| Introduction to Geo-mechanical Problems in Oil and Gas Production | read abstract | read presentation as pdf |
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| Tron Golder Kristiansen, BP Norge |
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| Formation evaluation and geo-mechanics - Development trends | read abstract | read presentation as pdf |
| Eiliv Skomedal, StatoilHydro |
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| Laboratory Measurements of Wave Velocities in Shale vs. 4D Seismic Response from the Overburden | read abstract | |
| Rune M Holt, Erling Fjær, SINTEF Petroleum Research / NTNU Jørn F Stenebråten, Eyvind F Sønstebø, Audun Bakk, SINTEF Petroleum Research |
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| High resolution 3D Mechanical Earth Model using seismic neural net modeling: Integrating geological, petrophysical and geophysical data | read abstract | |
| Farid Reza Mohamed, Andreas Rasmussen, Anke Simone Wendt, Andrea Murineddu, Schlumberger |
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| Investigation of the stress-dependence of static and dynamic moduli of sandstones using a discrete element method | read abstract | read presentation as pdf |
| Liming Li, Idar Larsen, SINTEF Petroleum Research Erling Fjær, SINTEF Petroleum Research / NTNU |
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| Stress inversion in nice holes in massive rocks, and stability of not-so-nice boreholes in laminated rocks | read abstract | read presentation as pdf |
| Carsten Vahle, Birger Hansen, Eriksfiord |
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| Depletion-induced reservoir compaction: Mechanism, models and their application | read abstract | |
| Peter Schutjens, Rob van Eijs, Dirk Doornhof, Shell |
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| Borehole Triple Breakouts - Fiction or a Reality in Nature? | read abstract | |
| Javier A. Franquet, Cormac Parsons, Baker Hughes Khalid Ahmed, Kuwait Oil Company |
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| Technology reduces Non-Productive Time (NPT) at Valhall - a step forward for 'No Drilling Surprises' | read abstract | read presentation as pdf |
| Roar Flatebø, Tron Golder Kristiansen, Pete Heavey, BP Norge |
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| Simulating the Stress Field around Salt Structures: an Integrated Approach | read abstract | |
| Wouter van der Zee, Martin Brudy, Cem Ozan, Marc Holland, Geomechanics International Inc. |
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| Shale mechanics for borehole stability: From generated input data to predicted stable mud weight window | read abstract | |
| O.-M. Nes, E.F. Sønstebø, J.F. Stenebråten, SINTEF Petroleum Research E. Fjær, R.M. Holt, SINTEF Petroleum Research / NTNU |
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| Anisotropic Wellbore Stability Calculations Using Enhanced Mechanical Earth Modeling | read abstract | |
| Anke S. Wendt, Marit Kongslien, Bikash K. Sinha, Badarinadh Vissapragada, Adam Donald, Schlumberger Eiliv Skomedal, Lasse Renlie, Erik Sandtorv Pedersen, StatoilHydro |
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This presentation will provide an overview of geomechanics applications and problems in the oil and gas industry. The presentation is written in simple terms. The presentation is aiming at providing the reader with a brief overview of typical applications of geomechanics in the oil and gas industry. Geomechanics is a fairly small discipline in the oil and gas industry compared to the more traditional disciplines like geology, geophysics, reservoir engineering, production engineering and drilling engineering. Many people in the oil and gas industry do also look at geomechanics as a fairly new discipline in the oil and gas industry, but this is maybe more because of a growing understanding of the importance of geomechanics in the oil and gas industry.
The presentation will point at important development trends in the area of formation evaluation and geomechanics based on business needs as seen from an operator's point of view. One of these trends is overburden characterization. The industry has to change from the current data starvation in the overburden to cope with challenges such as drilling failures due to charging of the overburden from injection out of target, drilling failures of highly deviated wells, well damage above depleted HPHT fields, general improvement of seismic interpretation and understanding 4D effects in shales. One ongoing activity in this area is direct pore pressure measurement in shale. The perspectives driven by this activity will be described. The audience will then be challenged on how to effectively identify and characterize sub seismic faults in the overburden. Such characterization is essential both to enable robust wells with respect to compaction induced well shearing and to avoid injection out of target.
An associated development trend is utilization of high quality shear logging to improve strength characterization in shales, sandstones and other lithology. The close connection between shear stiffness and the grain skeleton has been known for a long time but the recent improvements of shear data gives new opportunities. But still we have to deal with the old obstacle: how to transform from dynamic to static properties.
Several time-lapse ("4D") seismic field observations world wide indicate increased travel time of seismic waves in the overburden due to stress arching above depleting reservoir sections. We present controlled laboratory measurements on overburden field shale cores and compacted clay specimens where the in situ stress path is simulated. Multidirectional ultrasonic P- (and S-) wave velocities are measured as a function of stress changes reproduce at least qualitatively the response seen in the field. We underline the importance of performing tests along the representative stress path, and discuss the use of strain sensitivity through a dilation parameter (often referred to as the "R"-factor) as a 4D attribute. Finally, we point to possible sources of error when comparing laboratory measurements quantitatively with field data.
