The goal of petroleum exploration is to identify areas with the right geologic conditions for a profitable accumulation of petroleum.
Exploration involves detailed data gathering and modelling including seabed sampling and seismic surveying over smaller, more specific areas. If analysis of data collected from these activities looks promising companies may decide to drill an exploration well.
Once the final investment decision has been made to drill an exploration well to test a prospect, the focus quickly moves from evaluating the geologic risks to evaluating the operational risks. It is the point at which environmental, geological and financial risk all converge.
Drilling an exploration well is the only conclusive way to test for the presence of an oil and gas field.
While the desired result of an exploration well is the discovery of an oil and gas field, most wells drilled do not achieve this – as a rule of thumb; only 10% of wells make a discovery. And of these only 10% result in a producing field. As a well can cost between $5 million (onshore) and $100 million (offshore), it is a very costly decision.
Drilling experts from external consulting companies combine their knowledge with the petroleum company’s exploration team and begin to create a plan for operational drilling activities. The plan they create needs to be approved by the authorities prior to the drilling of the exploration well. The plan needs to take into account both the geologic and environmental challenges, and which engineering solutions best meet these.'
The environmental challenges of most concern to offshore drilling operations are the ocean conditions such as the wind, ocean currents, and swell (known as a metocean study). The operator, usually with help from experts, will analyse any historic data and forecasted seasonal conditions.
An impact assessment report, which is a requirement for a resource and marine consent, is undertaken in order to understand how the drilling operations might potentially impact on the marine environment, and how this can be mitigated. When undertaking an impact assessment 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 for securing an offshore 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 or any other hazards.
A site survey is not required onshore but surface considerations are taken into account in the wellpad location and well planning stage.
In order to drill the exploration well in the most effective and efficient manner, the drilling and exploration team plan a well trajectory aiming to intersect the target zone at the best angle and avoid any fracture zones or zones of high pressure.
If any of these features exist, 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.
Seismic data and prior drilling results are also used to help forecast the pressure as the well is drilled. This is used to decide on casing points, cement strengths and appropriate mud weights, which are essential safety measures to prevent the flow of hydrocarbons and retain ‘well control’.
Drilling a well
After the metocean, site survey, and well plans for the offshore site 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 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. In deeper waters, Semi-submersible rigs or drill ships are used. These rigs can either use dynamic position systems in very deep water or anchors and cables/chains to maintain their position in shallower water.
For onshore wells, the wellpad will have been constructed and the first step is to simply construct the drill rig.
Once a rig is securely positioned, the next process is to drill the first section of the well. 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. The casing isolates the low pressure upper sections from the higher pressure lower section of the well and the BOP helps control the flow of petroleum 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 next phase of drilling is initiated.
The hole is then drilled in sections to the target zone. During the drilling, the well diameter decreases at set depths, known as casing points. 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.
Casing is important as the pressure at the top of the well and bottom of the well are radically different - throughout drilling the pressure in the rock and the well need to be balanced. Balancing these pressures prevent petroleum entering the wellbore in uncontrolled way and protects the integrity of the surrounding rock. Drilling fluid (mud) is used to control these pressures.
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 rock 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.
When exploration drilling operations are successful and intersect a petroleum accumulation, the next step is to understand the size and commercial viability of the resource.
Appraisal operations will usually involve considerable amounts of 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.
These appraisal wells will be used to carry out a production test. This helps confirm reservoir size and determine the rate at which petroleum flows from the well.
Further analysis of the well results alongside consideration of market demand and possible oil/gas sales price will ultimately determine if such a discovery is economically viable.