Layer | Fill | Outline |
---|
Map layers
Theme | Visible | Selectable | Appearance | Zoom Range (now: 0) |
---|
Fill | Stroke |
---|---|
Collaborating Authors
Results
American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc. Abstract The type of fracture (i.e., horizontal, vertical or inclined) obtained in a fracturing process is often determined by instruments such process is often determined by instruments such as Borehole Televiewers, impression packers, etc. Such instruments only determine the fracture appearance at the wellbore and give no information about the fracture geometry away from the wellbore. At moderate depths (less than 15,000 ft), a fracture initiates when the maximum tensile stress induced at any point around the wellbore exceeds the tensile strength of the formation at that point. In this paper it will be shown that, theoretically, if none of the principal stresses are parallel to the borehole, the maximum tensile stress is attained at two diametrically opposite points along the circular periphery of the borehole. In three dimensions, periphery of the borehole. In three dimensions, the loci of these points are two straight lines parallel to the directrix of the borehole. parallel to the directrix of the borehole. If the smallest compressive principal stress is not the one which makes the least angle with the borehole, then these lines are likely directions for fracture propagation at the wellbore. Once the fracture extends sufficiently away from the borehole it becomes perpendicular to the least compressive principal perpendicular to the least compressive principal stress. At the wellbore this fracture may appear as vertical. The theoretical findings of the paper are verified experimentally. The paper also includes a discussion of the effects of these findings on fracture geometry evaluations. Introduction For over two decades the petroleum industry has been using hydraulic fracturing for well stimulation. The first comprehensive analysis of the mechanics of hydraulic fracturing was made by Hubbert and Willis. Assuming the wellbore to be parallel to the vertical principal stress, they calculated the stress principal stress, they calculated the stress distribution around the borehole. It was suggested that a fracture is initiated when the maximum tensile stress induced around the wellbore exceeds the tensile strength of the rock and that it extends in a plane perpendicular to the least in-situ compressive perpendicular to the least in-situ compressive principal stress. Scheidegger was first to principal stress. Scheidegger was first to consider the effect of fluid penetration on fracture initiation. He also modified the Hubbert and Willis equations by allowing the tensile strength of the rock to have a nonzero value. Kehle has analyzed the effects of the packers on stress distribution and fracture initiation. The mechanics of hydraulic fracturing in a porous permeable rock has been discussed in a porous permeable rock has been discussed in a series of articles by Haimson, Fairhurst and Haimson and Fairhurst. The stress distribution for the case when the borehole is not parallel to one of the in-situ principal stresses has been calculated by Fairhurst in an isotropic or transversely isotropic medium. Haimson conducted an extensive series of tests in the laboratory in order to investigate fracture initiation and orientation. He found that induced fractures are initiated when the maximum tensile stress exceeds the tensile strength of the formation and are oriented normal to the least compressive principal stress, as suggested by Hubbert and Willis. In Haimson's experiments the borehole was parallel to one of the principal stresses. A number of experiments have been conducted in the field for determining the fracture direction. This has been achieved by using instruments such as impression packers (Anderson and Stahl, Fraser and Pettit) or Borehole Televiewers (Zemanek et al.), etc.
