Internal Blowouts, Cratering, Casing Setting Depths, and the Location of Subsurface Safety Valves
- J.V. Walters (Shell Intl. Petroleum Mij. B.V.)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling Engineering
- Publication Date
- December 1991
- Document Type
- Journal Paper
- 285 - 292
- 1991. Society of Petroleum Engineers
- 1.14 Casing and Cementing, 1.6 Drilling Operations, 1.11.2 Drilling Fluid Selection and Formulation (Chemistry, Properties), 1.6.6 Directional Drilling, 1.6.10 Running and Setting Casing, 1.7.5 Well Control, 1.6.1 Drilling Operation Management, 1.7 Pressure Management, 4.1.2 Separation and Treating, 2.4.3 Sand/Solids Control, 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation, 5.1.1 Exploration, Development, Structural Geology, 4.1.5 Processing Equipment, 5.1.5 Geologic Modeling, 3 Production and Well Operations, 5.1.2 Faults and Fracture Characterisation, 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc)
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This paper promotes industry awareness of the potential mechanisms that may take place in the rare event of a loss of well control and how these mechanisms may be assessed for field-specific conditions. This is particularly important when exploration/appraisal drilling involves penetrating highly overpressured accumulations. Depending on the penetrating highly overpressured accumulations. Depending on the geological burial history, these over-pressures may be located in extremely competent or weakly consolidated formations, even at great depth. In-situ stress and rock-strength characteristics will play a major role with respect to the chances of reaching these target horizons successfully.
One of the most hazardous situations encountered during the drilling or completion phases of oil and gas wells is a loss of well control after an influx of highly overpressured formation fluids. This may arise through any one of the following events: (1) the sudden encounter of a highly overpressured interval, giving a significant well kick that causes fracturing of the openhole section, probably close to the last casing shoe, which may lead to significant mud losses to the fracture, further influx of formation fluids, and uncontrolled flow; (2) swabbing in the well when tripping out drill-string, allowing influx of formation fluids; or (3) fracturing of the openhole section by too high mud weights or by mud pressure surges when running in too fast (drillstring or casing). pressure surges when running in too fast (drillstring or casing). The unsuccessful control of a kick, with further influx of formation fluids, may lead to an external or internal blowout. External blowouts are clearly seen and nearly always result in significant rig damage apart from the risk of fatal injuries to personnel. The behavior of internal blowouts is much more difficult to predict and can range from containment of the overpressured fluids against a sealing layer located close to the blowout point to a migration of high-pressure fluids toward the surface (uncontained internal blowouts) with the potential risk of crater formation by near-surface sediment liquefaction. If liquefaction occurs directly below a rig site, rig loss is a possibility because of a loss of the soil load-bearing capacity.
Internal blowouts are the uncontrolled flow of high-pressure fluids (usually hydrocarbons originally contained in an interval underlying a sealing caprock) along an openhole section and into lower-pressured permeable intervals that become charged and pressurized with hydrocarbons. permeable intervals that become charged and pressurized with hydrocarbons. The key is to determine whether the hydrocarbons are likely to remain contained within the shallower interval or will continue to move vertically (for example, by rock fracture or the wedging open of fault planes). They may then reach near-surface unconsolidated horizons that can be liquefied by the high hydrocarbon pressures, resulting in crater formation. An example of an onshore uncontained internal blowout is shown in Fig. 1. The crater formed immediately below the drilling because of soil liquefaction, resulting in total rig loss beneath the ground surface. The mechanisms that may follow an internal blowout will be governed by a number of site-specific parameters: hydrocarbon fluid pressure at the blowout depth; hydrocarbon density; length of the openhole section; in-situ-soft values and the orientation of the minimum stress; fracture propagation pressure of an induced hydraulic fracture; location of fault propagation pressure of an induced hydraulic fracture; location of fault planes and sealing vs. nonsealing faults; rock-strength characteristics at planes and sealing vs. nonsealing faults; rock-strength characteristics at each lithological horizon above the blowout location; consolidated intervals; and unconsolidated intervals.
Some of these parameters can be obtained from drilling and field records. 1. The location of the consolidated or unconsolidated formations can be determined from drilling rates, the cuttings observed at surface, and logs. 2. The hydrocarbon pressure can be assessed from the mud weights used at the time of the initial kick and the magnitude of the kick. 3. Mud losses occurring because of fracturing of the openhole section give information on the fracture propagation pressure. Other parameters may be found from the local field setting and will be further discussed in the next sections. Collectively, the above parameters can be used to determine whether significant vertical movement parameters can be used to determine whether significant vertical movement of high-pressure hydrocarbons is likely to occur.
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