Exploration and development activities

Surveying the location

Geophysical surveys

These surveys help us to plan our subsea installation and well construction activities by understanding what is present on and below the seabed including:

• the shape and features of the ocean floor
• the materials that make up the seabed
• any unexpected or unusual features
• any dangerous features that could introduce a risk to our activities  

Different equipment is used depending on the type of geophysical surveys being completed. Equipment can include:

• Multi-beam echo sounder
• Side Scan Sonar
• Sub-bottom Profiler
• Magnetometer
• Sound Velocity Profiler (SVP)
• Conductivity, Temperature and Depth (CTD)

Geotechnical surveys

These surveys help us to understand the characteristics of the seabed before starting any projects. For example checking the seabed is stable for construction or installation of equipment and that any infrastructure to be installed can withstand the conditions of the seabed. These surveys involve:

• lowering a deployment skid to the seabed that contains the survey equipment
• using equipment to take small hole samples less than 1m3 through different processes (e.g. penetration testing, coring or grab sampling)
• analysing the samples to understand if the location meets engineering requirements

Constructing wells offshore

Well construction is the process of drilling and building wells in the seabed to extract natural gas or oil from the reservoir below the ocean floor. This involves a series of steps outlined below.

Positioning and installing a drilling unit 

There are different types of drilling units. In the Otway region, with its metocean conditions, we use a moored unit called a Mobile Offshore Drilling Unit (MODU). This unit is temporarily moored to the seabed and has a system to keep it in position without anchors (dynamic positioning). The MODU may move itself into position or be towed by anchor handlers. Other anchor handlers connect mooring lines from anchors on the seabed. 

Drilling operations

The drilling equipment is sent down to the seabed. The first part of the well (top-hole) is drilled, followed by each deeper section. Steel pipes (casings) are placed inside the drilled hole to stabilise it. For deeper sections, a long pipe (riser) and a safety device (blowout preventer or BOP) are added after the surface casing is secured with cement. A drill-through subsea tree is installed and tested to control the gas flow from the underground reservoir once the well is connected to the pipeline system and starts producing gas. The area directly affected by drilling the top-hole is about the size of a small table (2 m2).

Sidetracking operations

If the original bottom part of a well needs to be redone, a new section is drilled from an existing well to reach the same underground gas or oil reservoir. Sidetracking uses a new path from a well to get to the same general destination. 

Drilling cuttings and fluids

We use a special mixture called "mud" (made of seawater, clay/gel, and other additives) for several important tasks during drilling:

• keeping the drill bit cool and helping it move smoothly
• removing debris by carrying rock and dirt (drill cuttings) out of the well
• keeping the pressure in the well balanced to prevent gas or oil from entering the well unexpectedly
• ensuring borehole remains open and doesn’t collapse.

Cementing operations

Cement is used to hold the steel pipes (casings) in place during each stage of drilling and to temporarily or permanently close off the well.

Installing and testing the BOP

After the top part of the well has been finished, we install a Blowout Preventer (BOP). The BOP acts as a backup safety device to close off the well if there's an unexpected increase in pressure from the underground reservoir during drilling. It acts as a safety valve to shut off the well to prevent accidents. 

Completing the well 

This step sets up, cleans and protects the well from corrosion to prepare it for future gas production.

Steel pipes (casing or tubing) are placed in the well to control the flow of oil or gas from the underground reservoir. The well is cleaned out and filled with a salty water solution (brine). The space around the tubing is filled with a special brine that prevents rust. The inside of the tubing might be filled with a base oil to get the well ready for clean-up and testing.

Well clean-up and flowback

To prepare a well for production, we clean it out, gather important information, and run tests to ensure everything works correctly. This involves:

• clearing the well of drilling fluids and brines
• gathering data like well pressure, flow rates, and the makeup of the reservoir
• testing the well by allowing it to flow - this involves burning off some gas (flaring) while a test package is connected to measure the well's performance.

Well shut-in and suspension 

Wells are safely closed off which can be done in two ways:

• With a Subsea Tree installed: A subsea tree is put in place, and the well is shut off.
• With a Plug and Cap: A plug and a cap are installed to seal the well.

Both methods prevent condensate or gas from flowing out of the well.

Logging

To effectively plan for the future extraction of gas from the reservoir, we collect real-time data using special tools:

• Tools attached to the drill string to gather information as we drill.
• Tools lowered into the well on a cable collect data after drilling.

This data helps us understand the reservoir better and make informed decisions for efficient recovery

Installing, commissioning and testing well components

Once the well is safely closed off (shut-in or suspended), the next steps are to make sure all the infrastructure is installed correctly, tested and working properly before any gas is extracted. 

Pre-lay works

A stable foundation is set up to make sure the pipelines are securely placed on the seabed. This involves:

• Installing Concrete Mattresses or Grout Bags: These are placed where pipelines cross each other or where gaps (spans) might exist.
• Trenching or Excavation: Depending on the shape of the seabed, this might be needed to ensure the pipeline is properly supported.

Flowline and umbilical installation

Flowlines are pipelines that transport hydrocarbons from the wellhead on the seabed to onshore processing facilities. They can be rigid or flexible, depending on the specific needs of the operation and are installed using large spools (reels) or rotating platforms (carousels). Umbilicals are bundles of cables and tubes that supply necessary services to subsea equipment (e.g. provide power, communication, and inject hydraulic or chemical fluids). 

Installation of subsea structures

Other subsea structures are needed to help connect new wells to the flowlines and ensure everything works together. These are installed onto the seabed and can include:

• Manifolds: Central points where multiple flowlines connect.
• Skids: Frames that support and protect equipment.
• Hot-tap Tie-in Structures: Systems for connecting new pipelines to existing ones without stopping the flow.
• Diverless Hub: Connection points that don't require divers.
• Block and Bleed System: Safety systems to isolate and release pressure.
• Spools: Short sections of pipe used to connect equipment.
• SUDUs: Subsea Distribution Units for managing flow.
• Flying Leads: Flexible cables or pipes for connecting equipment.
• Stabilisation: Methods to keep everything securely in place.

Post-lay works 

Concrete mattresses are placed at intervals along a flowline or umbilical to keep them stable and in place on the seabed to meet specific design needs.

Testing, preservation and start-up

The equipment is prepared and protected before the gas flow starts and is controlled from land. This includes:

• Filling the inside of the structures with special fluids (inhibited water or MEG) and solid sticks to prevent rust.
• Releasing the fluids into the sea before the equipment is checked and tested.
• Inspecting and testing the system.
• Controlling the subsea equipment from the onshore gas plant.

When the system is ready and appropriate approvals are in place, production can start. This involves opening new wells to allow the gas to flow through the flowlines, pushing nitrogen gas back to the plant before operations can begin.