Drilling-induced Damage Research Articles

Pressure and temperature dependence of VP and VS in rocks from the superdeep well and from surface analogues at Kola and the nature of velocity anisotropy

P-wave velocities ( V P) and S-wave velocities ( V S) and their directional dependence (velocity anisotropy, shear wave splitting) were measured on eight cores samples from the Kola superdeep well and on eight samples collected from surface outcrops over a range of pressures up to 600 MPa at room temperature and from room temperature up to 600°C at 600 MPa. The core samples were recovered at depths of about 4670 to 11,720 m, and the outcrop samples represent lithologies similar to those of the core samples. Measurements were carried out on sample cubes of dry rocks (43 mm edge length) in a multianvil apparatus, allowing simultaneous measurements of V P and V S in the three structural directions X, Y, Z of the sample cubes and direct determination of length changes (volume change) of the sample with increasing pressure and temperature. Pronounced pressure (crack) sensitivity of P- and S-wave velocities indicates strong decompaction in the cores, due to drilling-induced damage and rapid pressure and temperature release during core retrieval. Marked velocity anisotropy and shear wave splitting is present in the foliated Proterozoic and Archean gneisses and amphibolites, due to oriented microcracks and lattice (crystallographic) preferred orientation (LPO) of the constituent mica and hornblende minerals. The experimentally determined intrinsic velocities and anisotropies compare fairly well with corresponding calculated data, based on the LPO and the single crystal properties of the major minerals. Measurements and calculations confirm strong relations of velocity anisotropy, shear wave splitting and shear wave polarisation to the structural frame of the rocks (foliation, lineation). The experimentally derived in situ velocities representing the averages of the velocities measured in the three structural directions (three P-wave and six S-wave velocities) compare fairly well with VSP data. The fit is even better when anisotropy (related to foliation and foliation dip) is included in the determination of the in situ velocities. From the relationship between experimental and calculated shear wave splitting data and the structural frame (foliation and lineation) of the Proterozoic and Archean rocks we interpret the VSP-derived shear wave polarisation at Kola ( Digranes et al., 1996) as resulting from the strong alignment of mica and hornblende, defining the foliation.

A high-pressure technique for determining the microcrack porosities of damaged brittle materials

The microcrack porosity of a damaged brittle material is found by accurately measuring the compressibilities under a hydrostatic pressure to 200 MPa. Crack apertures are sensitive to the stress applied normal to their plane and, as a result, at low confining pressures the bulk of the strain records the response of the progressive closure of these cracks. At higher pressures when much of the microcrack porosity is closed the observed strains are nearly the same as that expected for the solid compounds within the rock. The higher pressure response may then be used to estimate the expected solid strains that are removed from those observed to leave that portion of the strain due to microcrack porosity. This procedure differs from that of other workers who either ignore the portion of the strain due to intrinsic compression or account for it indirectly by taking the derivative of the observed strain curves. The final results may be analyzed in tensor form to show possible anisotropies in microcrack orientations within a given sample. An example of the experiment on a rock sample that was damaged during removal from the earth by drilling yielded a microcrack porosity of 0.79%, which was a substantial fraction of the total porosity near 5%. At 200 MPa, the principal crack strains had magnitudes of 1.86, 2.61, and 3.39 mstrains; the orientation of this last and greatest magnitude strain differing from the vertical direction by approximately 20°. This suggests that the planes of the predominant family of microcracks within the test piece were subhorizontal. This suggests that a portion of the microcracks result from drilling-induced damage and not solely from stress relief of the sample upon removal from the earth.