- Geophysics > Seismic Surveying > Borehole Seismic Surveying (0.44)
- Geophysics > Borehole Geophysics (0.44)
- Well Completion > Hydraulic Fracturing (1.00)
- Reservoir Description and Dynamics > Reservoir Characterization > Reservoir geomechanics (0.94)
- Well Drilling > Wellbore Design > Wellbore integrity (0.68)
American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc. Abstract A number of methods are used in the industry to detect and evaluate overpressured formations. The ability to detect and recognize overpressured formation is critical in conducting efficient and safe drilling operations. Observed changes in the properties of rocks, especially shales, can be used to evaluate the overpressured zones. Variations of rock properties can be detected by geophysical methods, wire-line logging techniques, surface measurements on the drilling mud and shale cuttings, and monitoring of several drilling parameters. The best approach for the detection and evaluation of overpressured formations is the study of a combination of several measured parameters since relying on one type of data parameters since relying on one type of data can result in misinterpretations. Introduction In the worldwide search for oil and gas, abnormally high pressure zones (geopressures) have been encountered in numerous countries on several continents. In our present discussion such geopressures, or abnormally high subsurface pressures, are defined as any pressure which exceeds the hydrostatic pressure of a column of water containing approximately 80,000 ppm total solids. ppm total solids. For the oil industry, occurrence of geopressures are important in many respects.Worldwide experience indicates a significant correlation between the presence and magnitude of geopressures and the shales and ratio of sedimentary sections penetrated by the drill bit. In some areas, the distribution of oil and gas appears to be related to the regional and local pressure environments. Knowledge of the expected pore pressure and fracture gradient is the basis pressure and fracture gradient is the basis to make the best of modern drilling techniques, i.e., efficiently drilling wells with correct mud weights and proper casing programs. This also prevents a breakdown of exposed formations and contains the high-pressure fluids in deeper formations, thereby reducing blowout hazards. Much of the extra cost in the search for and development of hydrocarbon reserves is for drilling fluid and casing programs. An additional, quite expensive item is the properly selected completion method which properly selected completion method which must be effective, safe and allow for killing of the well. Here, too, reliable pore pressure and fracture gradient data are a prerequisite. Last but not least, geopressures are also an important factor in several aspects of reservoir engineering. This includes consideration of compressibility and failure of the reservoir rock and the possibility of water influx from the overlying and underlying shale formations. For years the drilling industry dreamed of a method to predict and evaluate geopressures. Today, a number of methods, both surface and subsurface techniques, have become available for geopressures detection and measurement. These overpressured intervals and as much prior knowledge of them before drilling as is possible - are critical to conducting efficient and safe drilling operations.
- Europe > Norway > Norwegian Sea (0.44)
- North America > United States > Texas (0.28)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock > Mudrock > Shale (1.00)
- Geology > Geological Subdiscipline (1.00)
- North America > United States > Texas > Permian Basin > Yeso Formation (0.99)
- North America > United States > Texas > Permian Basin > Yates Formation (0.99)
- North America > United States > Texas > Permian Basin > Wolfcamp Formation (0.99)
- (21 more...)
- Well Drilling > Drilling Operations (1.00)
- Reservoir Description and Dynamics > Reservoir Characterization (1.00)
- Reservoir Description and Dynamics > Formation Evaluation & Management > Open hole/cased hole log analysis (1.00)
- Well Drilling > Drilling Fluids and Materials > Drilling fluid selection and formulation (chemistry, properties) (0.89)
American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc. Abstract The Hopkinson split bar has been used for about 20 years to study properties of rock and rock failure under dynamic loading conditions. The mathematical analysis needed to construct stress-strain curves from the measured waves requires that the rock specimen be very short compared to the wave length of the impact generated wave. In practice, specimens of several different lengths are often used with the same length wave, always making the assumption that the specimen is "short" enough for the analysis to apply. This paper describes the results of a study on the effect of the ratio of specimen length to wave length on the stress-strain curves derived from strain wave measurements. A computer simulation of the Hopkinson split bar has been made in which a rock having known elastic properties (ER, rho R) and given length is assumed to be placed between two steel bars also of known properties (ES, rho R). An incident