• Playing the field

    New methods have advanced the extraction of oil on the continent.

    Playing the field

    Back in 2000 the Economist called Africa ‘the hopeless continent’ on its cover. A decade later (December 2011 to be precise) that was updated to ‘Africa rising’. But what caused a continent to reverse direction in a decade? In a word: oil.

    In a recent Energy Intelligence report on Africa’s oil and gas potential, Christina Katsouris said that Angolan oil reserves alone have more than doubled over the decade from 6 billion barrels in 2000 to 13.5 billion in 2010, while Sudan has stormed in from nowhere, increasing to 6.1 billion from 0.6 billion.

    Production too is booming with Angola up from 0.7 million barrels a day to just over 1.8 million over the decade. Two new kids on the block have just started producing 95 000 barrels a day from the Jubilee deep water field off Ghana and 20 000 a day from Niger.

    The majority of reserves and production are concentrated in Nigeria, Libya, Algeria, Angola, Sudan and Egypt. There have been significant new discoveries in Ghana, Tanzania, Mozambique and Uganda, with extensive exploration projects under way in Sierra Leone, Mali and Kenya. That the exploration industry is in rude health is confirmed by a visit to the Cape Town harbour where the oil and gas rigs in port for repair and refurbishment far outnumber visiting cruise liners.

    Most African oil is exported as crude, because although many refineries have been planned, only three have been built, all by the China National Petroleum Corporation. While China is often criticised for neo-colonial treatment of Africans, when it comes to beneficiation, it is China and not the West that delivers the goods. As Katsouris points out, it’s the time-tested model of cheap exports and expensive imports.

    Production too is booming with Angola up from 0.7 million barrels a day to just over 1.8 million over the decade

    Sub-Saharan Africa has six major oil plays: the African Rift Valley fields in Uganda and Ethiopia, the West African Transform Margin off the bulge of the continent; an inshore zone stretching from Cameroon in the north down to Namibia in the south; the well-established Niger Delta off Nigeria; a deep water pre-salt zone stretching from Gabon in the north to Namibia in the south; and possible fields off Madagascar.

    Of the six plays, five are offshore, mostly in deep and ultra-deep water. By far the most interesting play consists of the pre-salt deposits, which have been energised by the enormous fields found recently off Rio de Janeiro. Estimated at 150 billion barrels, they are larger than all the African reserves put together and are well over 10% of the global total.

    Fit the Gondwanaland jigsaw back together and the African potential of pre-salt is clear. As you’d expect, there is a striking geological resemblance between pre-salt regions off the west coast of Africa and recent discoveries off Brazil, which transforms the whole of the western African littoral into elephant country.

    Deep and ultra-deep wells have implications for drilling, which explains why 50% of the Pre-Salt Development West Africa Congress was devoted to ‘identifying cost-effective drilling and completions strategies for achieving commercial flow rates in West African pre-salt wells: enabling an economic return on every well drilled’.

    Pre-salt is a geological layer located off continental shelves that accumulated salt above them during the break up of Gondwanaland. Oil deposits are trapped underneath this layer by a cap of salt, a phenomenon common off the coast of Africa and Brazil.

    According to Brazilian state oil company Petrobras its oil and natural gas deposits are located below a 2 km layer of salt, which itself lies under 2 km of rock, which lies beneath 2 km of Atlantic Ocean. Since 2006, Petrobras has sunk 11 wells in the Santos basin alone. All were successful. Salt layers extend for 800 km along the Brazilian coast – from the state of Santa Catarina in the south to Espirito Santo in the north and can be up to 200 km wide giving a total pre-salt area of 122 000 km2.

    Equally important is the identification of cost-effective drilling and completions strategies for achieving commercial flow rates

    Drilling through all this rock and salt to extract the pre-salt oil and gas is very expensive. This puts optimal drilling technologies at centre stage. To date, African pre-salt potential has been identified in basins offshore from Angola (Kwanza basin and Lower Congo basin), Namibia, Gabon, Congo, the DRC and Cameroon. The first discoveries in African pre-salt zones were made in 2011 off Angola and Gabon.

    Three companies have drilled test wells to date with different results, emphasising the importance of geological and geophysical understanding of pre-salts through advanced seismic acquisition and processing to accurately identify the locations of sweet spots (the low-hanging fruit of the oil world).

    Equally important is the identification of cost-effective drilling and completion strategies for achieving commercial flow rates of black gold. Factors to consider when designing an optimal drilling strategy for ultra-deep wells include identifying the correlation between rock measurements and rock response plus core-log integration to understand and improve drilling performance.

    New techniques from hyperspectral imaging of rock core may play a role here. These include detailing issues for better maintenance that will not only improve well integrity assurance but the whole well life cycle; reviewing technologies that can comply to a zero discharge policy in pre-salt drilling to avoid additional costs and pollution issues; and understanding if there are any tax or other incentives in place to improve contract strategy and competitiveness of offshore service companies.

    When assessing the most effective techniques for drilling through deep water, thick layers of salt and rock, cognisance needs to be taken of innovative drilling technologies in ultra-deep water and high pressure environments.

    The most suitable techniques for different rock formations can be determined by analysing the outcome of bilateral, radial and horizontal multi-fractured drilling. Optimal drilling bits used to drill through pre-salt carbonate formations to reduce the number of trips down-hole need to be identified. Zone specific drilling techniques for top-hole, salt and reservoir zones also need to be identified to increase efficiency.

    Africa has the potential to become the Texas and Saudi Arabia of the new millennium

    Water versus oil-based mud systems should be compared to ensure wellbore stability and optimise pre-salt drilling.

    An attempt should also be made to identify optimal casing systems. Additives and stabilisers that dissolve in salt zones but enhance the efficiency of water-based muds when drilling through rock need to be monitored as well as the effectiveness of both oil and water-based muds. Pressure prediction in pre-salt drilling may be one tool to optimise the timing for setting intermediate casing systems when hitting unexpected zones within the salt layer.

    An enhanced monitoring system to prevent potential wellbore failure should be imple-mented and various cementing methods to isolate zones of loss return need to be assessed.

    The effectiveness of different muds to control formation pressure when drilling into ultra-deep formations also needs to be investigated.

    Africa has the potential to become the Texas and Saudi Arabia of the new millennium. Energy will drive the continent to the future predicted by the Economist and optimal drilling is the way to get there while respecting both the environment and profits of investors.

    By Neil Pendock
    Image: Gallo/GettyImages