Heavy oil is difficult to produce for one simple reason: it is too viscous. Light oils flow easily, heavy oils do not. However, when any oil is heated it becomes much less viscous. Indeed a hot heavy oil can be much less viscous than a cold light oil. This is the fundamental reason that steam flooding of heavy oil reservoirs is attractive. Steam flooding is amongst the most efficient means possible for recovering oil – recovery factors for successful steam floods range from 50% to 80% and the technique has been widely used in onshore projects in California, Indonesia and the Neutral Zone whilst other steam techniques are commonplace in Venezuela and Canada. Steam injection has been applied offshore in the Emeraude field in Congo and in Bohai Bay in China as well as in the shallow waters of Lake Maracaibo.
Viscosity vs Temperature for a range of medium to heavy oils. Impact on viscosity of heating Pilot oil illustrated with animation.
There are three types of steam enabled production, which accounts for about 2 million bbls/day of heavy oil production around the world. The first of these techniques involves pumping of steam into a well for a few weeks and then producing back the condensed water and the oil. This technique is known as Cyclic Steam Stimulation (CSS, or colloquially "Huff & Puff") and boosts recovery factors on heavy oil fields from 5-10% to 30-40%.
Steam Assisted Gravity Drainage (SAGD) is a special variation of steam flooding in which two horizontal wells are drilled; a producer at the bottom of the zone and an injector approximately five metres directly above it. This is necessary for extra heavy oils, such as the tar sands in Alberta.
Finally, we have steam flooding, the technique we wish to adopt for the Pilot steam flood project. In a steam flood, the steam is injected continuously into dedicated injection wells, and oil, along with condensed water, is pumped out of production wells. Typically vertical wells would be arrayed very close to each other in a five spot pattern. The wells need to be close so that not too much heat is lost to the rocks above and below the reservoir before the reservoir is swept clean of oil.
As the steam condenses in the reservoir both the rock and the oil are heated up; and the, now mobile, oil is swept towards the producers by a bank of fresh water. But the benefit of steam flooding is not limited to improving the mobility of the oil. The high temperatures and the fact that the reservoir is being swept by a gas rather than a liquid conspire to dramatically reduce the residual oil saturation, from a typical value of 25% for a cold water flood down to just 5% in the steam zone. Combined the viscosity reduction and the improvement in the residual oil saturation mean that steam flooding has been found to boost recovery factors to 50% to 80% of the original oil in place.
The early steam floods used vertical injectors and producers but operators are finding that horizontal wells can be more efficient and effective. It is this innovation which enables the widespread implementation of offshore steam flooding, this is for two reasons:
- Firstly, horizontal wells are the key to achieving the well density that a steam flood requires. If we tried to develop Pilot with a conventional pattern of vertical well penetrations about 100 metres apart we would need to drill literally hundreds of wells; however, with long horizontal wells penetrating the reservoir, we can reduce the number of wells by an order of magnitude and achieve better sweep patterns than a conventional five spot pattern flood ever could.
- Secondly, if the formation has a high permeability, the injection rates into horizontal wells can be high enough that the heat losses in the wellbore and to the sea are no longer a significant problem. See this excerpt below from a talk we gave at DEVEX 2015 which deals head on with the conventional wisdom that steam flooding offshore is too inefficient to work.
We expect to be able to recover about 60% of the oil in place with a steam flood of the Pilot reservoir; if that seems high to you there is more detail on comparable projects in this article. However, the key to the economic viability of the project is the steam-oil ratio, that is the volume of steam that has to be injected to recover a barrel of crude. We will try to optimise the balance between recovery factor and steam-oil ratio by testing different well spacings, injection rates and well positions relative to the oil water contact. We will also be evaluating co-injection of steam and propane. This approach has the potential to further increase the recovery factor and to accelerate production, as well as to improve the steam oil ratio, which reduces fuel requirements.
Finally we have investigated the problems inherent in steam flooding reservoirs with significant amounts of bottom water, i.e. those reservoirs where the oil zone is underlain by a significant water zone rather than an impermeable shale. Typically operators have found that steam flooding reservoirs with this configuration is less efficient and the recovery factors are much lower. We have developed an innovative approach to this problem which achieves very high recovery factors.
We believe that proving that offshore steam works, and testing technologies that enable the application of steam floods at reservoir depths greater than 3000' will make a material contribution to maximising economic oil recovery in the UKCS.