wave of given shape and wave length is assumed to travel toward the steel-rock interface. Using plane wave assumptions, it is possible to compute the shapes of the reflected possible to compute the shapes of the reflected and transmitted waves. These are then used as if they were experimental results to compute the stress-strain relationship for the rock. This is then compared with the specified value. It is found that the slope and shape of the calculated curve for a given incident wave length is very dependent upon the specimen length. Similarly, the method of superposition of the measured waves is very critical. It is suggested that apparent rate of loading effects observed by some authors in comparing "static" and "dynamic" (Hopkinson bar) stress-strain curves may be a result of the analysis rather than a true rock behavior. A Hopkinson split bar was constructed and used to test four rock types. When the measured strain waves were analyzed using the techniques suggested from the computer simulation, it was found that the resulting stress-strain curves agreed well with the static curves. Introduction A knowledge of the dynamic properties of rock is important, for the development of improved rock fragmentation methods (percussive drilling, rock blasting, etc.) as well as the improved design of structures constructed in or on rock subjected to dynamic loading. Although several methods have been used to obtain dynamic rock properties, the Hopkinson split bar is perhaps the simplest for high stress level studies. This device, shown diagrammatically in Fig. 1, consists basically of two cylindrical steel bars to which strain gauges have been bonded, the sample which is sandwiched between the bars, and the loading system. In many laboratories including that of the present authors, the loading system consists of a striker that is accelerated down a long tube under the action of compressed air and impacts the end of the first steel bar. Quite a range of loading rates can be produced by merely changing striker length and controlling the velocity at impact. As the impact-produced strain wave,, reaches the sample, part of the wave is reflected and back part of the wave is reflected and back into the first bar and part is transmitted into the second steel bar. From measurements of the incident, reflected and transmitted waves, and assuming plane wave theory to apply, one can construct stress-strain curves for the rock sample. A comparison of these "derived" dynamic curves with those obtained by static methods can then be made.
- Geology > Rock Type (0.69)
- Geology > Geological Subdiscipline > Geomechanics (0.68)
- Geology > Mineral > Silicate (0.47)
- Well Drilling (1.00)
- Reservoir Description and Dynamics > Reservoir Characterization > Reservoir geomechanics (0.54)
Abstract Rock strength and mode of failure are functions of the rock pore pressures. Consequently, drilling rates should be relatable to the pore pressure and might be used to map the formation pressure profile while drilling. This paper summarizes a literature survey which was made to determine how pore pressure influences drilling response, and presents suggestions for considering these effects to minimize unnecessary well costs. Introduction Blowouts and lost circulation due to abnormal and subnormal pore pressures have plagued the drilling industry since its inception. The monetary losses are unknown but they must be astronomical considering the expensive precautions the drilling industry now takes to prevent their occurrence. Mud programs are designed and maintained to provide hydrostatic pressures in excess of assumed formation pore pressures. If the . actual pore pressures are much greater than the design assumptions, the flow of formation fluids into the wellbore is possible and a blowout may result. Conversely, if the actual pore pressure is considerably lower than the assumed pressures, drilling fluid is lost to the permeable zones. Fracturing of shallow, unprotected formations, with complete loss of returns, has occurred when using weighted muds. In addition to causing blowouts and lost circulation, formation pore pressures affect rates of penetration. Chip formation under a rotary rock bit and the hydraulic removal of these chips from their fracture craters are related to the formation pore pressure. The normal formation pressure gradient in the Gulf Coast is considered to be 0.465 psi/ft of dept)" however, pressure gradients as high as 0.85 Psi/ft have been encountered. The high pressure gradients were always encountered below zones of normal pressure gradients, and it was noted that the change in pressure gradients usually occurred across thick shale sections such as the Hackberry and the Discorbis D Shale Wedge. The changing pressure gradients in these thick, essentially homogeneous shales should produce measurable changes in the physical properties of the shale. This paper is a simulation of a literature survey which was made to determine the effects of pore pressure on the drilling characteristics of rocks, specifically shale. The effects of formation pore pressure on drilling response chip formation, and removal are considered in detail, and means of detecting changes in formation pressure