Analysing borehole breakouts is a wellknown method for obtaining horizontal stress directions. In this study dipmeter data from 46 wellbores in the Tampen area in the northern North Sea were analyzed for breakouts. The analysis identified a large problem of differentiating borehole breakouts from key- seats. Even at very low bole deviation elongations seemed to align with hole azimuth. This hampered breakout identification, and indicated that keyseating possibly is a much larger problem than implied in earlier breakout work from the North Sea From our findings and also based on other stress data from the area it seems as though the wellbore very often deviates in the direction of the least horizontal stress, thereby masking breakouts as keyseats. To examine this observation the effect of bedding and stress on the wellpath were examined, without yielding a solution to the problem. For future stress analyses in the northern North Sea enhanced use of imaging data will be important to avoid similar problems. Borehole breakouts identified by dipmeter analysis primarily seem to give an indication of the in-situ stress direction, as long as the number of observations are statistical significant. Traditional breakout analysis should not be trusted alone, but always be constrained by, for instance, image analysis.
Monitoring acoustic emission behaviour during core restressing is a means of collecting stress history information, but this effect (the "kaiser" effect) for rocks can not he used alone for paleostress reconstruction. Therefore, an integrated approach including regional geological and geophysical information such as magnetic, gravitational, and image data has been implemented in combination with core measurements and imaging logs. This analysis leads to a prohable history of far-field loading, and allows greater confidence in the interpretation in that numerous independent sources seem to suggest that the interpretation is consistent. For details of local stress distributions, Kriging methods are used, based on knowledge of the ancient landforms and erosional geometry. Practical applications of paleostresses in the context of basin analysis seem feasible if fractures can be linked to hydrocarhon migration and accumulation. To demonstrate this, the method is applied in the Ordos Basin in North China to establish the relationships between far-field and local stresses, and to link the stress and fracture history to the migration and accumulation of the large gas field found in the central Ordos area.
Compressibility of deep fluids-filled cavern is discussed. Compressibility is measured both through statical and dynamical tests. Statical compressibility is influenced by cavern shape and cavern fluids nature. This parameter plays an important role for such applications as the determination of stored hydrocarbons volume, of volume lost during a blow-out, and of pressure build-up rate in a closed cavern. Dynamical compressibility is measured through the periods of waves triggered by pressure changes. Both tube waves and longer period waves associated to the existence of an interface between a liquid and a gas can be observed. They can provide additional information, for instance the existence of trapped gas in the well-head.
Ringstad, Cathrine (IKU Retroleum Research) | Lofthus, Ellen Benedikte (NTNU) | Sonstebo, Eyvind F. (IKU Petroleum Research) | Fjaer, Erling (IKU Petroleum Research) | Zausa, Fabrizio (ENI Spa - AGIP E&P Divison and Giin-Fa Fuh, Conoco Inc.) | Fuh, Giin-Fa (Conoco, Inc.)
Micro-indentation measurements have been performed in order to investigate the possibility of extracting rock mechanical properties from small rock samples. The tests were performed with a 1 mm flat indenter. Two parameters were determined when analyzing the indentation measurements: The Indentation Modulus (IM) and the Critical Transition Force (CTF). IM is the slope of the force-displacement curve, corrected for deformations in the load frame. CTF is defined as the force level where the material deforms without significant change in the applied force.
The rock samples were casted in a mounting material (Demotec 30), in order to stabilize the sample during testing, and simplify surface preparation. The upper and lower surfaces of the samples were made flat and plane parallel by grinding. 15 materials were tested, including sandstones, limestones and shale/clay materials with different strengths and stiffness.
The micro-indentation measurements showed that both IM and CTF were significantly affected when the experiments were performed on small samples with volumes ranging from 0.04 Cm3 to 0.7 cm3. Correlations combining the Uniaxial Compressive Strength (UCS), Young's Modulus (E) and porosity ( ) with IM and CTF have been made. For measurements performed on small rock samples (embedded in Demotec 30) the following correlation with Uniaxial Compressive Strength was found:
UCS = 0.149 CTFR2=0.90
The correlation is only valid for small samples embedded in a mounting material with the same indentation properties as Demotec 30. It is not recommended to use the micro- indentation measurements to predict porosity and Young's Modulus.
Schutjens, P.M.T.M. (Shell Research and Technical Services Laboratory) | Blanton, T.L. (Mobil Research Laboratory) | Martin, J.W. (TerraTek, Inc.) | Lehr, B.C. (Shell Research and Technical Services Laboratory) | Baaijens, M.N. (Shell Expro)
P.M.T.M. Schutjens, SPE (1, T.L. Blanton, SPE (2, J.W. Martin (3, B.C. Lehr (1 and M.N. Baaijens (4
Laboratory experiments were done to investigate compaction of core taken from a 5 km deep, highly overpressured sandstone reservoir as a function of depletion, stress path, and time. Guided by a numerical analysis of the reservoir compaction and associated total stress change, a best-estimate and an unfavourable reservoir stress path were applied. Most samples showed a linear relationship between axial strain and total axial stress up to 65 MPa, which reflects the target depletion level. However, high-porosity samples compacted following the unfavourable reservoir stress path showed linear compaction up to only 52 MPa increase in total axial stress (i.e. up to 80% of target depletion). Non-linear (accelerating) compaction occurred in nearly all high-porosity samples with increasing total axial stress; total strains reached 4 to 23 ms after 65 MPa increase in total axial stress, typically followed by 20 to 50 axial ms creep at time periods of 11 to 3 weeks. Microstructural analysis revealed that the non-linear compaction is due to an increasing activity of grain-scale brittle deformation with increasing stress, which gradually leads to pervasive microfracturing including numerous intraand transgranular microcracks, collapse of weak minerals and grain size reduction. No strain localisation occurred. Analysis of the experimental data suggest that high-porosity layers loaded following the unfavourable stress path could show vertical compactions in the range 50 ms to 100 ms should they be depleted by more than 52 MPa. Monitoring of reservoir compaction and field stress path evolution is therefore advised.
