Thermal Oil Recovery With Horizontal Wells (includes associated papers 24403 and 24957 )
- S.D. Joshi (Joshi Technologies Intl. Inc.)
- Document ID
- Society of Petroleum Engineers
- Journal of Petroleum Technology
- Publication Date
- November 1991
- Document Type
- Journal Paper
- 1,302 - 1,304
- 1991. Society of Petroleum Engineers
- 5.4.1 Waterflooding, 5.3.9 Steam Assisted Gravity Drainage, 5.4.6 Thermal Methods, 5.2.1 Phase Behavior and PVT Measurements, 4.3.4 Scale, 5.1.1 Exploration, Development, Structural Geology, 5.7.2 Recovery Factors, 1.6 Drilling Operations, 7.1.9 Project Economic Analysis, 7.1.10 Field Economic Analysis, 5.8.5 Oil Sand, Oil Shale, Bitumen
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"A 2,000- to 4,000-ft-long horizontal well can replace several vertical wells, reducing investment and operational costs."
Horizontal-well technology is now being used in EOR projects, especially in thermal oil recovery. Although horizontal wells have recently been used in miscible and water-flood projects, the main EOR applications to date are in steam projects. Several horizontal wells have been drilled as pilot projects for thermal oil recovery in bitumen and heavy-oil reservoirs. A few field pilots show commercial viability. Thermal oil recovery is used to produce heavy oil or bitumen in high-permeability reservoirs. The main advantages of the use of horizontal wells are improved sweep efficiency, enhanced producible reserves, increased steam injectivity, and a decrease in the number of wells required for field development. This last point is especially important in thermal oil recovery projects, where several closely spaced vertical wells are required for economic development. A 2,000- to 4,000-ft-long horizontal well can replace several vertical wells, reducing investment and operational costs. The main disadvantage of horizontal wells is their initial cost. Moreover, horizontal wells are commonly completed in only one zone at a time, resulting in drainage of only one layer. Horizontal wells have been completed in multiple zones, although not yet in thermal oil recovery applications. In bitumen reservoirs, attaining reasonable production rates without steaming is difficult because of the high oil viscosity. (Bitumen is oil with less than 10API gravity and a viscosity of more than 10,000 cp at reservoir conditions.) Steam injectivity also can be a problem. Even with horizontal wells, initial steam cycling normally will be necessary to establish steam injectivity. In contrast, in heavy-oil reservoirs, wells can be produced at low rates even without steam injection. (Heavy oil has a gravity of more than 10API and a viscosity of less than 10,000 cp at reservoir conditions.) Here, steam injection with horizontal wells is primarily to enhance production rates, to improve sweep efficiency, and to increase ultimate oil recovery.
Injectors or Producers
Horizontal wells can be used as injectors or producers. The large contact area of horizontal wells can enhance injection rates in injection wells and improve production rates in producing wells. When horizontal wells are used as producers, they provide a large producing capacity with high production rates. Hence, high steam injection rates in surrounding injection wells are necessary to maintain reservoir pressure. When horizontal wells are used as injectors, then injecting steam uniformly along the well length occasionally may be difficult, espeially during the initial steam-injection period. This is because steam condensation in a cold horizontal well reduces the well length available for steam injection. This problem can be reduced or eliminated by preheating a long horizontal wellbore before steam injection. In field applications, excluding cyclic steam stimulation, horizontal wells are used more often as producers than as injectors. In horizontal producers, at least initially before the heat front arrives and the entire wellbore is heated, only cold heavy oil may be produced. The pressure drop in the long wellbore can be significant because of the high viscosity of the cold oil, which may cause uneven production along the well length.
Recovery Process Concepts
Horizontal wells provide flexibility in developing well patterns for enhancing the weep efficiency of thermal oil recovery. It possible to drill a long horizontal well, or to re-enter an existing vertical well and drill short drainholes 100 to 300 ft long) to enhance steam injectivity and/or oil productivity. This procedure can be espeially useful in multilayered reservoirs where a specific zone can be developed with a minimum number of vertical wells. In multilayered heavy-oil reservoirs, such is those in South America, production in some zones is delayed because of the unavailability of producing wells. In these multilayered reservoirs, the number of wells required to drain several zones simultaneously is too large.
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