bluuebunny/crossref_metadata_2025_split huggingface dataset:
From Core Compaction To Reservoir Compaction: Correction For Core Damage Effects.
Authors: Holt R.M. , Brignoli M. , Kenter C.J. , Meij R. , Schutjens P.M.T.M. 
Published: December 2007

Abstract
Core damage is a permanent alteration of rock properties as a result of drill-out and retrieval of the core from the in situ environment to the surface. For reservoir rocks, stress release during coring is thought to be a major core damage mechanism. In this paper, we present a systematic approach to correcting core compaction data for effects of such damage. Our core damage quantification strategy has the following 3 steps:
– A synthetic rock, analogous to the reservoir rock with respect to rock mechanical and petrophysical properties, plus texture, is created under stress. Comparative tests are performed to simulate virgin compaction behavior, and the behavior of an unloaded and reloaded simulated core. – Based on these experiments, a simple elastoplastic model has been developed, describing both the core and virgin compaction behavior of the synthetic analogues. The model is currently in a preliminary stage, but we present it here as an important element in our strategy. – Tests with natural reservoir rocks are performed. Obtained parameters are fed to the mathematical model above, permitting a prediction of the expected compaction of the same rock sample placed in the reservoir.
A field example illustrating the use of this approach is presented. The results, based on measurements on synthetic and real cores, indicate that core damage corrections may be substantial, reducing the expected compaction by 25 - 50 % during reservoir depletion.
P. 311
Ekofisk Revisited: A New Model of Ekofisk Reservoir Geomechanical Behavior
Authors: Rhett Douglas W. 
Published: December 2007

Abstract
The Ekofisk Field, a giant North Sea oil field, has produced more than 1.2 billion barrels of oil since the start of production in 1971. Sea floor subsidence, which was discovered late in 1984, has occurred at approximately a constant rate of about 38 cm/year since then. Full field water flooding began in 1987. Over the past decade numerous, long term laboratory studies of reservoir chalks were undertaken to understand the effects of water flooding on chalk strength and to provide input data for reservoir simulators and geomechanics codes used to predict reservoir compaction and sea floor subsidence.
Recent laboratory analyses suggest that some protective mechanism, possibly oxidized, polar hydrocarbons adhering to portions of the chalk matrix, may protect the calcite matrix from intimate chemical interaction with formation brines. This observation has led to a new constitutive model of Ekofisk rock mass behavior consisting of two sets of ‘end member’ stress strain curves. The first set represents the reservoir rock prior to water flood invasion, when the rock mass behaves as an assemblage of dominantly elastic, low compressibility matrix blocks capable of moving independently of neighboring blocks. The second set represents fully water-contacted, fully water-weakened chalk that has undergone the transition from elastic to plastic behavior. In its fully water-weakened state the chalk is highly compressible and displays the high strain rates and large strains observed in water flooded intervals of the Ekofisk reservoir. The two sets of stress/strain curves are related by water saturation.
Microseismic Logging of the Ekofisk Reservoir
Authors: Maxwell S.C. , Young R.P. , Bossu R. , Jupe A. , Dangerfield J. 
Published: December 2007

Abstract
Microseismic events or acoustic emissions induced by petroleum extraction in the Ekofisk field operated by the Phillips Petroleum Licence 018 group in the Norwegian sector of the North Sea' were monitored for 18 days in April 1997. The monitoring consisted of a 6 level, triaxial VSP wireline tool deployed within the reservoir in the 2/4-C 11 a observation well located near the crest of the field. 2100 microseismic events were recorded, corresponding to roughly 5 per hour. The rnajority of the events were located in the upper part of the reservoir, predominantly in low porosity layers overlying relatively porous layers which are undergoing water flooding and compaction. Events were most accurately located near the monitoring borehole, with the position of events further away being more uncertain. A cluster of activity surrounded the nearby well C 16, confirming the event positioning accuracy. Within 300 m of the well, most events were precisely located to an accuracy of better than 30 m, and found to cluster along discrete lineations. These lineations, which are parallel to major structures in the region, are attributed to induced definition of preexisting fractures. Comparisons with simulations of the water flood front indicated that the saturated zones around the injection wells were aseismic. The position of the water flood front was not accurately enough known to test if any of the clusters of microseismicity were associated with it. However, compaction of the chalk may be strongest at the water front. If a significant proportion of the total acoustic activity is caused in association with the water front, then time-lapse microseismic monitoring could represent a useful tool for tracking the position of the water flood.
P. 387
Bedding-Related Borehole Instability in High-Angle Wells
Authors: Okland D. , Cook J.M. 
Published: December 2007

