Page 66: of Offshore Engineer Magazine (Feb/Mar 2015)

65 67

Dry gas reservoirs characterized by a very narrow conti- nental shelf. To the south, it is less than Although oil has been discovered 5km wide. However, to the north, it can offshore Kenya, the major fnds to date extend out to about 50km. Water depths have been dry gas reservoirs offshore on the shelf are up to 400m. Mozambique and Tanzania. One of the

Challenges

This is followed by a steep drop to the development options being considered is

East Africa ocean foor and water depths of more ‘subsea to beach’ full wellstream transfer than 2000m. Slope gradients are typically with no foating production systems. up to 10%, although these can exceed However, dry gas reservoirs are defned in the deep 15% in places. The seabed then slopes as having no hydrocarbon liquids pres- more gently downwards to water depths ent. They will contain formation water.

in excess of 4000m. As these reservoirs are pro-

The foor itself has a large number of duced, pressures and temperatures ridges and sea mounts that are hundreds within the production systems of meters in height. There is also a series will fall. Water, initially present of deep underwater canyons running out as vapor, will begin to condense. perpendicular to the shoreline that are hun-

These systems will therefore need dreds of meters wide and kilometers long.

to operate, at least in part, in the

All of these obstacles, the very undu- multiphase region.

In addition, since water will be lating terrain and steep slopes will pose present, it is likely that liquid cor- signifcant challenges for identifying rosion and hydrate inhibitors will suitable routes and landfalls for future need to be injected for transporta- fowlines and pipelines.

tion over relatively long distances.

In the Indian Ocean to the north of

This will increase the volume of

Madagascar, the surface current circula- liquids in the production systems tion is normally clockwise, although and push them further into the there is an equatorial counter-current multiphase region. in winter. However, to the south of

Madagascar, the current changes direc-

Multiphase fow tion and circulates counter-clockwise.

As pressures and temperatures

Between Madagascar and the mainland, fall, the gas velocity increases. the Mozambique current fows from

Although a production system with north to south and is magnifed by the low liquid loading might start in channeling effect between the two land the stratifed fow regime, the effect masses. of the increasing gas velocity will

There are also two monsoons. The be to move it into stratifed wavy fow and, north-east monsoon or kaskazi brings eventually, into slug fow. Each of these dry air in from the Persian Gulf from fow regimes has a greater pressure drop

November to April and the warm, moist

The East African coastline. Image from the GRID-Arendal Continental Shelf Programme.

per unit length, so pressure drops will kusi monsoon blows in from the south- been delayed by the formation of a new increase as the fuid moves closer to shore.

east from April to October. The slightly

Gas has been found in great

With the pressures and temperatures government administration and negotia- cooler kusi brings the heavier rains, from quantities ofshore East Africa. decreasing, more liquids will condense. tions with Kenya over the maritime border.

late March to early June. There is then

The presence of more liquids increases the

While no offshore feld developments a second rainy season in November and

What will be the challenges likelihood of a change in fow regime, lead- have as yet been sanctioned, a number of

December. These may therefore place around getting it to shore? ing to increasing pressure drops. These in pre-FEED concept selection studies have limitations on installation activities.

turn cause more liquids to condense. The been awarded. Final investment deci-

Because this is a frontier area, the oil

Alex Hunt takes a look.

situation therefore escalates.

sions for the frst projects are likely to be and gas industry currently has limited made within the next three years. Almost operational experience and knowledge. ffshore East Africa is rapidly When the fuids reach the base of the all of the current discoveries are in water

The seabed bathymetry is now being emerging as an exciting new continental shelf, the production systems

O depths greater than 1000m. It is possible surveyed and mapped. There is maritime frontier exploration play. Major will need suffcient pressure for the that both subsea facilities and foating information available on surface currents, gas discoveries, with volumes cur- fuids to be able to climb the slope and production systems will be required.

wind speeds and directions. rently estimated to exceed 100 Tcf, have reach the shore. The back-pressure in the

However, the region has some very been made offshore Mozambique and system will limit natural fow and reduce

However, more data is needed on challenging characteristics that will Tanzania. An oil discovery has also been reservoir recovery unless some form of the strengths and directions of currents require an integrated multi-disciplinary made offshore Kenya. pressure boosting is provided.

through the water column. Long-term data

Further south, exploration licenses have gathering campaigns using acoustic dop-

Slugging has two main causes. approach. Offshore production facilities been awarded offshore Madagascar and pler current proflers (ADCPs) will provide

Hydrodynamic slugging is governed by will need to be designed for both installa-

South Africa. Seismic data is currently the information required for the design of gas to liquid ratios and fow velocities. tion and operation.

being acquired and processed. In the far risers and moorings for foating production

However, terrain-induced slugging is

Metocean challenges north, Somalia has been considering an systems and will also support the planning related to the seabed topography. The major

The seabed topography of East Africa is offshore licensing round, although this has of optimized installation programs. undulations of the East African seabed will

February 2015 | OE oedigital.com 68 068_OE0215_GeoFocus2_Hunt.indd 68 1/20/15 6:57 PM