Page 48: of Offshore Engineer Magazine (Sep/Oct 2015)

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Designer

Drilling mud

Designer mud is helping Total tap pressure depleted

HPHT reservoirs in the UK North Sea.

Elaine Maslin takes mud. The mud contains particles that plug any micro fractures as soon as it reaches them, strengthening the formation. The a look. 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 differ- or most, designer clothes are ent pore pressures, fracture gradients and which zones will be a luxury you could do with- exposed during drilling. Total has a geo-mechanics team tasked

F out. Not so with designer mud, with this job. Brian says they have become good at predicting especially for Total’s high-pressure, the reservoir characteristics. By interpreting logging data from high-temperature (HPHT) felds in the existing wells and then providing the data required to design

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

UK North Sea. the mud needed, which is then procured from Halliburton

Designer mud, sometimes also called Baroid. Total has been using Halliburton Baroid’s Steelseal and a 1100 bar, 190°C temperature res- drilling, because you still need a mud intelligent mud or stress caged mud, has and Baracarb, made from graphite and calcium carbonate, ervoir. There are also 175 g/L formation weight heavy enough to balance the been a key enabler on the frm’s assets respectively.

chlorides. higher pressure formation still exposed in the Elgin Franklin area, specifcally The particles range from 5 microns, looking like powder,

The next challenge was one the com- above,” Brian says. While in theory, around drilling infll wells, in depleted to 1500 microns – currently the largest used by Total – which pany initially thought may not be pos- reservoir zones should deplete at the reservoir zones, where steep and rapid looks like coarse salt granules. Reaching this size has only sible – drilling infll wells – due to pres- same rate, they don’t, creating complex changes in pressure could otherwise been achieved by the frm using the mud over the years and sure depletion in the reservoir. However, systems that have to be navigated. “You prove very diffcult to drill. increasing its understanding of what it can do. But it’s not as

Total did it. In 2008, Total drilled its frst have to fnd a balance to stop any fuids “It [designer mud] really is key. These simple as using mud with one particle size in it – as fractures infll well on Franklin, “opening a new coming in, but not so that it is so over- formations – sand and chalk – we know will not form uniformly – a particle size distribution has to horizon,” says Total E&P UK managing balanced above the pore pressure or how to drill them and we know how to be calculated so that the different sized fractures that develop director Phillippe Guys. fracture gradient that you can fracture effciently manage the rock formation,” can be plugged. In addition, using managed pressure drill-

Thanks to the success in this area, the formation, induce losses or become says Tom Brian, senior drilling engineer, ing allows the frm to more closely navigate the bottom hole

Total has brought on stream two new differentially stuck.” Total E&P UK. “We understand what pressure. platforms at the Elgin Franklin complex The alternative is to isolate the depleted tools we can run. The challenging piece While using designer mud enables Total to drill through these – Elgin B and West Franklin, extending zones, with liners, and carry on drill- is the pore pressure and to manage the sections, because it means there are specifcally sized particles the development’s life from 22 years, ing with a different mud weight, but change in pore pressure.” in the mud, it cannot just be recycled after the drilled cuttings as initially anticipated, to more than 32 because of the number liners required for are removed. The mud has to be constantly treated before being

Elgin Franklin years. the multiple zones drilled through, you recirculated back into the well in order to maintain the proper would run out of hole sizes to continue, Total E&P UK has been a front-runner in and designed particle size distribution.

Pressure depletion The latest well drilled using designer mud was West Franklin D

Brian says. HPHT feld development in the UK, with ,

The concern over drilling these wells had Wells have been lost by not getting this its Elgin Franklin HPHT development in and another is due to be drilled this summer. West Franklin D been around pressure depletion. Drilling right, resulting in geo-mechanical defor- the central North Sea, producing since was the second well drilled on West Franklin, to a vertical at virgin pressure, while still providing a mation, which also potentially results 2001. It was challenged with pressures depth of 5600m, to the top of the reservoir. It had to go through

HPHT challenge, is simpler, due to hav- in resources which can no longer be up to 15,500psi and temperatures up to a heterogeneously depleted Fulmar interval reservoir, which ing a consistent pressure. It is a different reached unless another way can be found 350°F on the felds. meant encountering changes in pressure from 1100 bar to 600- story once the reservoir has started to be to access them. Franklin was discovered in 1986 and 700 bar, over the space of a few meters. depleted. Elgin in 1991. It took 15 years and £20 “We have been increasing our understanding, over the years,

Mud, glorious mud “At the reservoir, you have got a million of research investment, before of just how much we can do with the designer mud,” Brian reduction in pore pressure, which Total uses managed pressure drilling both could be produced. Initial develop- says. “We don’t think we’re yet at the limit [of particle size that reduces fracture gradient and weakens and expandable liners, but, Brian says, ment challenges included 3-4% carbon can be used] and work to understand what we can achieve con- the formation, which creates diffculty the key is the designer, or stress caging dioxide, 30-50ppm hydrogen sulphide, tinues. It is all part of the infll drilling problematic.”

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