Pressure In Wells Research Articles

Stress orientations from oil-well fractures in Alberta and Texas

Many oil wells in Alberta exhibit spalling of the walls (known as breakouts), which elongates the holes with the longer axis aligned northwest–southeast. This alignment is observed over an area in excess of 4 × 105 km2, in siltstones, sandstones, carbonate sediments, and one shale formation, through the stratigraphic column from Devonian to Cretaceous. It is unrelated to dip of the beds. We have elsewhere suggested that these breakouts are produced through stress concentration near the hole walls, in a stress field having large, unequal horizontal principal stresses and with the larger compression oriented northeast–southwest. It is probable that this northeast–southwest principal stress is σ1. This paper adds new data from oil wells in Alberta and northern British Columbia and shows that the breakouts, and by inference the stress orientations, are consistent through much of the western Canadian sedimentary basin. It also uses evidence from hydraulic fracturing in west-central Alberta and from steam-injection fracturing in eastern Alberta to support the view that σ1 is aligned northeast–southwest. The breakouts are consistent with either a thrust stress field or a strike–slip stress field, but the fractures formed by excess pressures in wells favour the latter. Recently Schafer has reported oil wells systematically elongated with a mean azimuth of N39°E in the Austin Chalk of southern Texas. We regard these elongations as formed by breakouts in a stress field with greater horizontal stress near N51°W, and this view is supported by overcoring measurements by Hooker and Johnson in granite of the Llano Uplift, 225 km from Schafer's wells, that reveal large horizontal compressions at the surface with the larger oriented N33°W.

Natural and Induced Fracture Orientation: ABSTRACT

Natural fractures in rocks comprise: (1) joints which are commonly closely spaced, are of limited linear extent, and have negligible tangential displacements; and (2) faults across which opposite blocks have tangential displacements ranging from millimeters to tens of kilometers. Induced fractures are principally those produced in surrounding rocks by fluid pressures applied within well bores. The orientation of fractures with respect to applied stresses may vary widely, depending upon the amount of finite strain the rock has experienced after the fractures were formed. The present discussion is limited to fractures in rocks that have experienced only minor strain subsequent to the fracturing. In virtue of the fact that rocks are elastic solids, there exists in three-dimensional space beneath the surface of the ground a field of stress definable at each point by three mutually perpendicular principal compressive stresses and by the space orientation of these stress axes. On the three planes perpendicular to the principal stresses, shear stresses are zero; on all other planes, if the principal stresses are unequal, nonzero shear stresses exist. Parallel with the ground surface the shear stresses must be zero. Hence, at each point of this surface, one of the three principal stress trajectories must terminate perpendicularly. Therefore, in regions of gentle topography and simple structure the underground stress field is usually characterized by a system of principal stress trajectories one of which is nearly vertical and the other two nearly horizontal. When rocks are subjected to compression under unequal triaxial stresses, for certain stress combinations failure by fracture and tangential slippage occurs. Usually, conjugate sets of slip surfaces are formed whose lines of intersection are parallel with the intermediate axis of stress and whose acute angle (commonly about End_Page 2086------------------------------ 60°) is bisected by the greatest principal stress. This affords the guiding principle relating the orientation of common faults--normal, reverse, and transcurrent--to the associated stress fields. Hydraulically induced fractures, whether by fluid pressure in wells or by the intrusion of igneous dikes, tend to follow surfaces parallel with the greatest and intermediate principal compressive stresses and perpendicular to the least stress. Therefore, the orientations of hydraulic fractures, or of igneous dikes and sills, are strongly influenced by the prevailing stress state in the ambient rocks. In particular, in tectonically relaxed regions characterized by normal faulting, the greatest principal stress is nearly vertical, and the intermediate and least principal stresses nearly horizontal, with the intermediate stress in the strike direction of the local normal faults. In such a region, the preferred orientation of hydraulic fractures is vertical and perpendicular to the least rincipal stress, and parallel with the strike of the local normal faults. Hydraulic-fracture orientation may also be influenced by anisotropy or planar inhomogeneities in the rock such as bedding, schistose cleavage, or a system of parallel joints. If such a planar system does not depart too far from perpendicularity to the axis of least stress, hydraulic fractures may follow such a zone of weakness, across which the shear stress will not be zero. In this case, provided the rocks are also stressed tectonically, slippage along the fracture with possible resultant earthquakes is an expectable consequence of increasing the fluid pore pressure in the rock. End_of_Article - Last_Page 2087------------

Measurement and transmission of steam in Matsukawa geothermal power plant

Matsukawa geothermal power plant has been under smooth operation since its establishment in October 1966 as the first power plant using geothermal steam in Japan. The most important problems in setting up the geothermal power installation were the pressure in the well and the selection of the materials to be used. Both were decided by the characteristics of the well. In Matsukawa the characteristic of the steam was tested in wells Nos. 1, 2, 3. The items of the investigation discussed in the paper are the following: • — amount of the steam output, pressure and temperature in wells; • — chemical contents of the discharge; • — hot water and corrosion tests; • — problems in the specifications of the geothermal steam pipeline and its design.

Water‐level fluctuations caused by Montana earthquake

The major earthquake of August 17, 1959, near the Montana‐Wyoming border had marked effects on water levels and artesian pressures in wells throughout the United States.Preliminary reports from field offices of the U. S. Geological Survey in 21 states show that water‐level fluctuations were automatically recorded in 136 observation wells. These wells for which records are available, and the maximum double amplitude of the fluctuations, are listed in Table 1.

Rock Rupture as Affected by Fluid Properties

Abstract This paper concerns the rupture or breakdown of rock formations as related to drilling, completing, and stimulating production of wells, and comprises data compiled from a study of literature and records of treatment of oil and gas wells, and from tests conducted in bores drilled into rock cores and outcrops of rock. Results of the investigation indicate that the internal pressure to rupture cylinders of rock and to break-down rock formations surrounding a bore in the earth is dependent upon the extent of intrusion of fluids, the position of bedding planes, the ratio of internal to external diameter, the tensile strength of rock, and magnitude of confining pressure, and is independent of the size of bore, degree of fluid saturation, and temperature of rock within practical limits. It is concluded that the mathematical relationship of pressure in bores and stresses in the surrounding rock must not be limited by the simplifying assumptions of homogeneity, isotropy, and impermeability; that the incidence of lost circulation of drilling fluids to induced fractures may be reduced by preventing intrusion of fluids into the small intrinsic fractures along weak bedding planes; and that the magnitude of the breakdown pressure of wells to be treated may be lowered by removal of mud cake. Introduction The purpose of this paper is to present and discuss the results of tests which may serve to broaden the understanding of the phenomena of rupture or breakdown of rock and thus contribute to the improvement of the techniques for drilling and completing wells, including such operations as preventing lost circulation, stimulating production, and placing cement. Since the early recognition of the possibility of rupturing rock adjacent to a well by fluid pressure, the distribution and magnitude of stresses around a well and the internal pressure to cause failure have been expressed mathematically by application of the principles defining the elastic and inelastic behavior of thick-walled cylinders of a homogeneous, isotropic, impermeable material. In either the elastic or plastic state, if the conditions of homogeneity, isotropy, and impermeability were the normal characteristics of rock, there would be no reason to question the validity of the above principles when applied to rock.

Differential Gas Pressures in Wells

Differential Gas Pressures in Wells