Introduction to Deepwater Field Development Strategies

Babs Oyeneyin

1.1 Introduction to Global Energy and Hydrocarbon Development

Petroleum is a complex mixture of hydrocarbons that can occur in liquid and gaseous forms within the pore spaces of conventional reservoir rock or unconventional shale matrix at shallow or great depths on land or in an offshore environment, depending on composition, impurities, and prevailing conditions of pressure and temperature.

The process of hydrocarbon exploration and production starts with a search for the presence of hydrocarbons within the rock matrix, drilling of an appropriate conduit from the surface through various rock types to the depth of interest where the hydrocarbon is ‘stored’, installation of subsurface and surface process facilities and eventual production, and processing and sale.

The operation is capital intensive and needs to be carried out efficiently and safely with little or no impact on the environment. Thus, the operator must possess the ability to extract the hydrocarbon fluids efficiently and economically from the reservoir rock through the wellbore to the topside production facility all the way to the beach via the pipeline (tieback) over the life of a field within any environment. For land locations, the pipeline tieback is normally in the form of a pipeline from a process facility to the export terminal. In offshore/deepwater environments the subsea tiebacks connecting all deepwater wellhead manifolds to the process facility are now recognised as one of the cheapest ways to develop in deepwater environments.1

Therefore, the objectives of oil company operations can be classified into the following key areas:

(a) Safety, Health, and Environment and Energy Supply Security

(b) Maximisation of Capital Outlay
One major objective of any enterprise is to maximise cash flow and recoverable reserves through:

• Maximisation of production rate

• Maximisation of recovery

• Minimisation of downtime through effective prevention and control of operational problems

(c) Minimisation of Costs
Another key objective is the minimisation of overall costs in an attempt to maximise profit through:

• Minimisation of capital cost: Ensuring that optimum capital expenditure (CAPEX) is maintained to ensure efficient production with minimum downtime

• Minimisation of operating expenditure (OPEX) including:

• Minimum production cost – Today, the emphasis is on minimising lifting cost per volume (per barrel or m3) of produced fluids, thereby increasing production of fluid

• Minimum treatment and workover costs

To achieve these key objectives, today's trend is to set up an integrated project team of different specialists comprising the geology and geophysics team, production geologist, reservoir engineer, drilling engineer, production technologist, etc., who will be responsible for effective reservoir management through strategic planning and optimum well design. The integrated team, and most especially the production technologist, must therefore be very conservative with the different facets of development and operation of the well including:

• Drilling (casing design, drilling/completion fluids selection)

• Completion (design/installation of completion string)

• Production (monitoring well and completion performance)

• Workover/Recompletion (diagnosis/installation of new or improved production system)

• Eventual abandonment (planning depletion profile and identifying candidates and procedure for abandonment)

Today, energy security is a serious challenge with the global demand for hydrocarbons (oil and gas) outweighing other sources of energy such as coal, nuclear, and renewable energies (Figure 1.1).

The demand for gas (Figure 1.2) is growing faster than the demand for oil (Figure 1.3), especially in developing countries (Figure 1.4).

To meet the ever-increasing gap between the demand and supply of hydrocarbons – especially oil and gas – the petroleum industry is sourcing advanced technologies to enhance production and recovery from the existing mature and marginal fields with more than 50% of recoverable reserves of conventional oil and gas still in place, and carrying out aggressive exploration to new frontiers such as the ultra deepwater environments and arctic regions. Additionally, there is also an increasing trend toward the exploitation of unconventional hydrocarbon reservoirs including shale oil and especially shale gas as well as coal-bed methane and heavy oil. The deepwater environment (from 2000 m to over 3000 m), which includes major assets in the Gulf of Guinea in the West African subregion, the Campos Basin in South America, and the artic regions, now forms the cornerstone of many oil/gas field developments. These deepwater environments are characterised by very hostile environmental sea conditions and complex temperature and geological features such as high pressure and high temperature (HP-HT) with a very narrow margin of rock pore pressure-fracture pressure window (which makes drilling by conventional overbalanced methods difficult), shallow unconsolidated reservoir sands with massive sand production problems, and a combination of challenging and ‘flexible’ rock lithologies.

The economic viability of the mature and marginal fields, deepwater and ultra deepwater environments, and even the new frontier of the arctic regions as well as the unconventional shale reservoirs requires the depletion of these reservoirs with a minimum number of wells. To achieve this, engineers are being challenged to use more long horizontal, extended reach, and multilateral wells and long subsea tiebacks. In addition to trying to minimise problems associated with fluid production, there is the added benefit of using these wells to potentially reduce field development costs. In deepwater environments and arctic frontier fields as well as in mature fields, multiphase fluid production accompanied by increasing sand production is inevitable.

The major challenges for companies are how to maximise individual wells and field development performance, minimise operational cost and non-productive time, and guarantee flow assurance across the composite production system from reservoir through wellbore, long subsea tieback to topside for effective asset integrity.

1.2 Examples of Deepwater Developments of the World

The deepwater environment can be categorised into three main depth tiers:

i. Deep – Tier I

• 500 m – 2000 m of water depth

ii. Very Deep Water Depths – Tier II

• 2000 m – 3000 m of water depth. These represent the current exploration frontiers.

iii. Ultra Deepwater – Tier III

• Greater than 3000 m of water depth

At these water depths, the seabed temperatures can be as low as 100 °F, creating major challenges for flow assurance with cold flow of produced fluid and impact on material integrity.

The major ultra deepwater environments include:

• Arctic

• Gulf of...