push ahead with tight gas projects has not abated in Saudi Arabia, in fact, it is proving a powerful draw for new recruits, explains Ramsey.

“Firstly the domestic energy needs of GCC countries are powerful motivators to learn more about this type of resource. And because it is so important, and such a challenge, with a great opportunity, we are actually being approached by people who are ready for a new challenge in their career, and this is where they want to be.”

In North America Shale gas, once a classic “non-conventional” resource are now becoming the “new conventional”.

Similarly with tight gas, people are now delivering such encouraging results that in certain circles it’s discussed as the new conventional. “That’s probably a bit of bravado and going a bit far,” laughs Ramsey. For tight gas specialists and the companies that master the know-how the future appears bright indeed.

“Today it is possible. It’s challenging, but by building the expertise to tackle the challenges of the future we are unlocking valuable reserves,” concludes Ramsey.

Units of measurement

A darcy (or darcy unit) and millidarcies (mD) are units of permeability – a measurement the ability of fluids to flow through rock or other porous media. A medium with a permeability of 1 darcy permits a flow of 1 cm³/s of a fluid with viscosity 1 cP (1 mPa•s) under a pressure gradient of 1 atm/cm acting across an area of 1 cm². A millidarcy (mD) is equal to 0.001 darcy.

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