Careful stewardship of oil fields is necessary if the industry is to make the most of its reserves. Clifford Jones, a reader in engineering at the University of Aberdeen, explains why.
A well, once yielding oil, cannot be abandoned for resumption of operation without loss of productivity. Why is this so?
Once a well comes into service oil is raised by pressures of natural gas. This pressure is a characteristic of the entire field not just the well, so the productivity of a well is influenced by activities at neighbouring wells. During a period of abandonment the internal gas pressure of the field will drop if other wells within it have remained operational, causing the previously abandoned well to release oil less copiously.
A further factor is that an entire field will have been depleted of oil during inactivity. During this period operators of neighbouring wells can draw oil from the inactive well and recover it at their own. This is known as the Law of Capture: a well does not have a catchment area and oil produced at a well is the property of the well operator. Of course, all wells might belong to one operator in which case the Law of Capture is irrelevant.
It sounds as if the fortunes of a well operator are dependent on the activities of its neighbours.
This is true. At the time that the Middle East oil industry was beginning this had been recognised as a problem in the US. Laws therefore came into being regulatory authorities, and had the entitlement to check the oil-to-gas production ratio of any well; a low value of this ratio signifies potential to reduce the gas pressure of the field to a degree disproportionate to the quantity of oil being recovered. A well with a lower value of the oil-to-gas production ratio than that set by law would be deemed to be a threat to other wells in the field and compulsorily closed down.
What if the gas becomes sufficiently depleted for there to be no oil recovery possible by reason of its pressure?
This often happens. It is then necessary to install a pump, the most common is the beam pump, aka a pumping jack. Such a device will provide pressure previously created by the natural gas, but at a cost in energy terms since a beam pump has itself an energy requirement. Substitution of a beam pump for the field’s own gas pressure causes a drop in the Energy-Return-on-Energy-Invested (EROEI) for the oil being obtained. The economic basis of oil production is finely balanced and such a drop can have a significant effect.
Surely the gas not only has a role in oil production but is itself an important product.
Quite so. Gas occurring with oil is called associated gas, while that which occurs without oil is non-associated gas. Sometimes, especially at offshore fields, associated gas has been sufficient for collection to be economic but has been simply flared off. In this regard Nigeria (an OPEC country) and Trinidad and Tobago (non-OPEC) have been criticised although the latter at least has implemented improvements in this regard.
Can you identify an oil field in the Middle East where associated gas is particularly abundant?
Yes, the Ghawar field in Saudi Arabia, which happens to be the largest oil field in the world! It has been productive for almost 60 years. The associated natural gas at Ghawar accounts for a third of Saudi Arabia’s known natural gas reserves.
What alternative is there to use of a pump to enhance oil yield?
Injection of sea water. This is done both at offshore and onshore fields, including Ghawar. Another oil field in the Middle East where injection of seawater takes place is the Burgan field in Kuwait, the second largest oil field in the world.
Can injection of seawater have any adverse effects on an oil field?
Yes, the chief one is scale deposition on to the geological structure by salts present in sea water. Such scale can cause flow restriction. It is in fact possible that both Ghawar and Burgan have already been affected.
Given that Ghawar and Burgan are the largest and second largest oil fields in the world, is there cause for concern at this?
Quite possibly. Ghawar produces about 5 million barrels of oil per day (bpd) and Burgan 2 million bpd, making their combined yield about 9% of the world requirement. Any factor that harms these fields has the potential for significant effects at international level. Bad stewardship of an oil field in the form of gas loss reducing the internal pressure causes the EROEI value of oil produced at the field to drop. While use of seawater in oil recovery raises production in the short term it can cause damage. An alternative, used in the North Sea, is to inject carbon dioxide instead of seawater so that enhanced oil recovery and carbon sequestration go hand in hand.
