Bottomwater Drive in Tarmat Reservoirs
- Abdul Aziz Al-Kaabi (King Fahd U. of Petroleum and Minerals) | Habib Menouar (King Fahd U. of Petroleum and Minerals) | Muhammad Ali Al-Marhoun (King Fahd U. of Petroleum and Minerals) | Hasan S. Al-Hashim (King Fahd U. of Petroleum and Minerals)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Engineering
- Publication Date
- May 1988
- Document Type
- Journal Paper
- 395 - 400
- 1988. Society of Petroleum Engineers
- 4.1.5 Processing Equipment, 5.2.1 Phase Behavior and PVT Measurements, 4.3.4 Scale, 5.2 Reservoir Fluid Dynamics, 4.1.2 Separation and Treating, 5.5 Reservoir Simulation, 5.4.1 Waterflooding, 5.1.1 Exploration, Development, Structural Geology, 6.5.2 Water use, produced water discharge and disposal
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This paper addresses the class of tarmat reservoirs subject to bottomwater drive. Different shapes of tar layers are simulated physically and numerically to study the behavior of WOR and oil recovery. Four different cases were studied: a square barrier beneath the well, a disk beneath the well, a hollow square or disk beneath the well, and a half plane. The results showed that breakthrough time occurs earlier in the case of hollow tarmat barriers, while it is delayed considerably in the case of tarmat barriers shaped in the form of a disk beneath the well. Paradoxically, in this last case, the WOR increases more rapidly and Paradoxically, in this last case, the WOR increases more rapidly and becomes higher toward the end of the depletion than in any other case. Among all the cases studied, the no-barrier case gives the highest recovery, while the hollow-tarmat-barrier case leads to the lowest recovery.
Many reservoirs in the Middle East and north Africa are characterized by a tarmat barrier at the bottom of the oil zone on top of the aquifer. This tarmat barrier is in general very thick and could be as thick as the oil column in some reservoirs. The tar viscosity is sometimes on the order of more than 1,000 times the oil viscosity. Although this tar is believed to have originated from the same source that generated the oil during the migration time, the present characteristics of the tarmat are clearly different from the characteristics of the reservoir oil. More specifically, the tarmat is so viscous that it is considered practically immobile. In general, the tar layer is considered impermeable and its extension over the aquifer is so large that it isolates the oil zone from the aquifer locally in some reservoirs. In this respect, the tar layer acts like any natural barrier and impedes the bottomwater drive from the aquifer beneath. This paper presents the effect of such tar barriers on oil recovery and WOR behavior in a tarmat reservoir. Different geometries of tar barrier have been assumed. A physical laboratory model is used to study the effect of each type of barrier. After it is verified that the physical model and the numerical simulator are in good agreement, the generalization of the results is conducted on the numerical model.
Although abundant literature on recovery of heavy oil in North America and Venezuela has been published, work done on tarmat reservoirs is very limited. Tarmats are reported in some reservoirs in south Iraq and Kuwait. They are also present in Eid El Shergi field in Qatar, where the Arab IV formation is characterized by a massive tar column more than 250 ft [76 m] thick. Also in Qatar, the Dukhan, Maydan, Mehzan, and Bulhamine fields present similar features as in Eid El Shergi. In Saudi Arabia, large accumulations of tar are also reported in Khursaniyah, Manifa, Ghawar, and some other fields. In Ghawar field, for example, the tar zone extends more than 15 miles [24 km] and in the Uthmaniya region reaches up to 500 ft [150 m] in thickness. In Libya, some of the largest oil reservoirs, such as Sarir, present similar tarmat problems. In one of the few technical papers related to tarmat reservoirs, Bashbush et al. discussed methods of waterflooding El Bundug field in the offshore area of Abu Dhabi and Qatar. In this reservoir, both geologic and reservoir studies showed that the tarmat layer was acting as a barrier to fluid movement without being a complete seal. It was also shown in this study that, because of the presence of the tar barrier, peripheral water injection leads to poor presence of the tar barrier, peripheral water injection leads to poor ultimate oil recovery, as low as 15%. In another paper related to Minagish field in Kuwait, where the tarmat thickness varies from 30 to 115 ft [9 to 35 m], Osman discussed the conditions under which the tar layer breaks to allow communication between the oil zone and the aquifer. In a recent study, Shamsaldeen and Farouq Ali tested different recovery techniques in laboratory models simulating tarmat reservoirs. They discussed the possibility of using steam and solvent to displace tar. They also examined the efficiency of different types of waterfloods and concluded that some communication between the aquifer and the oil zone is highly desirable to improve the recovery. Once communication is attained, an internal waterflood can be very effective. The common feature of all tarmat reservoirs is the isolation of the oil zone from the aquifer. In the primary stage of depletion, the oil reservoir behaves like a finite lens where the pressure decreases rapidly, leading to an alarming increase of GOR. This was observed in Minagish, El Bundug, Sarir, and other fields. The decision to start a waterflood process will be subject to where to inject the water: above, below, or within the tar zone. In any of these injection schemes, there is a strong possibility of losing part of the recoverable oil. part of the recoverable oil. In the present paper, where the tar layer is considered immobile, different geometries of the tarmat barrier are assumed. The type of geometry is inspired by either the original configuration of the tar above the aquifer or by the possible shape the tarmat layer can have after some EOR process has been used to displace the tar. In the first case, the tar may extend over part of the aquifer in a continuous manner, as in the case of Ghawar field. In the second case, it may be assumed that the tar layer presents "holes" as a result of steam or other EOR methods. The objective of this study is to examine the implications of tarmat layers with different geometries on oil recovery and WOR.
Physical Model Physical Model A 12 x 12 x 4-in. [30 x 30 x 10-cm] laboratory Lucite TM model was constructed to simulate a fraction of the tarmat reservoir with the production well at the center. A sliding packer inside the well was production well at the center. A sliding packer inside the well was used to simulate different well penetrations. The porous medium was constituted of 30-mesh glass beads, which were washed by a diluted solution of sulfuric acid and chromium hydroxide after each nin. The glass beads were then dried in a constant-temperature oven at 450deg.F [232deg.C) to ensure the same conditions of wettability for all the runs. In all displacements, the oil phase was represented by kerosene mixed with lube oil to increase its viscosity and the water phase is distilled water. The physical properties of the fluids are presented in Table 1. The experimental setup is represented schematically presented in Table 1. The experimental setup is represented schematically in Fig. 1, and the characteristics of the porous medium, which were measured in a separate linear model, are summarized in Table 2.
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