Is the San Andreas Fracture a bayonet-shaped fracture as inferred from the acoustic body waves in the SAFOD Pilot hole ?

The method using the propagation of acoustic body waves within the stress modified areas around a vertical borehole has been applied to the granitic formation penetrated by the SAFOD Pilot hole near the San Andreas Fault trace. This method allows us investigating the horizontal in situ stresses. Only P waves supplied useful and surprising information. A depth of 1270 m separates an upper region of uniform thickness of stress modified areas, possibly corresponding to a shear domain, and a lower region where there are simultaneously two values of the thicknesses of the stress modified areas (particularly between 1500 and 1600 m of depth) possibly corresponding to a compressive and a shear domain. In order to integrate the contradictory effects of the simultaneity of shear and compressive domains at some depths, as well as the presence of three shear zones at particular depths, we propose that the San Andreas Fault could be bayonet-shaped instead of planar. Other recent available information in the literature about this fault, such as the presence of a fault zone of low shear wave velocity, stress rotation measured with depth, and the large angles of the frictional coefficients, can be logically explained by this kind of fault geometry

Underreamer mechanics

In the oil and gas industry, an underreamer is a tool used to extend and enlarge the diameter of a previously-drilled bore. The problem proposed to the Study Group is to obtain appropriate mathematical models of underreamer dynamics, in forms that will lead to feasible computation. The modes of dynamics of interest are torsional, lateral and axial. This report describes some initial models, two of which are developed in more detail: one for the propagation of torsional waves along the drill string and their reflection from contact points with the well bore; and one for the dynamic coupling between the underreamer and the drill bit during drilling

Borehole Stoneley Wave Propagation Across Heterogeneous And Permeable Structures

This study investigates the propagation of borehole Stoneley waves across heterogeneous and permeable structures. By modeling the structure as a zone intersecting the borehole, a simple one-dimensional theory is formulated to treat the interaction of the Stoneley wave with the structure. This is possible because the Stoneley wave is a guided wave, with no geometric spreading as it propagates along the borehole. The interaction occurs because the zone and the surrounding formation possess different Stoneley wavenumbers. Given appropriate representations of the wavenumber, the theory can be applied to treat a variety of structures. Specifically, four types of such structures are studied, a fluidfilled fracture (horizontal or inclined), an elastic layer of different properties, a permeable porous layer, and a layer with permeable fractures. The application to the fluid-filled planar fracture shows that the present theory is fully consistent with the existing theory and accounts for the effect of the vertical extent of an inclined fracture. In the case of an elastic layer, the predicted multiple reflections show that the theory captures the wave phenomena of a layer structure. Of special interest are the cases of permeable porous zones and fracture zones. The results show that, while Stoneley reflection is generated, strong Stoneley wave attenuation is produced across a very permeable zone. This result is particularly important in explaining the observed strong Stoneley attenuation at major fractures, while it has been a difficulty to explain the attenuation in terms of the planar fracture theory. In addition, by using a simple and sufficiently accurate theory to model the effects of the permeable zone, a fast and efficient method is developed to characterize the fluid transport properties of a permeable fracture zone. Tills method may be used to provide a useful tool in fracture detection and characterization.Massachusetts Institute of Technology. Full Waveform Acoustic Logging ConsortiumUnited States. Dept. of Energy (Grant DE-FG02-86ER13636

Assessment of blasting operations effects during highway tunnel construction

Blasting operations are one of the fundamental parts of daily civil engineering. Drilling and blasting still remain the only possible ways of tunnelling in very adverse geological conditions. However, this method is a source of various disadvantages, the main one being tremors propagating through the geological environment which not only affect buildings, but also disturb the comfort of living in the vicinity of the source. Designing this procedure is mostly done using standardized empirical relations. This article shows the possibility of using a FEM technique in predicting blast effects. This approach is demonstrated in a simple case study on the impact of blasting operations on steel pipes