Abstract
In the Oseberg field in the Norwegian sector of the North Sea, extended reach drilling (ERD)has been employed to increase oil recovery. Wells up to 9,327 m total depth (TD) have been drilled. Hole stability problems were experienced in the Upper Jurassic Draupne Formation (Kimmeridgian shale) in a number of very highly inclined wells; these problems had not been experienced at lower inclinations. A rock mechanical study was carried out to explain and overcome the stability problems.
A series of thick-walled hollow cylinder (HC) tests were carried out on core material and analogous outcrop samples in order to identify the mechanisms of the hole problems, together with triaxial tests to obtain modelling parameters. Hole deformations were measured during external pressurization of the cylinders, and after testing the samples were sectioned and exiunined for damage. A very strong influence was found of the angle between the HC axis and the bedding plane of the shale. Specifically, if the HC axis lay more than approximately 100 away from the bedding plane, the hole was relatively stable; if the HC axis was parallel to bedding, the hole was highly, and catastrophically, unstable, forming very large regions of cavings. An anisotropic elastic model for the stresses around the hole did not predict such a sudden change in behaviour.
It was finally concluded that the borehole instability related to strength anisotropy associated with the pronounced bedding of the organic-rich Draupne shale. Spallings from the high side and the low side of the hole occurred at a number of different mud densities when the inclination was sufficientiy high. The decision was made to modify the wellpath so that the "angle of attack" between the well trajectory and the bedding planes in the Draupne Formation always exceeded 200. The world record well C-26 (9,327 m; 7,853 m horizontal reach) was sidetracked due to stuck pipe after an unsuccessful high-angle Draupne penetration. The sidetrack was stable at practically the same mud density, but with a lower inclination.
The reported type of hole instability is widely recognized in hard-rock applications of rock mechanics (mining, tunnelling), but is relatively new to petroleum-related rock mechanics.
P. 413
Rock Mechanics Applied to Drilling – An Operational Review
Authors: Svennekjær M. , Bratli R. K. 
Published: December 2007

Abstract
This paper gives effective procedures to drill wells in a depleted reservoir. From the fact that depletion result in decreased horizontal stresses and fracture gradients, the working hypothesis is that a depleted reservoir should be drilled by keeping the Equivalent Circulating Density (ECD) as low as possible. This have been achieved by using a low mudweight and applying the downhole pressure sub actively; which imply focus on the combination of hole cleaning, lower pump rates, avoiding backreaming, and careful breaking of circulation before pumps are started.
The pressure sub and theoretical calculations show that bottom hole pressure give peak values under the following circumstances: Too quick start of pumps/breaking circulation, too high mudweight, too high mud viscosity, too high tripping velocity, poor hole cleaning, too high pump rates, and pressure pulses. Other factors which may have some influence on the bottom hole pressure are: Well stability problems leading to excess fill and loading of the well, and unstable mud. An additional method to reduce the ECD is to increase the fluid column diameter by installing liner instead of casing or by drilling holes with a larger diameter. Also, an exact calculation of the ECD is usefull.
Three cases from three different wells are discussed, and theoretical calculations and observations are compared. We also observed that the fracture gradient of the borehole wall increased with time, which is usefull if it is not possible to go below the minimum fracturation gradient.
Prediction of Rock Parameters from Micro-Indentation Measurements: The Effect of Sample Size
Authors: Ringstad Cathrine , Lofthus Ellen Benedikte , Sonstebo Eyvind F. , Fjaer Erling , Zausa Fabrizio , Fuh Giin-Fa 
Published: December 2007

Abstract
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.
P. 487
Production as a Tool to Control the Efficiency of Reservoir Fracturing: The Lacq Case- Study
Authors: Lahaie F. , Boyer E. , Grasso J.R. , Fourmaintraux D. 
Published: December 2007

Abstract
We analyze the feedback loop between production and the fracturing of a reservoir. We focus on a case of hydrocarbon recovery (Lacq, France), well constrained both in terms of production parameters and fracturing pattern, as a result of a 20-year monitoring of induced seismicity. Production, which plays the role of a stressing rate for the rocks surrounding the depleted zones, appears to have profound impact on the fracturing pattern of the reservoir, that ranges from a localized damage with relatively large fractures, to a diffuse damage with relatively small fractures, for high and low values of the production, respectively. As the fracturing pattern has been recently proposed to have far-reaching importance for the permeability and the increase of productivity of the reservoir through time, this study suggests the possibility to use production as a knob, so as to tune the response of the reservoir to a level of most efficient fracturing, in terms of hydrocarbon recovery. A careful monitoring of seismic indexes may help pinpoint the ongoing type of fracturing, and adjust production accordingly. This paper provides new routes for taking into account fracturing impact in production objectives.
P. 501
Frictional Contact On Cutters Wear Flat and Evaluation of Drilling Parameters of a PDC Bit
Authors: Geoffroy H. , Minh D. Nguyen , Bergues J. , Putot C. 
Published: December 2007

