The below somewhat dated article on in situ combustion (ISC) makes interesting reading for PBG stock holders:
1. Back in 1982 11 of 18 Alberta oil sands projects were using vertical ISC with disappointing results. Perhaps this experience taints the memories of oil companies thinking of using THAI.
2. THAI could be applicable to North Sea medium to heavy oil.
3. Today there are 16 successful air injection projects in operation worldwide.
4. There exists a worldwide body of experience in oil companies on how to implement ISC.
BELOW IS THE ARTICLE
World Perspective Heavy oil is a vast hydrocarbon resource, with 5 trillion barrels to be exploited worldwide, especially in North and South America, and with some 10 billion barrels in the North Sea. The conclusion of Meyer and Schenk of the US Geological Survey, in 1988, that: “heavy hydrocarbons – certainly those heavier than 16° or 17° gravity API – can never compete with lower molecular weight hydrocarbons in terms of production capacity and net energy yield”, still essentially holds true (1).
However, after nearly 20 years, the amount of heavy oil resource has hardly changed, since less than 1 per cent of it has been recovered. Certainly, the special technologies needed will be tried in onshore locations first. No real attempt is likely to be made to recover the heavier oil in the North Sea, (Mariner B -15°API and Bressay 11-12° API) until more advanced, proven techniques become available.
Currently, the main dynamism and technology thrust is in the Oil Sands region of Alberta in Canada – and likely to remain so for the foreseeable future. This area contains more than 1 trilllion barrels of bitumen, which makes the North Sea resource look quite tiny by comparison. About 80 per cent of UKCS heavy oil is medium heavy (2). More advanced technologies will eventually be applied, because of the large amount of oil left unrecovered after waterflooding. One thing, however, which is not on the UK’s side is time. The UKCS oil offshore infrastructure is ageing rapidly, so that there may only be another10 to 15 years left to exploit this remaining oil. The ‘time window’ to do this could be extended by 10 to 15 years, if we target much higher recoveries, nearer 80% of the original oil-in-place. In situ heavy oil technologies, such as SAGD (steam assisted gravity drainage) are already approaching 50% recovery. The new THAI process (Toe-to-Heel Air Injection) could possibly increase this to 80%. The additional 5 billion barrels of extra heavy oil reserves to be gained would extend the operating horizon by 10 years, providing energy security and time to adjust to new scenarios.
Why THAI?In 1982, eleven out of 18 heavy oil projects in the Lloydminster area of Alberta, Canada were firefloods - in situ combustion (ISC). ISC incorporates all of the main displacement mechanisms found in other enhanced oil recovery schemes: steam, hot water, immiscible gas, and miscible solvent drives, and was then, and is still, considered by many to be the ‘best’ process.
However, despite more than 130 pilot and development projects during the 1970s and 80s, ISC fell-out of favour. This is partly explained by the schematic in Figure 1. Conventional ISC is operated as a long-distance displacement process, using vertical wells. Although the interwell distances are generally not very large, 100-200m, compared with the kilometre distances used in North Sea oil fields, the process usually suffers from severe gas override and advective instabilities, unless the reservoir geology is especially favourable. Premature gas breakthrough can occur, leading to an early cessation of operation, as occurred in some of the earlier projects. This ‘bad experience’ has tended to blight the image of ISC as a viable process. In conventional, long-distance processses (Figure 1), fluids tend to bank-up downstream of the displacement front. For the ISC process, this can reduce the gas permeability, leading to the worst possible scenario: a reduction in air injectivity and eventual loss of vigorous combustion activity, causing the process to become locked in a low temperature oxidation (LTO) mode, from which it is almost impossible to recover.
Moore et al (3) indicate that the key to success in operating an ISC process is to operate it in a high temperature oxidation (HTO) mode, in order to sustain vigorous, propagating combustion. THAI – Toe-to Heel Air Injection, is able to achieve this because it operates as a short-distance displacement process (Figure1). Thus, the mobilised oil only has to be displaced a few metres into the horizontal producer below, rather than 100s of metres in conventional long-distance displacement.
Figure 1: Short-Distance Displacement vs Long-Distance Displacement
Figure 2 is general schematic showing the main features of the THAI process. After startup and ignition, the communication pathway for injection gas (air) is established via the combustion front, and all of the mobilised fluids and combustion gas are produced into the open-section of the horizontal producer well. The downstream, cold oil layer, remains in this state until it is affected by the leading edge of the combustion front-mobile oil zone (MOZ). This special characteristic of the THAI process enables it to operate in a steady production manner, since fluid saturations ahead of the combustion front remain at their original reservoir condition.
Figure 2: Schematic of Steady-State Operation of THAI
THAI also has its own in-built ‘guidance system’, by virtue of the horizontal producer well acting as a ‘pressure sink’, in the direction of the vertical production riser. Overall stability of the process is governed by three effects (see Fig. 2 schematic):
Controlled gas override due to pressure draw-down into the horizontal well,
Gravity drainage, and
The formation of a heavy residue or coke plug in the horizontal well
The latter is a dynamic process, whereby heavy crude draining into the horizontal well ahead of the approaching combustion zone is pyrolised. The temperature in the horizontal well can, however, be controlled by injecting water (steam), if needed. The ‘gas seal’ mechanism diverts gas communication from the ‘toe’, during startup, directly to the combustion front, and there is subsequently no danger of oxygen bypassing into the horizontal well (4).
THAI is currently in the startup phase of a first field pilot, which is being conducted by Whitesands In Situ Ltd (Heavy Oil Division of Petrobank Ltd), at Christina Lake, Alberta, Canada. Some of the main potential benefits of the process are listed below:
Stable, robust process, due to controlled gas overriding and formation of a ‘gas seal’ in the horizontal producer well.
Unlimited availability of injection fluid (air), and no ‘capture and transport,’ costs. ‘Flue gas/CO2 produced is available for other large-scale light oil and heavy oil IOR projects, with eventual sequestration.
High sweep efficiency leading to high oil recovery, up to 80 % of oil-in-place.
Substantial in situ upgrading produces lighter oil – preserved via short-distance displacement operation.
High air injectivity maintains process in HTO, vigorous combustion mode.
Reduced sulphur (>30%) and heavy metals (>90%).
Reduced number of wells for repeated line-drive operation.
Applicability to North Sea?In addition to proving THAI technology onshore, the big question mark is whether the short interwell spacing is appropriate for North Sea conditions because of the much larger cost of drilling wells offshore. If we consider three direct line-drive sections (Figure 3), similar to the Christina Lake THAI pilot, a rough idea can be gained of the economic potential.
Figure 3: North Sea THAI Development for Medium/Heavy Oil
Well costs for 1 new horizontal producer well (HP) with two horizontal well extensions (2x HP) and 1 new vertical injector well (VI), are $125MM. Over 10 years, charging interest and depreciation at $18.75MM/yr - not including royalties and taxation, using an interwell spacing S = 100 m, a production rate per well pair of 100 m3/day, an operating cost of $8/bbl and a crude oil price of $50/barrel:
Revenue = 300 x 7 x ($50 –$8) x 340 = $30 MM/yr
Profit = 30 – 18.75 = $11.25 MM/year
Thus, there appears to be scope applying the THAI process in the North Sea, although considerable optimisation is needed before we can say that it will be applied. It is also worth noting, that air injection technology has been around for 40 years, so a wide body of experience exists in oil companies on how to implement ISC projects.
There are currently sixteen successful air injection projects in operation worldwide, split between deep light oil and heavy oil. The lower viscosities of North Sea medium-heavy oils may allow wider interwell spacings and possibly higher productivities than currently envisaged for Athabasca oil sand.
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