Designer mud

Designer mud is helping Total tap pressure depleted HPHT reservoirs in the UK North Sea. Elaine Maslin takes a look.

The West Franklin well head platform installation last year.  Photos from Total.

For most, designer clothes are a luxury you could do without. Not so with designer mud, especially for Total’s high-pressure, high-temperature (HPHT) fields in the UK North Sea.

Designer mud, sometimes also called intelligent mud or stress caged mud, has been a key enabler on the firm’s assets in the Elgin Franklin area, specifically around drilling infill wells, in depleted reservoir zones, where steep and rapid changes in pressure could otherwise prove very difficult to drill.

“It [designer mud] really is key. These formations – sand and chalk – we know how to drill them and we know how to efficiently manage the rock formation,” says Tom Brian, senior drilling engineer, Total E&P UK. “We understand what tools we can run. The challenging piece is the pore pressure and to manage the change in pore pressure.”

Elgin Franklin

Total E&P UK has been a front-runner in HPHT field development in the UK, with its Elgin Franklin HPHT development in the central North Sea, producing since 2001. It was challenged with pressures up to 15,500psi and temperatures up to 350°F on the fields.

Franklin was discovered in 1986 and Elgin in 1991. It took 15 years and £20 million of research investment, before both could be produced. Initial development challenges included 3-4% carbon dioxide, 30-50ppm hydrogen sulphide, and a 1100 bar, 190°C temperature reservoir. There are also 175 g/L formation chlorides.

The next challenge was one the company initially thought may not be possible – drilling infill wells – due to pressure depletion in the reservoir. However, Total did it. In 2008, Total drilled its first infill well on Franklin, “opening a new horizon,” says Total E&P UK managing director Phillippe Guys.

Thanks to the success in this area, Total has brought on stream two new platforms at the Elgin Franklin complex – Elgin B and West Franklin, extending the development’s life from 22 years, as initially anticipated, to more than 32 years.

Pressure depletion

Walk to work on the West Franklin well head platform.

The concern over drilling these wells had been around pressure depletion. Drilling at virgin pressure, while still providing a HPHT challenge, is simpler, due to having a consistent pressure. It is a different story once the reservoir has started to be depleted.

“At the reservoir, you have got a reduction in pore pressure, which reduces fracture gradient and weakens the formation, which creates difficulty drilling, because you still need a mud weight heavy enough to balance the higher pressure formation still exposed above,” Brian says. While in theory, reservoir zones should deplete at the same rate, they don’t, creating complex systems that have to be navigated. “You have to find a balance to stop any fluids coming in, but not so that it is so over-balanced above the pore pressure or fracture gradient that you can fracture the formation, induce losses or become differentially stuck.”

The alternative is to isolate the depleted zones, with liners, and carry on drilling with a different mud weight, but because of the number liners required for the multiple zones drilled through, you would run out of hole sizes to continue, Brian says.

Wells have been lost by not getting this right, resulting in geo-mechanical deformation, which also potentially results in resources which can no longer be reached unless another way can be found to access them.

Mud, glorious mud

Total uses managed pressure drilling and expandable liners, but, Brian says, the key is the designer, or stress caging mud. The mud contains particles that plug any micro fractures as soon as it reaches them, strengthening the formation. The most common particles are made from graphite and calcium carbonate.

The important part is the work done to determine the size of particles necessary for each reservoir according to the different pore pressures, fracture gradients and which zones will be exposed during drilling. Total has a geo-mechanics team tasked with this job. Brian says they have become good at predicting the reservoir characteristics. By interpreting logging data from existing wells and then providing the data required to design the mud needed, which is then procured from Halliburton Baroid. Total has been using Halliburton Baroid’s Steelseal and Baracarb, made from graphite and calcium carbonate, respectively.

The particles range from 5 microns, looking like powder, to 1500 microns – currently the largest used by Total – which looks like coarse salt granules. Reaching this size has only been achieved by the firm using the mud over the years and increasing its understanding of what it can do. But it’s not as simple as using mud with one particle size in it – as fractures will not form uniformly – a particle size distribution has to be calculated so that the different sized fractures that develop can be plugged. In addition, using managed pressure drilling allows the firm to more closely navigate the bottom hole pressure.

While using designer mud enables Total to drill through these sections, because it means there are specifically sized particles in the mud, it cannot just be recycled after the drilled cuttings are removed. The mud has to be constantly treated before being recirculated back into the well in order to maintain the proper and designed particle size distribution.

The latest well drilled using designer mud was West Franklin D, and another is due to be drilled this summer. West Franklin D was the second well drilled on West Franklin, to a vertical depth of 5600m, to the top of the reservoir. It had to go through a heterogeneously depleted Fulmar interval reservoir, which meant encountering changes in pressure from 1100 bar to 600-700 bar, over the space of a few meters.

“We have been increasing our understanding, over the years, of just how much we can do with the designer mud,” Brian says. “We don’t think we’re yet at the limit [of particle size that can be used] and work to understand what we can achieve continues. It is all part of the infill drilling problematic.”