A 3D Mechanical Earth Model (3D MEM) of a high-pressure high-temperature field was built integrating geological, petrophysical and geophysical data. The 3D MEM represented the close-to-initial property field conditions, and is an ideal basis for e.g. wellbore stability and formation integrity forecasting during initial and depleted field conditions. Using a neural networking approach, detailed dynamic rock properties at the wellbore level were integrated with dynamic rock properties derived from seismic inversion to obtain a "true" 3D property model. The lateral resolution of the model was driven by seismic trace density; the vertical resolution was driven by upscaled well log information.
The workflow was developed for constraining regional property trends that honor structural elements in the field, and allow to derive realistic rock property values and distributions even in areas where only limited well log information exists. The 3D MEM demonstrated that (1) Neural networking driven by seismic inversion produces rock property values that are nearly identical with the upscaled logs. This is a remarkable improvement compared to wellbore centered field models; (2) The distribution of the rock properties is controlled by main structural elements such as faults, erosional surfaces, layering and topography; and (3) The distribution of the rock properties has a high lateral and vertical resolution even in areas where wellbore information was sparse.
A new constitutive contact model which governs the contact behavior between bonded or unbonded elements has been developed and implemented in a discrete element model. Such a discrete element model mimics the non-linear elasticity and plasticity at grain contacts. The model also captures the effect of the development of microcracks due to stress alteration. The model has been used to simulate sandstone specimens which were loaded under different stress paths and had experienced different stress histories. The static moduli and the dynamic moduli of the model at different stress states were calculated. The simulation results show how the moduli were influenced by the stress states, stress path, and stress history. In particular, the simulations show the growing difference between the static and dynamic moduli as failure is approached. The simulations thus support the assumption that microscopic failure events play a major role for the difference between static and dynamic moduli, as well as for the failure process.
Under ideal conditions of round borehole in sealed elastic homogeneous rocks, one can, with interactive methods, quickly find a stress field which would produce the observed hole instabilities. Borehole imagers and dipole sonic tools form the ideal pair of instruments for stress observation in boreholes.
The field-wide stress field is essential knowledge not only to predict fracture and fault behavior in the reservoir, but also to plan new wells which tend to get more and more challenging through field life (longer reach, "shrinkage", branching...).
Examples will be given, in the form of stress distributions, of highly asymmetric situations involving irregular-shaped wells communicating with dipping laminated soft rocks at arbitrary angles. Analytical methods can be used in some situations, else numerical methods are applied.
Birger Hansen, the founder of the www.eriksfiord.com consulting group, is a geologist who has spent decades building methodologies for borehole imagery and reservoir modeling.. Carsten Vahle is a structural geologist who joined Eriksfiord AS two years ago. He works with structural and geomechanical interpretation of borehole images in Stavanger.
Production-induced depletion in water and hydrocarbon reservoirs leads to deformation, compaction, displacement, and stress change both inside and around the reservoir. Potentially serious consequences for production include well damage, permeability reduction, and vertical displacement at the Earth surface (subsidence). Well damage can occur inside the reservoir by e.g. buckling of the casing due to compaction-induced along-well shortening. Wells can also be damaged by compaction-induced fault slip, which is reported to occur mainly above the reservoir. Permeability reduction caused by compaction is typically a few percent of the pre-production value in consolidated rocks like sandstone, but can be tens of percent in poorly-consolidated granular rocks. Subsidence on land is a problem when operating in low-lying areas, certainly given observations and recent predictions of sea level rise due to global warming. Compaction drive is one of the few positive effects of reservoir compaction, but can be essential in high-compressibility granular rocks with low-viscous oil. The business impact of reservoir compaction in many fields drives our efforts to model it with analytical and finite-element tools. We will present some published examples of our technology, and discuss how the model results were used in our operations.
This paper presents field evidences of Triple Breakouts occurring around vertical boreholes drilled in a weak formation in Kuwait. Three distinct breakouts developed at the borehole wall 120-degrees apart each other in shallow oil reservoirs. The breakouts were identified on acoustic and resistivity wireline image logs from three independent imaging tools. The manuscript does not fully explain why these breakouts are formed nor how to predict their size; instead, it shows real examples of this unique and very rare borehole shear failure. Triple breakouts have been created in the laboratory by drilling small borehole in cubical rock samples under isotropic horizontal stress conditions. Furthermore; borehole triple breakout failures under isotropic stress boundary conditions have been also predicted from discrete element modeling by previous researchers. To our knowledge, Triple Breakouts have never been observed in vertical boreholes drilled into the Earth?s subsurface. No field evidence of their existence has been published in the oil industry. The triple breakout evidences obtained from acoustic and resistivity wireline image logs were meticulously processed and analyzed in order to discard any other artificial cause that may create these borehole failures.
