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Exploration drilling and discovery appraisal
Once explorers have identified a region with the potential to host a petroleum accumulation and defined any prospective targets, they move into the second two phases of petroleum exploration:
3. Exploration drilling operations
Drilling an exploratory well in the hope of encountering petroleum.
Appraising all discovery and well data to determine if a discovery is economically viable.
Exploration drilling operations
Pre-Drilling
Once the final investment decision has been made to commit to drill an exploration target, the focus quickly moves from evaluating the geologic risks to evaluating the operational risks.
The reason for this shift is that drilling an exploration well is the only means by which they can test their theories. It is also the point at which environmental, geological and financial risk all converge.
Drilling related experts, often from external consulting companies, combine their knowledge with that of the exploration team and begin to create a plan for operational drilling activities.
The plan they create needs to be submitted to and approved by the authorities prior to the drilling of exploration well. The plan takes into account both the geologic and environmental challenges, and which engineering solutions best meet these.
Environmental Considerations
The environmental challenges of most concern to offshore drilling operations are the metocean conditions (the wind, ocean currents, and size of waves). The operator, usually with help from an external expert, will analyse any historic data and forecasted seasonal conditions.
An Environmental Impact Assessment (EIA) may also be undertaken in order to understand how the drilling operations might impact marine flora and fauna, and how this can be mitigated.
When undertaking an EIA the drilling team generally work with local environmental experts to understand the environment they will be working in. These analyses allow for the most appropriate drilling rig and support vessels to be selected, and how best to operate them in the chosen environment.
Surface/near surface site survey
Understanding the surface/near surface is of vital importance before commencing any drilling activities, i.e. the securing of drill rig to the seafloor, and the drilling of the conduit (the very top part of the well).
Prior to these activities, a site survey is carried out to ensure the site is safe. This survey scans the sea floor to image its bathymetry (sea floor terrain) and collect very high resolution 2D seismic data in order to understand the near surface geology and detect the presence of shallow gas.
The image above is of the MV Tangaroa, a NIWA vessel, doing a multibeam bathymetry survey. Visit the NIWA website.
The surface/near surface geology needs to be understood to ensure that the rig is securely positioned and to prevent the rig’s supporting feet slumping or sliding or the anchors being pulled out because they were placed on a soft or rough surface.
Subsurface activities
In order to drill the exploration well in the most effective and efficient manner, the drilling and exploration team combine all their knowledge and plan a well trajectory aiming to intersect the target zone at the best angle and avoid any fracture zones or zones of high pressure.
By analysing the seismic data very carefully the team can identify if any of these features exist. If a fracture zone or zone of high pressure is predicted the team will adjust the trajectory of the well in order to drill through them efficiently.
Due to the complexities of the subsurface strata (rocks) the exploration well is often drilled as a curve or ‘s’ shape instead of a straight vertical line.
The seismic data is also used to help decide on casing points, cement strengths and appropriate mud weights, which are essential safety measures to control the flow of hydrocarbons.
Drilling a well
After the metocean, site survey, and well trajectory studies are complete and approved, the exploration company is ready to test their theory that the prospect contains a commercial accumulation of hydrocarbons.
The first step is to safely secure the rig on site according to the site survey results. The drill rig used for drilling the exploration well is dependent on both the water depth and water conditions. Where wells are in relatively shallow water (up to 120 m), Jack-up rigs are commonly used; these rigs have feet that rest on the sea floor in order to support them and keep them in place. In deeper water, Semi-submersible rigs or Drill ships are used. These rigs can either use dynamic position systems or anchors and cables/chains to maintain their position.
Once a rig is securely positioned, the next process is to drill the conduit (the very top part of the well). Once the conduit is drilled it is cased with steel pipe that is cemented in place and a blowout preventer (BOP) is installed. Both the steel casing and BOP are safety measures to control the flow of hydrocarbons when the well reaches the target zone.
Before any further drilling is carried out, the BOP and conduit casing are tested by pressurising the well with higher than expected pressures. If the BOP and conduit pass the tests then the well is re-entered and the next phase of drilling is initiated.
The hole is then drilled ahead bit by bit to the target zone. During the drilling, the well diameter decreases at set depths, which are known as casing points. The decreasing diameter is due to the physical limits of drilling at depth. The casing points are predicted during the well trajectory planning phase and finalised during the drilling according to the geology they encounter.
At each casing point the drill bit is pulled out of the well and a steel casing (pipe) is inserted and cemented in place. The well is then pressure tested to ensure that the cement and casing are secure.
Throughout the drilling operation, drilling mud is introduced to the well, this is an important safety measure to ensure that any pressurised liquids encountered in the rock formation are contained.
The drilling mud also cools and lubricates the drill bit and as it returns to the surface, carries with it the small rock fragments or chips produced by the drilling. These are separated from the drilling mud and analysed by a geologist to determine the actual geology being drilled and ensuring that the trajectory plan predicted the rocks correctly.
When the well reaches the target zone, fluids and gas are collected (if encountered) to evaluate the commercial potential of the zone. The well is also wireline logged; this is a process where geophysical tools are lowered down the well to help determine the geology and the presence of hydrocarbons. If the well encounters hydrocarbons it enters the discovery appraisal phase.
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Discovery appraisal
In the event exploration drilling operations are successful and intersect a hydrocarbon accumulation, the next step is to appraise the discovery so that an assessment of the size and ultimate commercial viability of the resource can be gained.
Appraisal operations will usually involve considerable amounts of post-well technical analysis of the discovery well. In some cases, further seismic (often 3D) will be deemed necessary to further map the target structure, or to identify additional targets in the area.
For almost all offshore discoveries, additional well drilling operations will be required. These wells will be drilled with the same high levels of safety and environmental considerations as the initial exploration well.
One or more of these appraisal wells will also be used to carry out a production test of the reservoir. This helps confirm reservoir size and determine the rate at which the petroleum resource is producible.
Further analysis of the appraisal results alongside consideration of market demand and possible sales price will ultimately determine if such a discovery is economically viable.