95/01956 Climate change: The UK programme

P-wave velocities (VP) and S-wave velocities (VS) and their directional dependence (velocity anisotropy, shear wave splitting) were measured on eight cores samples from the Kola superdeep well and on eight samples collected from surface outcrops over a range of pressures up to 600 MPa at room temperature and from room temperature up to 600°C at 600 MPa. The core samples were recovered at depths of about 4670 to 11,720 m, and the outcrop samples represent lithologies similar to those of the core samples. Measurements were carried out on sample cubes of dry rocks (43 mm edge length) in a multianvil apparatus, allowing simultaneous measurements of VP and VS in the three structural directions X, Y, Z of the sample cubes and direct determination of length changes (volume change) of the sample with increasing pressure and temperature. Pronounced pressure (crack) sensitivity of P- and S-wave velocities indicates strong decompaction in the cores, due to drilling-induced damage and rapid pressure and temperature release during core retrieval. Marked velocity anisotropy and shear wave splitting is present in the foliated Proterozoic and Archean gneisses and amphibolites, due to oriented microcracks and lattice (crystallographic) preferred orientation (LPO) of the constituent mica and hornblende minerals. The experimentally determined intrinsic velocities and anisotropies compare fairly well with corresponding calculated data, based on the LPO and the single crystal properties of the major minerals. Measurements and calculations confirm strong relations of velocity anisotropy, shear wave splitting and shear wave polarisation to the structural frame of the rocks (foliation, lineation). The experimentally derived in situ velocities representing the averages of the velocities measured in the three structural directions (three P-wave and six S-wave velocities) compare fairly well with VSP data. The fit is even better when anisotropy (related to foliation and foliation dip) is included in the determination of the in situ velocities. From the relationship between experimental and calculated shear wave splitting data and the structural frame (foliation and lineation) of the Proterozoic and Archean rocks we interpret the VSP-derived shear wave polarisation at Kola (Digranes et al., 1996) as resulting from the strong alignment of mica and hornblende, defining the foliation.

X-ray Computed Tomography (CT) Applied in Routine Core Analysis: Examples from New Guinea and the North West Shelf, Australia: ABSTRACT

X-ray Computed Tomography (CT) is a powerful technique for nondestructive 3-D analyses of cores. It provides sedimentological and structural information that is difficult or impossible to obtain by other means. Nevertheless, x-ray CT has until recently lacked acceptance by the petroleum industry as a routine core analysis tool. The reasons for this stem mainly from resolution limitations of second generation CT scanners used to carry out early research on cores, a lack of image analysis software specific to core analysis, and the relatively high cost of scanning. Increased emphasis is now being placed on detailed reservoir characterization and the need to better define heterogeneity on the core scale. Core analysis has become more sophisticated and expensive. These factors, combined with more advanced CT scanner hardware and image analysis software, have resulted in a resurgence of company interest in x-ray CT. Indeed, the technique has now evolved to where it plays an increasingly important part in the process of routine core analysis. The authors present examples from New Guinea and the North West Shelf, Australia, where x-ray CT is being successfully used to scan sealed core for selection of core plug sites, determination of plug integrity prior to special core analysis,more » and assessment of formation damage through testing of various mud systems flushed through core plugs. Drilling-induced damage is also being quickly assessed through the scanning of sealed core. In addition, minipermeameter results have been related to CT images as a means by which core-scale permeability pathways can be mapped in 3-D.« less

The Effects of Fastener Hole Defects

The influence of drilling-induced defects, such as delamination, on the fa tigue life of a pin bearing joint in a toughened carbon epoxy system was investigated. In a pin bearing joint, the parameters of interest are the bearing stress and the pin position or tolerance. Tension bearing by-pass tests were conducted on coupons under ambient and elevated temperature wet conditions. Specimens were tested in a bearing tension frame to static failure in order to measure the failure load and to calculate pin bearing stress. From static test results, a fatigue load was selected as 66% of the static pin bearing failure load. Coupons were then tested under pin bearing fatigue loading at ambient and elevated tem perature wet conditions. Two-dimensional finite element models were used to simulate the pin and tension loads applied to the coupons. These models simulated the stress distribu tion around the loaded pin. The loads in the finite element models represented the failure load incurred in the static pin bearing tests as well as the maximum load during a selected cycle of the pin bearing fatigue tests. Experimental results showed the pin movement to vary with respect to delamination diameter, specimen thickness, and configuration. Numerical results compared with the static failure levels and indicated the form of the delaminated regions. Results give an indication of the effects of environment and hole quality under pin bearing load. Large variations in drilling-induced damage tended to cause variations in the pin bearing response of like specimens.