PREDICTING FRACTURE EXTENSION AROUND A BOREHOLE USING THE NUMERICAL DISPLACEMENT DISCONTINUITY METHOD

Prediction of blast damage radius is integral in the optimization of mining safety and production. The damage radius can be related to blasting variables such as fragmentation size as well as overbreak in hard rock. Various methods have been developed to predict and assess damage radius extension for individual holes to aid in blast design. These methods range from experimental and observational techniques to theoretical applications. Some of these methods have been used with a degree of success. However, many of them lack a comprehensive combination of experimental and theoretical contributions. Since post-blast environments are not conducive to assessing whether the predicted damage radius was correct, this research actively strives to determine the extension of fractures post-blast and compare it to the predicted value. The predictions are based on the use of current fundamental methods in conjunction with a novel methodology proposed in this research. Numerical modeling has become a promising field within fracture extension analysis and prediction. While methodologies such as Discrete Element Modeling (DEM) and Finite Element Analysis (FEA) have proven effective in fracture prediction, this research delved into the Displacement Discontinuity Method (DDM). The DDM setup is simple as only the crack (fracture) itself is discretized, saving computational time and increasing efficiency when solving. The crack, which for these purposes can be visualized as an infinitesimally small line crack, is divided into an arrangement of nodes and elements. From there, pressure, such as an explosive detonating in a borehole is applied, and the iterative time step process begins to propagate the fracture. This methodology is based upon S.L. Crouch’s book “Boundary Elements Methods in Solid Mechanics.” While the code itself has been used extensively in the petroleum fracking industry, in this research, it has been adapted to blasting. Traditionally only one fracture could be tracked, but this research modified the algorithm to now attempt multiple fracture propagation through coordinate system combination of each fracture. The current build utilizing DDM analysis within this research is highly convenient, and time-consumption for processing is for all intents and purposes almost zero. Observations of experimental results, as well as modeling analyses, create a large scatter of crack length results depending upon time and loading. In blasting, hard-rock inherently has initial flaws and cracks of which are random in nature. The cracks can be present between grain boundaries in addition to physical voids. These initial natural flaws influence the damage radius around the blasthole in an arbitrary manner; therefore, a random analysis is required to represent any patterns that may be created. This research introduced the use of random parameter distributions and observed the effects these parameters had upon the overall damage radius of the blasthole. Validation and calibration is a necessity when modeling real physical problems. The dissertation included research for two (2) small-scale experiment that investigated the damage radius of a borehole loaded with an appropriate explosive and calibrated the explosive use in the large-scale test. One (1) six-foot cube of high-strength concrete was manufactured as an analog for in-situ rock. A small-diameter hole (7/8”) was drilled through the center, loaded, and shot. Various instrumentation was utilized to capture the damage radius produced, and this information was used as feedback into the DDM program to model the blast. The DDM model provided accurate results to assess the extension of the cracks for a single-crack condition, while the multiple-fracture two-crack models proved more conservative when assessing the damage radius. The DDM damage envelope developed from randomization of fracture angle using the two-fracture model provide results closer to that of the single-crack model. The two-small scale experiments were successful in terms of fracture and fragmentation of the block, as both were completely fractured, and in explosive calibration. The large-block scale test needs improvements; Recommendations and future work are provided for both model and scale-testing improvement

Structure and in Situ Stress Analysis of the Tazhong Uplift, NW China: Implications for Fault Reactivation

The Tarim Basin in northwest China is an intracratonic, poly-phase basin with a subsurface structure that records a protracted tectonic history associated with crustal accretion and amalgamation. Currently, the basin is bounded by actively deforming mountain belts but displays little evidence of active deformation within the basin. Here, detailed interpretation of 3D seismic reflection data and analysis of drilling-induced deformation in deep boreholes (e.g. borehole breakouts) are used to resolve uncertainties about the timing and distribution of past deformation, the effect of pre-existing structures on subsequent deformation, and the current in situ stress state in the Tazhong Uplift of the Central Tarim Basin. The geometry and kinematics of Ordovician thrust faults and folds, Silurian-Permian strike-slip faults, and Triassic igneous bodies and normal faults, along with stratigraphic relationships, suggest that creation of new faults, and reactivation of pre-existing faults occurred during tectonic events in the Paleozoic and Early Mesozoic; however, no evidence of faulting is observed in Late Mesozoic or Cenozoic strata in the Tazhong Uplift. The current in situ stress should favor extensional and strike-slip tectonics with maximum horizontal compression directed NE, which contrasts with past stress states in the basin inferred from Paleozoic and Mesozoic structures. In situ differential stress magnitude in the Tazhong Uplift (ranging from 94 to 170 MPa) is insufficient to reactivate the most optimally-oriented faults in the Central Tarim Basin, even though the basin is bounded by the active Tian Shan and Kunlun Shan thrust belts to the north- and south-west, and the left-lateral strike-slip Altyn Tagh fault to the south, all associated with the ongoing Himalayan-Tibetan orogeny. The low differential stress may be understood if the basin-bounding faults (particularly the Altyn Tagh fault) operate at low absolute shear stress, similar to continental transform faults such as the San Andreas fault, CA