Numerical Simulation of Complex Fracture Network Development by Hydraulic Fracturing in Naturally Fractured Ultratight Formations

The creation of large complex fracture networks by hydraulic fracturing is imperative for enhanced oil recovery from tight sand or shale reservoirs, tight gas extraction, and Hot-Dry-Rock (HDR) geothermal systems to improve the contact area to the rock matrix. Although conventional fracturing treatments may result in bi-wing fractures, there is evidence by microseismic mapping that fracture networks can develop in many unconventional reservoirs, especially when natural fracture systems are present and the differences between the principle stresses are low. However, not much insight is gained about fracture development as well as fluid and proppant transport in naturally fractured tight formations. In order to clarify the relationship between rock and treatment parameters, and resulting fracture properties, numerical simulations were performed using a commercial Discrete Fracture Network (DFN) simulator. A comprehensive sensitivity analysis is presented to identify typical fracture network patterns resulting from massive water fracturing treatments in different geological conditions. It is shown how the treatment parameters influence the fracture development and what type of fracture patterns may result from different treatment designs. The focus of this study is on complex fracture network development in different natural fracture systems. Additionally, the applicability of the DFN simulator for modeling shale gas stimulation and HDR stimulation is critically discussed. The approach stated above gives an insight into the relationships between rock properties (specifically matrix properties and characteristics of natural fracture systems) and the properties of developed fracture networks. Various simulated scenarios show typical conditions under which different complex fracture patterns can develop and prescribe efficient treatment designs to generate these fracture systems. Hydraulic stimulation is essential for the production of oil, gas, or heat from ultratight formations like shales and basement rocks (mainly granite). If natural fracture systems are present, the fracturing process becomes more complex to simulate. Our simulation results reveal valuable information about main parameters influencing fracture network properties, major factors leading to complex fracture network development, and differences between HDR and shale gas/oil shale stimulations.

Safety at Sea: Improving Search and Rescue (SAR) Operations in the Barents Sea

In this paper, we select some of the crucial issues for future search and rescue (SAR) operations in the Barents Sea. The different nations that are involved and the resources necessary to build emergency preparedness due to the climatic conditions are thus important factors. This paper summarizes the state of the art within these areas while also indicating future development needs. The special requirements for life saving equipment on vessels due to the climate and requirement on personal protective equipment related to accidental immersion are also essential and thus presented in this paper. In addition, safe haven designs where the vessel itself is designed to provide shelter for personnel in distress is also a topic chosen to be addressed.

Development of a New Physical Model for Experimental Assessment of Expandable Casing Technology Effect on Wellbore Cement Integrity

The focus of this novel research was to develop a physical model and apply an experimental approach to explore the effect of casing expansion technology on wellbore cement integrity.

Dimensioning of Twistlocks in Modular Offshore Platform Designs for Arctic Regions

To mount service modules in a flexible manner on offshore platforms twistlock systems are used. If the application is not in accordance with standard conditions the existing regulations are not sufficient. Therefore new regulations with appropriate dimensioning equations must be generated. The results of the OMAE 2013 paper “Assessment of Container Mountings in Modular Offshore Platform Designs for Arctic Regions” are applied in this paper to reach such equations. The stress on twistlocks which mount standard containers to the offshore platform deck in arctic regions depends on roll angle of the offshore platform and friction coefficient — depending on temperature — and is analyzed by regression analysis in this paper. The results show that a local twistlock system can be used to simplify the calculation. Also the application of a crane module is simulated. The regression analysis is divided into two parts — the cone and the base of the twistlock: these parts take most of the load again depending on roll angle and friction coefficient. The result of this paper consists of dimensioning equations for these two different applications of twistlock systems in arctic regions.

Probabilistic Modeling of Extreme Value Distributions of Ice Loads on “Molikpaq” Platform for Sakhalin-II Project

Characteristics of the drifting ice cover and the scenarios of the ice loads on offshore structures are the major parameters defining durability and reliability of the ice-resistant platforms on the Sakhalin offshore. The study is devoted to the problems of probabilistic and numerical modeling of the process of interaction between the ice cover and the ice-resistant concrete structures on the Sakhalin offshore zone. Geometry of the “Molikpaq” (PA-A) platform for Sakhalin-II Project is used as an example. The input statistical data were received on the basis of full-scale observations of the ice conditions in the Piltun-Astokhskoe deposit area during 1989–2002. The distribution of probability exceedance of ice loads for various ice scenarios on the “Molikpaq” (PA-A) platform was received. A probabilistic estimation of extreme values of ice loads was carried out, taking into account return period of ice conditions.