Abstract
Wear development during bit life modify the mechanical response of PDC bits and reduce their efficiency. Contrary to cutting mechanism, friction mechanism on wear flats which develops on the cutters, remains badly understood. A new friction law based on numerical and experimental studies has been stated by the authors. It depends on the material behavior, and relates the applied normal pressure to the wear flat to the subsequent dip angle a into the rock surface. Drilling parameters for PDC bits can then be evaluated, provided a reasonable wear law be postulated. Two different experimental benches have been built up, one for determining the parameters of cutting and friction mechanisms on a single worn cutter, and the other one for determining the global parameters on a small scale drill bench. Preliminary data allow to discuss some hypotheses of the model.
P. 87
Sand Production on Water Injectors: Just How Bad Can It Get?
Authors: Santarelli F.J. , Skomedal E. , Markestad P. , Berge H.I. , Nasvig H. 
Published: December 2007

Abstract
This paper presents a field case based upon a reservoir operated by Statoil in the Norwegian Sea. The case concerns a series of water injectors - i.e. both sub-sea and platform - that underwent extreme losses of injectivity over short periods of time. When worked over, the wells showed extreme amounts of sand fill that sometimes were several hundreds of meters above the top perforation. Furthermore, the link between well shut-ins and injectivity losses was clear right from the onset of the study.
The life of the injectors is thoroughly reviewed and the reasons for the injectivity losses are established.
First, it is shown that even under no flow conditions corresponding to shut in periods, the rock around the wells is too weak to sustain the stresses and fails. Second, it is established that because of permeability heterogeneity, the wells are cross flowing during shut in periods hence allowing sand to be produced in front of the perforated interval. Third it is shown that under routine operation conditions, the produced particles in front of the perforated intervals are not able to settle down in the rat-hole before injection restarts and hence plug the perforation tunnel upon injection restart. Finally, it is demonstrated that during a standard shut in, pressure waves as large as 80 bars are generated because of the so-called water hammer effect that hits the formations as would a seismic wave do. As a consequence, the formation already weakened by sand production undergoes liquefaction that triggers large amounts of sand to be released in the well, hence killing totally its injectivity.
Finally, the paper presents how the operating conditions of the wells were successfully changed to avoid the repetition of the problems experienced previously.
P. 107
Onset of Sand Production and Open Hole Stability Near Heated Horizontal Wells
Authors: Wang Yarlong 
Published: December 2007

Abstract
Steam injection through a vertical or horizontal well has been commonly exercised in heavy-oil reservoirs to enhance oil recovery. Thermally-induced effects may enhance the mobility of the formation by increasing the reservoir permeability and reducing the oil viscosity, but it can also crush the sand grain and damage the formation, rupture the casing, and eventually destroyed the well. Sand production and wellbore stability of the heated horizontal well are of great concern in well completion and perforation design. Formation damage due to stress concentrations induced by in- situ stresses and the temperature difference is responsible for sand production and instability. To analyze the formation damage and the induced stresses, a geomechanics model fully coupled to energy transport and fluid diffusion process is employed. Both the sanding risk and stability for open-hole cases subject to a non-isothermal, non-hydrostatic in-situ loading, and underbalanced/balanced drilling conditions are investigated. Critical drawdown is calculated based on the thermo-poroelastic theory. It is concluded that changing bottomhole temperature and injection rate can impose a significant impact on the wellbore stability and sanding potential. Specifically, the risks for sand production and that for wellbore instability can be reduced significantly by controlling wellbore temperature and injection rate.
P. 117
Application of Probabilistic Techniques for Assessing Sand Production and Borehole Instability Risks
Authors: McLellan P.J. , Hawkes C.D. 
Published: December 2007

