Why, despite the application of horizontal drilling, water injection and a host of gadgets and techniques, does the majority of oil remain in a reservoir when the wells are shut down for the last time? No pressure, no pay-day.
After a sustained period of low oil prices and the related impact on many mature oil basins from the North Sea to the Niger Delta, you may well ask ‘why should we care about unrecoverable oil?’
Under the present paradigm the response is largely ‘we don’t’. However what if technology are driving winds of change.
Why oil flows and then stops
Going back to first principles, oil is trapped in sedimentary layers below the earth’s surface. The weight of these layers stacked on top creates pressure in the oil reservoir. If the pressure differential between the reservoir and the earth’s surface is sufficient, the oil flows upward for production. As the oil is produced there is less material in the reservoir and the pressure decreases until a point is reached where it ceases to flow or the quantities produced no longer satisfy the accountants.
The amount of pressure needed reduces if the oil flows more readily from the pores that have been its home for the last million or so years. Opening the pores make this journey easier (fracking), lubrication can grease this journey (traditional MOER) and pressure provides the driving force (gas and water injection).
So once nature’s pressure bubble has been burst what viable means of pressure replications exist? There are a number, some tried and tested, some new and potentially more profound.
Feeling the force
Water injection is a long established technique, introduced at pressure through injection wells it pushes the oil from its pores toward oil production wells. It works until a saturation point is reached when the reservoir become water saturated and oil is longer pushed toward production wells in sufficient quantities.
In heavy oil fields pumps are used to suck oil to the surface. Heavy oil fields tend to be at shallower depths making this feasible although the overall recoveries are consequently substantially below those for conventional oil. This is used with standard cold production or even with thermal recovery depending on how heavy the heavy oil is.
Another form of pressure introduction relates to gas injection. This can be natural gas or Carbon Dioxide. The injected gas creates pressure in the reservoir. The impact varies according to how the gas dissipates and the extent to which it is absorbed into oil and/or water. Injection of gas has to be undertaken repetitively as renewed oil production will reduce pressure again. The quantities required can be substantial to pressurize an entire reservoir which can make it logistically and economically unfeasible. This is because natural gas is a valuable commodity plus infrastructure is required to capture and transport the gas for injection. This is the case unless there is a source of gas in the proximity of the reservoir or infrastructure already connecting such a source.
The environmental benefit of capturing CO2 and injecting it out of harms way for the economic benefit of increased oil are obvious. The trouble is the reality frequently fails to match the beauty of the headline. CO2 is a smaller molecule compared to natural gas meaning it is absorbed into the liquids of the reservoir to a greater extent, reducing pressure impact. Being a molecule minor also makes it prone to dissipating into the reservoir meaning the pressure impact is harder to harness and difficult to control. Finally to use large scale CO2 sources requires collection, storage and transportation infrastructure. Unfortunately lashing a pipe to the exhaust of every truck and Ferrari is not without challenge and power stations tend to be located for convenience to the electricity grid and power source not for oil fields that have lost their fizz.
Today’s artificial pressure sources for our tired oil fields all have merit and in the correct projects can bring worthwhile benefit to oil recovery and the associated economics. Arguably the major potential prize of pressurization still remains evasive, namely a source of pressure to restore reservoirs to their former production glories in a sustainable manor allowing the majority of the remaining at production decline to be profitably extracted. New technologies that boldly go where other pressure pretenders have failed to tread perhaps hold the keys to the oil chest right under everyone’s nose.