Tarmats is a topic that has not been studied very often, leading to a lack of published literature on the subject. The limited literature can very well be explained by the concentration of this problem in the Middle East.
One of the big contributors to the tarmat literature is Moor who studied tarmat presence, distribution and nature, as well as asphaltic sands and bitumens in reservoirs. He found four different organisms that contribute to the tarmats’ formation (Moor 1984).
(1)
Water Washing:
The removal of a portion of light hydrocarbons, and allowing the asphaltic fraction to locate itself at the foundation of oil accumulation.
(2)
Gravity Segregation:
In this procedure the resistance attracts the heavier hydrocarbons towards the foundation, and the lighter hydrocarbons move upwards.
(3)
Natural Deasphalting:
The entrance of natural gases from source rock and their rise through the hydrocarbon column due to buoyancy. Such an action would result in a lower solubility and case the asphaltic fraction to precipitate and rest at the foundation of the reservoir.
(4)
Biodegradation:
Meteoric water moves beneath the pooled reservoir, along transmitting bacteria use to metabolize crude oil’s lighter fraction. Thermal currents located in the reservoir would distribute lighter fraction to the oil/water located at the base where the bacteria is active. As a result, the formation of a tarmat is witnesses near the foundation of the reservoir.
Moor’s research extends to other areas, such as the five different groups of subsurface tar seal occurrences due to the level of concentration, continuation, and the structural position. The distribution of hydrocarbon within entire basis or individual traps is controlled by Tar seals associated with unconformities. Additionally, the tar seal that do occur at the unconformities are categorized in five different groups (Figure 2.1).
(i) Tar seals with four-way closure located above traps.
(ii) Tar seals located alongside the margins of overly matured basins
(iii) Oil first trapped by tar seals and then reallocated through basin deformation.
(iv) Trapped oil by tar seals and deeper structures.
(v) Tar seals advantageously traps the oil.
Those reservoirs which have many levels of these characteristics are known as tarmat reservoirs. Such type of reservoir is come across the World mainly in Middle East (Moor 1984).
Figure 2.1 Tar seal Classification (Moor 1984)
Abdul Aziz Al-Kaabi et al tell that there are so many searches done in WOR and oil recovery and many shapes of layers of tar are observed physically and also numerically in order to study the behaviour and working of WOR and recovery of oil. From all these researches four cases were found which are studied as square barrier beneath the well a disk beneath the well, a hollow square or disk beneath the well, and a half plane. The research conducted on these four cases shows that in hollow tarmat barrier case, the breakthrough time comes earlier, and if we consider the disk beneath the well case breakthrough time is delayed as well because WOR shoots very rapidly. No-barrier case got the highest recovery from all the cases discussed. And hollow tarmat barrier got the least recovery. Many of the major oil reservoirs in Middle East have the issue of tar barrier of oil zone and the underlying water zone, which have a very strong bottom water drive. Many investigations are done which can be used for increasing oil recovery from such type of reservoirs. There is no work published on this issue in Iraq, Kuwait and Saudi Arabia. Many models and schemes are made; initially three zones were set in the models namely oil zone which is at the top, water zone which is placed in the middle and tar zone which is placed at the bottom. The oil and tar zone have the thickness which is varied in order to fulfil the variety of conditions. And the water zone is protected with water drive. There are many different types of techniques which is done named internal water flood with bottom water drive, internal water flood without bottom water drive, injection of solvent, injection of steam into (a) water zone, (b)oil zone, (c)tar zone (Abdul Aziz Al-Kaabi et al 1988)
In Venezuela and North America many literatures were published on recovery of oil. Tarmats are introduced in reservoirs in Kuwait, South Iraq and Qatar. In Saudi Arabia huge accumulations of tar are reported in fields named Manifa, Khursaniyah and in many others fields. In Ghawar the tar zone exceeds from more than 15 miles and in Uthmaniya tar zone goes up to 500 ft with respect to its thickness ( Abdul Aziz Al-Kaabi et al 1988).
Osman during 1985, published a study regarding Minagish field located in Kuwait. The case of Minagish field in Kuwait represented a very typical case of tarmat reservoirs in which tar is in
cluded at the contact of oil-water and usually has a thickness that ranges between 30 feet and 115feet. In Figure 2.2 presents the average rock properties and the structural cross-section of the MN-26 injector showing the tarmat (Osman 1985).
Initially, the Minagish field was supposed to have water flooding below the tarmat. This was also the reason, whey the discussion of a possible tarmat breakdown due to the waterflood below the tar zone. Figure 2.3 demonstrate the graphical method that Osman used in order to predict the different pressure rates at the tarmats depending on the injection rate and time. In comparison, Figure 2.4 represents the curves of differential pressure of the water that was injected versus injection time depending on the distance of the injector. Osman’s study overall was fascinating; however one of the most important discoveries was that water injection was the main effect on differential pressure across tarmats, than the oil production. Lastly, Osman recommended a way of finding the response time at the well that can be observed, and allow for time to complete the switch injection from below to above the tarmat. (Osman 1985).
Regardless, of all the quantitative results that Osman presents, his model is very simplistic to represent the such a complicated problem. Osman made a few assumptions that were questionable, such as:
1) The consideration of a tarmat as a rigid barrier breaking at 15psi/foot as a pressure gradient.
2) The increase in pressure due to water injection is preeminent, while the decrease in pressure because of oil production is insignificant.
3) The way he applied the superposition theory is uncertain in this study at least.
4) Osman fails to mention the rheology and the characteristics of the tar.
5) Lastly, he fails to provide and discuss the geometric description of the tarmat that was broken.
An extension of Osman’s work examines the results from having a sealing fault close to the water injection and the influence of the sealing fault on the behaviour of the tarmat. This above mentioned study resulted in a technique that was able to calculate the time of the tarmat break down, what the response time was at the nearest well, and lastly the differential pressure at the tarmat located anywhere in the reservoir (Osman 1986).