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ABSTRACT: Drilling around salt structures means coping with a wide range of stress and pore pressure conditions, sometimes over a relatively short vertical distance. Zones with exceptionally low shmin, with open fractures, with overpressured fluids, or with exceptionally high sHMAX can be encountered. It is often possible to make semi-quantitative predictions of stress orientations and magnitudes based on the geological history of the salt structure emplacement, the general tectonic regime, and the displacement history of the sediments around the structures. For example, in the case of a diapiric structure that has pierced through overlying strata, the outward thrust placed on the sediments surrounding the dome shaft imprints the region surrounding the shaft with a highly compressive radial stress and a low tangential stress. These stress regimes not only affect drilling strategies and tactics around salt structures, they also affect completion approaches involving perforation placement, hydraulic fracture design, and horizontal well placement. 1. INTRODUCTION Salt structures in extensional sedimentary basins are associated with large hydrocarbon deposits. Traps may be found in the anticlinal structures and normal fault blocks above piercement and non-piercing salt domes, in updip traps in upwarped and occasionally overturned strata that terminate against the salt dome flank in the piercement region, in gentle flank anticlines, or under salt tongues (Figs 1 & 2). Accessing these resources presents problems including massive lost circulation, fractured shale sloughing, serious gas cutting of mud, and so on. These problems are related to current stress state, gas migration, and rock properties and fabric alteration arising from large deformations. Issues during salt drilling are discussed elsewhere [1]. Initial drilling of an anticlinal structure above a Gulf of Guinea salt dome in the 1990's resulted in 92 lost drilling days because of an exceptionally low shmin value, a MW window<0.05 density units, and (available in full paper) Figure 1: Shallow Salt Dome with Trap Locations massive lost circulation that re-initiated at each attempt to continue advancing. Similar though less severe lost-time cases are reported wherever extensive drilling takes place around salt structures, and the magnitude of the problems are larger the younger and less consolidated the sediments. Salt dome growth is complex; structures may grow and punch through sediments (piercement), others develop while sediments are emplaced around them, others deform strata above so that thinned anticlines form, and so on. Even in "simple" stress conditions (e.g. pure extensional regimes for millions of years), salt ridges, domes, and isolated salt pods now unconnected to underlying sources of salt can form. (available in full paper) Figure 2: Drilling Beneath Salt Tongues in Deep Water In listric fault conditions such as the Gulf of Mexico (GoM), offshore equatorial West Africa and eastern South America, salt tongues linked to deep salt deposits through a "stock" can form (Fig 2), much like laccolith structures observed in igneous rocks. These structures have recently become exploration targets because of better sub-salt seismic imaging. In thick sedimentary salt beds, mobilization can be observed at all stages of deformation, such as in the Kungurian Salt in the Pre-Caspian Basin.
- Europe (1.00)
- North America > United States > Louisiana (0.48)
- Geology > Structural Geology > Tectonics > Salt Tectonics (1.00)
- Geology > Structural Geology > Tectonics > Compressional Tectonics > Fold and Thrust Belt (1.00)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock > Mudrock > Shale (0.38)
- North America > United States > Colorado > Raton Basin (0.99)
- Asia > Kazakhstan > Atyrau Oblast > Caspian Sea > Precaspian Basin > Kashagan Field (0.98)
- North America > Cuba > Gulf of Mexico (0.89)
- (5 more...)
ABSTRACT: Drilling through salt sections requires that the particular properties of salt, its creep behaviour and high solubility, be recognized and incorporated in the drilling plan. Salt is a viscous material and creeps under differential stress; the creep rate is a strong function of both temperature and stress difference (actually underbalance between the mud pressure and the vertical stress). A simple model approach to account for these effects in a reasonably quantitative manner is described. Problems encountered in drilling through salt include hole closure leading to stuck tools, differential dissolution of beds of carnallite, bischofite and other halides, encountering stiff and non-viscous stringers in salt strata, and exiting salt into non-salt rocks, always a challenging phase of the drilling. Strategies for successful salt drilling involve recognizing salt closure behavior, stresses, and adjusting drilling fluid density and temperature to minimize problems. Casing design issues in salt are also discussed. 