Stress trajectories around faults have been simulated with a 3-D finite element model, based on 8-model brick elements in five layers. Preliminary simulations imply that stress trajectory deflections in the neighbourhood of fault zones can be expected if there is a large geomechanical contrast between the fault zone and the adjacent rocks and if far field stress anisotropy is small. On the other hand, if the horizontal stress anisotropy is large, significant stress deflections appear unlikely to occur near a fault. Within a softer' fault zone H will be aligned approximately parallel to the trend. Magnitudes of maximum and minimum stress are modelled for soft and hard fault simulations. Significant modifications in stress magnitudes developed in the vicinity of the fault zones.
Borehole failures like drilling-induced fractures and breakouts observed inimage logs offer the possibility to derive high quality stress orientations.Image logs from 16 wells in the Northern North Sea are analysed for theoccurrence of borehole failures. While in wells to the west of the VikingGraben very few stress related borehole failures are observed abundantdrilling-induced fractures are detected in all of theimage logs in wells to theeast of the graben. Also the orientation of SHappears to beslightly different for the western and the eastern part of the investigatedarea. An SHorientation of approximately NI000E is found inthe western part, while the SHorientation is found to beapproximately N800E for the wells in the eastern part. Investigating theconditions for fracture initiation in three of the wells allows to constrainthe possible magnitude of SH. For all three wells theoccurrence of drilling-induced fractures can only be explained if theSHmagnitude is at least slightly greater than the verticalstress, which indicates a strike-slip tectonic regime(Sh
The specific surface of particles' and the properties of the pore fluid control the sensitivity of shales to a given change in the host environment. In this study, dielectric permittivity measurements are suggested to assess the reactivity coefficient, which is a measure of the physico-chemical sensitivity of a clay-fluid system. The effects of specific surface and pore fluid properties on permittivity data are discussed using dielectric spectra of ideal systems and real shale cores. Normalized high-frequency real permittivity data and the percentage of free water (estimated using high- frequency imaginary permitivity spectra) are shown to decrease with the increase in the reactivity coefficient. Similarly, high low-frequency permittivity values normal to the bedding plane indicate systems with high reactivity coefficients. Vertical strains measured during osmotic consolidation tests confirm the previous trend. Finally, general guidelines are presented to help in identification of reactive systems using complex permiflivity measurements.
Compressional tectonics in the Eastern Cordillera foothills are investigated using a large strain, two-dimensional finite clement method. The main purpose is to calculate the stress regimes within the Cusiana field and to compare the results with field data.
In the foothills, the NW-SE tectonic push is clearly confirmed by borehole breakouts. The present-day state of stress in the Cusiana field, is investigated through a footwall-hangingwall model with a detachment/ramp thrust fault. The tectonic push was simulated by applying horizontal displacement to the vertical boundary of the model. Simulations with both elastic and elastoplastic rheologies were performed.
To obtain a realistic horizontal stress gradient (in the range of 1.25 to 1.5 psi/fi) at depth, the stiffness of the hangingwall block has to be sufficiently low to accomodate the thrust geometry. With a perfectly sliding fault (zero friction), the tectonic push localizes the plastic deformation into a shear band which can be interpreted as the backthrust observed on the seismic section. Moreover, in the vicinity of the Cusiana thrust, the major principal stress rotates. This rotation is in good agreement with orientation found by core DSCA measurements.
To determine factors controlling permeability variations within and adjacent to a fault-hosted geothermal reservoir at Dixie Valley, Nevada, we conducted borehole televiewer observations of wellbore failure (breakouts and cooling cracks) together with hydraulic fracturing stress measurements in six wells drilled into the Stillwater fault zone at depths of 2 to 3 km. Measurements in highly permeable wells penetrating the main geothermal reservoir indicate that the local orientation of the least horizontal principal stress, Shmin, is nearly optimal for normal faulting on the Stillwater fault. Hydraulic fracturing tests from these wells further show that the magnitude of Shmin is low enough to lead to frictional failure on the Stillwater and nearby subparallel faults, suggesting that fault slip is responsible for the high reservoir productivity. Similar measurements were conducted in two wells penetrating a relatively impermeable segment of the Stillwater fault zone, located 8 and 20 km southwest of the geothermal reservoir (wells 66-21 and 45-14, respectively). The orientation of Shmin in well 66-21 is near optimal for normal faulting on the nearby Stillwater fault, but the magnitude of Shmin is too high to result in incipient frictional failure. In contrast, although the magnitude of Shmin, in well 45-14 is low enough to lead to normal faulting on optimally oriented faults, the orientation of the Stillwater fault near this well is rotated by 40 from the optimal orientation for normal faulting. This misorientation, coupled with an apparent increase in the magnitude of the greatest horizontal principal stress in going from the producing to nonproducing wells, acts to inhibit frictional failure on the Stillwater fault zone in proximity to well 45-14. Taken together, data from the nonproducing and producing wells thus suggest that a necessary condition for high reservoir permeability is that the Stillwater fault zone be critically stressed for frictional failure in the current stress field.