Norway's recent New Well Delivery success was the drilling of water injector G-14 on Valhall. Due to extreme reservoir compaction and resulting subsidence, the planned trajectory was predicted to have major stability issues and a high "trainwreck" potential. By integrating advanced computational modelling to quantify stress-changes and historical well comparisons coupled with ground-breaking visualisation, the well was delivered 61 days ahead of budget and schedule.
Wellbore stability problems while drilling close to salt structures are common. While in general for wellbore stability analyses the assumption is used that the vertical stress is a principal stress and equal in its magnitude to the overburden load, this is not a valid assumption anymore drilling close to a salt body. Because salt structures create, due to their shape and rheological behavior, a perturbation of the stress field with strong spatial variation of the principal stress magnitudes and orientations. To provide realistic stress input data for wellbore stability predictions the stress fields around salt structures are simulated using realistic 3D geometries.
This paper presents results of 3D finite-element simulations of stress fields around several salt structures ranging from a classic salt dome to allochthonous salt sheets. In the finite element models the salt is simulated as a creeping material which means that it can?t withstand any differential stress, and the adjacent clastic sediments are simulated as porous-elastic materials. The initial unperturbed stress situation is derived using well log data, pressure tests, and wellbore failures. 3D seismic data are used to derive interval velocities, based on which density, overburden and pore pressure can be estimated.
The presentation outlines the workflow used to create the finite-element model from structural information. We discuss the meshing algorithm developed by the authors which allows for high density meshing in the area of interest. The various formations are populated with mechanical properties using either log-derived or seismic data. In the final modeling step the initial stress state is applied to the model to calculate the detailed stress field around the salt dome.
The resulting fully three-dimensional stress tensor (which describes the stress orientations and magnitudes) is extracted from the finite element model and exported to a 3D visualization system. The visualization system allows us to extract the stress tensor along arbitrary well trajectories. To validate the model output we compare the stress outputs with available stress measurements and indicators such as (extended) leak-off tests or losses during drilling. After this calibration step we successfully used the input for wellbore stability predictions.
The workflow described in this presentation provides an efficient way to create realistic 3D finite-element simulations from complicated structural data. It is optimized for the best resolution around the area of interest (well trajectory) while limiting the size of the numerical problem to an order that can be handled in reasonable times. Most importantly, it is set up to reflect the variability of material properties both in horizontal and vertical direction in a geologically realistic way. Thus, this workflow allows for a detailed simulation of the stress field around salt bodies that is new to the hydrocarbon industry and helps to significantly reduce the risk for wellbore failures of increasingly costly wells drilled to exploit e.g. sub-salt plays in the Golf of Mexico.
This presentation will illustrate our integrated effort on providing relevant input data to borehole stability modeling in shales, as well as how the borehole stability modeling can be done utilizing such input data. Laboratory measurements of stress dependent mechanical and petrophysical properties of a number of shales have been performed on samples from cuttings to core size. The data provided by the various experimental methodologies - which are partly developed in-house - constitute a platform for analysis and modeling of time dependent borehole stability during drilling, using in-house developed software.
Focus will be on how mechanical and petrophysical key parameters for shales can be provided directly or indirectly from various data sources - be it through measurements on shale material stemming from cuttings to cores, or through models and correlations - depending upon which input data are available. In particular, experiments addressing fluid exposure effects on mechanical behavior, using brines with various concentrations of various salts will be described.
Geomechanical interpretations of inhomogeneous anisotropic materials are enhanced by three dimensional (3D) sonic measurements that provide compressional, fast shear, slow shear, and Stoneley wave slownesses in (an)isotropic drilled formations.
We investigated the Kvitebjørn high pressure high temperature (HPHT) gas/condensate field for enhanced Mechanical Earth Modelling (MEM), and wellbore stability analyses using three-dimensional sonic measurements in overburden and reservoir formations. The Kvitebjørn field is situated on the Norwegian continental shelf west of the Viking Graben.
In the HPHT Kvitebjørn field, major development challenges are the determination of the drawdown rate and the maximum depletion for optimum production while maintaining rock integrity. These challenges are directly related to the effect of changes in reservoir pressure on the in-situ formation stress state. Using advanced processing of sonic data, we estimated the amount of stress anisotropy, stress orientations, and stress magnitudes at particular reservoir intervals. These stress data together with measured in-situ minimum horizontal stress values were used to calibrate horizontal stress logs previously estimated in the MEM. Furthermore, we constrained rock elastic properties (deformation, strength) using sonic slownesses and elastic shear moduli calculated from the sonic dataset. These data were later used to estimate rock strength and the onset of wellbore deformation. The two approaches allowed us to calculate (1) isotropic rock properties from the sonic slownesses, and (2) vertical and horizontal rock properties from the calculated shear moduli.
Wellbore stability calculations were based on the isotropic rock property and stress data, and the vertical and horizontal rock property and stress data sets . We are able to demonstrate the superiority of anisotropic wellbore stability calculation, and the control of horizontal rock properties on formation integrity.
The work that we present here is intended to open a new direction for geomechanical modelling: To demonstrate a new measurement allowing practical consideration of rocks as an anisotropic material.