Effectiveness of Geophysical Methods in Calcareous Harbor Fills

The effectiveness of electromagnetic (EM), ground penetrating radar (GPR) and seismic refraction (SR) were evaluated by surveying a shallow trench in which a number of objects of varying composition and size were buried. The trench was excavated in granular calcareous fill material. An experienced geophysical contractor was asked to provide blind predictions of object locations using each of the techniques in turn. GPR with a 400 MHz antenna was the most successful, followed by SR and EM surveying. GPR and SR were also carried out at the port of Hilo to investigate complex subsurface conditions.

Three Dimensional Fully Localized Waves on Ice-Covered Ocean

We have recently shown [1] that fully-localized three-dimensional wave envelopes (so-called dromions) can exist and propagate on the surface of ice-covered waters. Here we show that the inertia of the ice can play an important role in the size, direction and speed of propagation of these structures. We use multiple-scale perturbation technique to derive governing equations for the weakly nonlinear envelope of monochromatic waves propagating over the ice-covered seas. We show that the governing equations simplify to a coupled set of one equation for the envelope amplitude and one equation for the underlying mean current. This set of nonlinear equations can be further simplified to fall in the category of Davey-Stewartson equations [2]. We then use a numerical scheme initialized with the analytical dromion solution of DSI (i.e. shallow-water and surface-tension dominated regimes of Davey-Stewartson equation) to look for dromion solution of our equations. Dromions can travel over long distances and can transport mass, momentum and energy from the ice-edge deep into the solid ice-cover that can result in the ice cracking/breaking and also in posing dangers to icebreaker ships.

Two-Dimensional Model for Wave-Induced Pore Pressure Accumulation in Marine Sediments

In this paper, a two-dimensional (2D) porous model is established to investigate the predication of the wave-induced pore pressure accumulations in marine sediments. In the new model, the VARANS equation is used as the governing equation for the wave motion, while the Biot’s consolidation theory is used for porous seabed. The present model is verified with the previous experimental data [1] and provides a better prediction of pore pressure accumulation than the previous solution [2]. With the new model, a 2D liquefied zone is formed at the beginning of the process, and then gradually move down. After a certain wave cycle (for example, 30 wave cycles in the numerical example), the liquefaction zone will become one-dimensional (1D) and continuously move down and eventually approaches to a constant. Numerical results also conclude the maximum liquefaction depth increases as wave height increases and in shallow water.

Cementitious Sealing Material: 3D Digital Image Based Characterization of Pore Size Distribution and Modeling of Transport Properties

Exhausted oil and gas reservoirs are one of the most potential storage facilities to sequestrate the worlds CO2. These reservoirs are sealed with cementitious materials, that should have a long time performance. Therefore, this paper emphasizes the characterization of the evolving capillary pore network and transport properties of the cementitious microstructure used to seal the wellbore. The Hymostruc numerical model is employed to simulate the development of an evolving virtual microstructure of cementitious materials. The simulated 3D microstructures were then digitized to form a matrix of cubic voxels. The pore-size distributions of the obtained virtual microstructures were calculated using a combination of three-dimensional digital image processing algorithms: 1) distance transform and 2) medial axis thinning algorithm to obtain a 3D skeleton of the pore structure. Transport properties of the simulated microstructures are analyzed employing a finite difference 3D transport model. The modeling results are compared with available literature results.

Uniaxial and Triaxial Stress-Strain Behaviour of Ductile Cement Pastes Reinforced With Wollastonite Microfibers

When an oil well is submitted to cyclic steam injection the heating process induces tensile stresses in the cement sheath due to the thermal gradient that take place leading to cement-steel debonding and/or cement cracking. Similar problem can occur if the cement sheath is submitted to high creep deformations coming from the adjacent rock (this is the case for example of oil exploration in salt domes). In both cases sheath cracking can result in loss of hydraulic isolation and consequently in excessive water production with undesired economic and environmental consequences. In order to deal with this challenging scenario oil well cementing systems of special properties (e.g. high tensile strength, low elastic modulus and elevated toughness) should be used as an alternative to conventional high compressive strength systems. In this study cement pastes of high ductility were developed using wollastonite micro-fibers as reinforcement. The mixtures were developed within the framework of the Compressive Packing Model [1] and wollastonite microfibers were added in volume fractions of 2.5, 5.0 and 7.5 %. Uniaxial and triaxial compressive tests were carried out to obtain the unconfined and confined stress-strain behavior of the composites. The crack initiation stress and strain and the fracture process of the pastes under unconfined stress will be reported in this paper. Triaxial tests were performed under confining pressures of 0, 600 and 1200 psi and the Mohr-Coulomb criteria assumed to determine the internal frictional angle and cohesion. The results show that the addition of wollastonite microfibers increased the compressive strength of the pastes keeping the same strain capacity of the matrix. The internal frictional angle was also increased with the increase in the fiber volume fraction. However, the cohesion of the paste was reduced with the fiber addition.