gradients are postulated. BASIC ROCK MECHANICS Structural failure is induced in sedimentary rocks by applying stresses which exceed the shear strength of the rock frame. The type of failure produced and the ultimate strength of the rock are variables which are functions of the pressure conditions surrounding the rock at the time of failure. Mode of Failure Rocks, like any other solid, will deform elastically with increasing stress until the yield strength of the rock is reached; at this point the rock fails in shear. If the shearing failure is accompanied by a complete loss of cohesion along the shear plane the rock is said to have failed by brittle fracturing. When forces of adhesion existing along the shear plane after fracturing exceed the shear strength of the rock frame before failure, the mode of failure is said to be ductile. P. 73^
- Geology > Rock Type > Sedimentary Rock > Clastic Rock > Mudrock > Shale (1.00)
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Well Drilling (1.00)
- Reservoir Description and Dynamics > Reservoir Characterization > Reservoir geomechanics (1.00)
Abstract A study of the point-load breaking strength and the three-point bending strength of three rock types has been made at room [75F] and liquid nitrogen [-320F] temperatures. A factorial design experiment, using three variables at two levels, was made on a limestone rock to evaluate the effects of these variables on the mechanical properties of this rock. The factorial design offered simplicity and economy in this type of testing. The same mechanical tests were applied to basalt and charcoal granite specimens supplied by the USBM. In general, average increases in the point-load breaking strength at the low temperature level were 46 per cent for the limestone, 52 per cent for the basalt, and 21 per cent for the granite. The average modulus of rupture strength increased 59 per cent for limestone, 49 per cent for basalt and 19 per cent for granite. analysis of the data from the factorial experiment indicated that the main effect was temperature, the intermediate effect was sample preparation, and no effect was observed for the two rates of loading used. No interaction between variables was evident. A determination of Young's modulus at the two temperature levels was made on the limestone. The increase in the modulus at liquid nitrogen temperature was approximately 2.5 times that at room temperature. Introduction The advent of lunar exploration is creating interest in the physical properties of rock at very low temperatures. This interest extends not only to the various engineering and space-oriented groups but also to those with an interest in the general field of rock mechanics. Penn and Gaudin indicated that the thermal environment on the lunar surface varies from approximately -250F to +250F. Associated with the temperature effect are the additional complicating environmental factors of reduced gravity in a vacuum. Our discussion, while dealing with only room and liquid nitrogen temperatures, indicates some interesting changes in the point-load breaking strength and modulus of rupture of three rock types. A search of the literature failed to turn up any information on the behavior of rocks at very low temperatures. The most related paper is that of Monfore and Lentz who studied the effects of low temperatures on several different. concrete mixes. Their measurements included compressive strength, splitting strength, Young's modulus, Poisson's ratio, thermal contraction and freeze-thaw resistance over a temperature range from, room to -250F. Of particular interest are the effects-of temperature, the compressive and splitting strengths of concrete. In general, the various concrete samples reached a maximum compressive strength at approximately -150F and decreased with further decreasing temperature. The splitting strengths followed a similar pattern but rather sharply at the freezing joint and a maximum, at -175F, somewhat higher than that observed for compressive strength specification. The lack of data on low temperature effects on rock properties prompted this investigation. The testing program consisted of the point-load breaking strength test after -Reichmuth and the three-point bending [modulus of rupture] test. Test specimens used were limestone obtained from a quarry near Valders, Wis., and basalt and charcoal granite supplied by the Twin Cities Mining Research Center of the USBM. In addition to the above tests, which were run at both room and liquid nitrogen temperatures, Young's modulus was determined for the limestone at the two temperature levels. EXPERIMENTAL PROCEDURE Bulk samples of the limestone as obtained from the quarry were randomly cored using 1/2-in. and 1-in. diameter diamond bits. P. 189^
- Research Report > Strength High (0.74)
- Research Report > Experimental Study (0.74)
- Geology > Rock Type > Sedimentary Rock > Carbonate Rock > Limestone (1.00)
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Geology > Rock Type > Igneous Rock > Granite (0.97)
- Geology > Rock Type > Igneous Rock > Basalt (0.88)
- Well Drilling > Wellbore Design > Rock properties (0.54)
- Reservoir Description and Dynamics > Reservoir Characterization > Reservoir geomechanics (0.54)