1. INTRODUCTION Large oil and gas reservoirs are associated with salt structures. Domal structures in the Gulf of Mexico (GoM - Jurassic salt emplaced during the Tertiary), Williston basin (Mid US Continent Devonian age) the North Sea (Zechstein age salt emplaced in the Cretaceous), Iran (Zagros salt plugs, which in some areas outcrop), Brazilian and West African offshore basins, and other areas, provide targets for exploratory oil and gas drilling. Sub-salt resources are found in the GoM salt tongue regions, in large areas in Kasakhstan (Kashagan and Tengiz), and in other areas. These may involve drilling through as much as 1500-2500 m of salt to depths of 5-9 km. ]Drilling through salt is rapid if there are few nonsalt beds. Typical ROP of 15 to 40 m/hr means that a 1000 m section can usually be drilled in two or three days with a PDC bit. ROP is important because speed minimizes hole closure from creep. Salt is essentially impermeable, so the effect of drilling fluid density (MW) on ROP is small. MW management can be used to control closure rate while sustaining reasonable penetration rates. However, high MW carries risks of lost circulation in non-salt zones, and this risk must be properly managed through knowledge of stresses. Salt does not present as serious drilling problems as fractured shale, but there are challenges such as washouts, rapid borehole closure, mud weight control issues, and casing placement decisions. Subsalt overpressure or pressure reversion may exist, and extensive rubble or sheared zones are common underneath salt tongues or adjacent to diapirs. It may be difficult to decide where salt ends and nonsalt sediments start: salt-infilled rubble zones and salt with 30-40% non-salt shale and sand inclusions can exist within salt beds, or at the boundaries of salt structures. However, most drilling problems within salt are managed relatively easily by considering salt properties during planning and drilling. Issues arising in drilling around salt structures are discussed elsewhere [1]. Salt is found as salt tectonics structures (domes, ridges, salt tongues, pillows...) as undeformed bedded sedimentary salt, and as mixed domains, as in the GoM, PreCaspian Basin, South Atlantic margin basins (Brazil, Angola...), Canadian Scotian Shelf and the Central Graben area and more southern parts of the North Sea.
- North America > United States > Texas (0.46)
- Europe > Netherlands > North Sea (0.45)
- Europe > Denmark > North Sea (0.45)
- (5 more...)
- Phanerozoic > Paleozoic (0.48)
- Phanerozoic > Mesozoic (0.34)
- Geology > Mineral > Halide > Halite (1.00)
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Geology > Structural Geology > Tectonics > Salt Tectonics (0.86)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock > Mudrock > Shale (0.45)
- North America > United States > South Dakota > Williston Basin (0.99)
- North America > United States > North Dakota > Williston Basin (0.99)
- North America > United States > Montana > Williston Basin (0.99)
- (30 more...)
ABSTRACT: Based on research and model development, including a new rock strength model that considers the effects of both chemical reactions and capillarity changes, a new pressure model that calculates fluid pressure variations with water saturation, an improved nonlinearity model for rock deformation modulus, and a coupled analytical elastoplastic model for stress estimation, the mechanisms for sand instability have been identified, quantified, and compared to address the question of why sand often fails after water breakthrough in an oil well. Model calculations indicate that, with increase of water saturation, sands tend to become weaker (strength reduction) and softer (stiffness reduction) while the loading stresses are elevated and the maximum shear stress moves outward into the reservoir (i.e. a larger zone is affected). For the case discussed, losses of both rock strength and modulus can be up to 80%, while the shear stresses can double because of fluid relative permeability changes and strength loss. Furthermore, after shear failure the sands are more easily detached from the rock matrix because of a decrease in tensile capillary strength with an increase of water saturation. Since the capillary strength is shown to depend only on water saturation, the sanding rate for each value of saturation is constant until destabilizing forces are changed, which leads to so-called episodic sand production after an oil well starts to produce water. These analytical tools can serve as a basis to develop more useful sand stability tools for multiphase fluid flow environments. 1. INTRODUCTION Sands become unstable and start to flow after water intrusion even though no preceding sand production was observed [3,4], and massive sand production occurs when Sw reaches a particular value [6]; For sanding wells, the average sanding rate during water breakthrough is higher than before breakthrough [2]; The critical global pressure gradient that activates sanding drops when Sw increases [5]; and, Sanding appears as an episodic phenomenon: at a given Sw, a sand cavity starts to grow and then becomes stabilized; additional cavity growth episodes require either an increase of pressure gradient or a change in the water saturation value [5,6]. It is estimated that, on average, companies produce three barrels of water for each barrel of oil [1], seventy percent of which comes from weakly consolidated or unconsolidated sandstone. The intrusion of formation water into water-wetted but oil-saturated sand, which is the usual case in oil fields, may trigger or worsen the sand instability that has been frequently observed both in the field [2, 3] and in the laboratory [4-6]. Some characteristics of water-related sand production are: Chemical reactions between water and solids and the dissolution of cementitious materials may weaken the rock; Changes in the surface tension and capillary force may lower the cohesive strength; Extensive experiments have been carried out to study the effect of changes in Sw (or moisture content, humidity, etc.) on different rock samples, such as shale [7, 8], chalk [9-11], and sandstone [2, 5, 12-15]. The various possible mechanisms may be generalized as follows [16]:
- Europe (1.00)
- North America > United States > Texas > Kleberg County (0.24)
- North America > United States > Texas > Chambers County (0.24)
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock > Sandstone (0.46)