Abstract
The mechanical stability of a borehole or perforation tunnels can have a critical influence on petroleum drilling and recovery processes. For instance, extensive failure of rock surrounding an open-hole completion may result in massive sand production. Furthermore, it is often necessary to evaluate the risk of borehole instability when drilling through weak shale cap-rocks, or when drilling underbalanced through poorly cemented reservoir rocks. Numerous geomechanical modeling techniques have been developed to address these issues. However, the existing models are limited in that they fail to account for the inherent variability of mechanical and petrophysical properties of rock, as well as uncertain or poorly constrained values for these and other critical input parameters.
This paper describes the adaptation of probabilistic techniques for quantitatively evaluating the risks of borehole instability or sand production. This technique involves the use of a versatile poro-elastoplastic model, implemented within the framework of a spreadsheet-based Monte Carlo simulation tool. The typical results generated by this model indicate that rock strength parameters, bottomhole pressure, reservoir pressure and near-borehole permeability changes have the most significant influence on the extent of yielding for underbalanced drilling or production conditions in weak sandstones.
P. 143
Ekofisk Field Overburden Modelling
Authors: Nagel N.B. 
Published: December 2007

Abstract
In weak formations, significant pressure depletion may lead to compaction. Transmission of compaction through the rock layers overlying the depleted formation may, in turn, cause subsidence. At the Ekofisk field, a large producing oil and gas reservoir located in the Norwegian sector of the North Sea, subsidence of the seabed due to reservoir compaction led to extensive facilities modifications in the mid-1980's and has significantly contributed to the need for the on-going Ekofisk II field redevelopment project.
While reservoir compaction is the source of the volume loss that leads to subsidence, transmission of compaction through the overburden strata often is a critical governing factor on its impact. Theoretical modelling work has shown, for example, the relationship between reservoir depth or width to the transmission of compaction to subsidence. Building upon the theoretical work and utilizing available field data, a numerical study was conducted to evaluate the impact that overburden behavior at Ekofisk field has on compaction and subsidence.
For the study, available field data for validating the modelling of the overburden were reviewed, including reservoir compaction data, platform subsidence data from the center as well as the edges of the subsidence bowl, data from eight bathymetry surveys providing information on the extent and change of the subsidence bowl with time, and overburden core and log data. The results of the study shed light on the impact of overburden stiffness, the extent, nature, and development of reservoir compaction, and the compaction to subsidence transfer ratio (C/S ratio) for the field.
P. 177
Integrating Wellbore Data and Geomechanical Modeling for Effective Characterization of Naturally Fractured Reservoirs
Authors: Olson J.E. , Hennings P.H. , Laubach S.E. 
Published: December 2007

Abstract
The quantification of fracture spacing and length are crucial for determination of effective block sizes to be used in reservoir simulation. Outcrop and modeling studies have shown that average values of these parameters do not adequately describe a fracture pattern, however. Observational work suggests that the cumulative number of fractures larger than a given size (in length or aperture) can be described with power-law relationships, and these empirical models are used to extrapolate from the observational scale to other scales of interest. Fracture spacing distributions are commonly described relative to their "saturation level," a measure of the regularity of the spacing between fractures of a given set that is thought to be related to strain level. As a spacing population approaches a normal distribution it is said to become more saturated or "well-developed," and this progression is assumed to be related to increasing strain imparted to a rock layer. A complication observed in sandstone outcrops from Oil Mountain, a Laramide anticlinal structure in Wyoming, is that fracture sets can reach saturation at different levels of strain even in the same bed. Assuming that average spacing indicates the level of strain accommodated by a given fracture set, the cross-fold fracture set from Oil Mountain requires very high strain to become saturated (spacing on the order of ift for a 30 ft thick bed). In contrast, the fold-parallel set displays a very high saturation at extremely low strain level (30 ft average spacing for a 30 ft thick bed). With a geomechanical model, we investigate the flindamental processes that control fracture propagation to explain spacing variability and suggest how to develop predictive models of fracture spatial arrangement.
In addition to discussing the mechanics of fracture network development, we address the sampling problems inherent in reservoir characterization. For subsurface fractures, measuring the saturation, length distribution, or connectivity is highly problematic because boreholes rarely encounter the fractures of interest. This is a serious impediment to acquiring sufficient data to constrain predictive characterization models that feed into reservoir simulations. Using Oil Mountain as an outcrop analog of a sandstone reservoir, we explore the use of microfractures as indicators of the attributes of large fractures. The advantage of microfractures is they are so much more abundant than macrofractures that they can be readily sampled from weilbores (a single thin section can produce thousands of measuments). If microfractures can be used as proxies for large fractures, then the results of predictive characterization modeling can be applied to subsurface cases much more effectively and accurately than has hitherto been possible.
P. 221