ABSTRACT: Geomechanics plays a vital role in reducing drilling problems. Parameters such as Young's modulus and Poisson's ratio are calculated from acoustic logs (sonic wave transit time). Occasionally, these logs are not recorded because of many factors such as time saving or cost cutting. In such cases, empirical correlations are used to calculate the sonic transit times. But none of these empirical correlations are universally acceptable. As a result, inaccurate values can potentially raise major concerns throughout the life of the well. The objective of this paper is to develop a robust empirical model for compressional and shear wave transit times using artificial intelligence techniques for unconventional reservoirs. For this purpose, well logs data was used from a tight sandstone formation to predict the transit times. Artificial neural networks (ANN) was used in this study. The ANN models predicted the sonic waves with very high accuracy with a correlation coefficient (CC) up to 0.96 and an average absolute percentage errors (AAPE) as low as 2%. The shear wave transit time prediction from the new model was also validated using available sandstone empirical equations.

ABSTRACT: Matrix acidizing in carbonate reservoirs can enhance productivity by injecting acid into formation to dissolve carbonate mineral and create long highly conductive channels known as wormholes. Nevertheless, the carbonate rock after dissolution reaction is susceptible to mechanical failure under reservoir depletion since its mechanical strength becomes weaker. This paper investigates the effects of wormhole geometry and rock strength weakening on wormhole stability by coupled hydro-chemo-mechanical simulation. The acidizing process is firstly simulated using in-house acidizing simulator to get wormhole geometry. Next the reservoir depletion process for the acidized formation is simulated using in-house fully coupled geomechanics-reservoir simulator. Simulation results have shown that most of wormholes can sustain the mechanical stability during the production phase. The stress relaxation in the wormhole region due to the arching effect prevents the damaged rock from failure despite low mechanical strength of the wormhole region.

ABSTRACT: Biogenic coalbed methane (BCBM) reservoirs aim to convert the carbon in u coal to methane using microbial techniques. Given the nascency of the technology, there is a dearth of reservoir scale investigations of the effects of bioconversion. This paper presents the results of high resolution microscopy, aimed at capturing the changes in pore structure as a result of bioconversion. The images were processed to measure pore-strains due to bioconversion, which was used as an input parameter for a new poro-mechanical model, developed to describe the stress and permeability behavior of BCBM reservoirs. Imaging results showed that the fracture porosity varies significantly post-bioconversion. Fractures wider than 5 microns, widened post-bioconversion, whereas fractures narrower than 5 microns reduced in width. Reservoirs with wide fractures exhibited significant enhancement of the state of effective stress and modeled permeability with depletion. Reservoirs with narrower cleats indicated the exact opposite. These results clearly demonstrate the importance of geologic/reservoir assessment prior to selecting a reservoir for biogenic treatment. Reservoirs with a large distribution of wide cleats would not only enhance gas production rates by providing the microbes with more surface areas, but also boost subsequent flowrates of methane.

ABSTRACT: Cement paste is used in several areas of civil and petroleum engineering. In oil/gas wells, it forms the cement sheath that provides mechanical support to wells and contributes to fluid isolation in different formations. Evaluation of the stress state in order to preserve well integrity during, for example, a casing test, requires simulations of hydration and loading history from placement until final hydration and also the knowledge of poroelastic parameters of the cement paste at every degree of hydration. Conventional laboratory experiments, such as drained or undrained isotropic compression, are extremely time-consuming and costly, and they are practically impossible to be performed at every degree of hydration. Micromechanical model and homogenization method are usually used to estimate the properties of porous materials. The poroelastic parameters of a class G cement paste were simulated assuming a microstructure in which the C-S-H HD cover some clinker phases and are covered by C-S-H LD. The results are validated on static and dynamic undrained oedometric modulus as well as the static oedometric Skempton coefficient.

ABSTRACT: We measure acoustic travel-times in Green River shale both parallel and perpendicular to bedding to observe the effects on petrophysical properties of interactions with water (fracture fluids) for different durations. X-ray diffraction shows the samples dominantly comprise carbonate and quartz. Scanning electron microscopy shows morphology of the minerals and pore network of the shale. Horizontal and vertical helium permeability measurements are conducted to analyze permeability evolution under different stress conditions. The samples record very low permeabilities at recreated confining stresses – indicating the need to be stimulated. Permeability decreases with increasing confining stress. Permeability increases with increasing pore pressure due to gas slippage effects. Increasing effective stress generally decreases the permeability. Acoustic travel-time measurements show that compressional and shear wave velocities increase with confining stress. Shear, Young's and bulk modulus all increase with confining stress with the more rigid samples exhibiting higher fracture conductivity. Compressional and shear wave velocities decrease as the shale is exposed to water and quartz dissolves from the pore structure, reducing moduli and acoustic velocities of the samples that exhibit lower fracture conductivity. Measured porosities suggest greater accessibility of the pores from a direction parallel to bedding.

ABSTRACT: Characterization of rock permeability change during the fracture network evolution is critical to the design and assessment of geological storage facilities. Under geo-storage conditions, rock fractures propagate during damage processes and re-bond during healing. However, natural heterogeneities in rocks significantly reduce experimental repeatability. Moreover, healing is a time-consuming process and requires high pressure or temperature to achieve. In this study, we resort to the 3D printing technology for specimen preparation so that complex rock microstructure changes are precisely controlled. We validate the specimen preparation by comparing experimental permeability test results with analytical solutions. Channel networks with different tortuosity, porosity, fracture branch, and structural anisotropy are designed to mimic various microstructure changes in rocks. To fabricate fractured rocks, we also embed heterogeneous fracture networks into 3D printed specimen and modify fracture geometry to account for anisotropic microstructural changes. Experimental findings revealed the evolution trend of permeability with these structural characterization parameters. They also match well with those obtained from computational fractured rock models under identical hydro-mechanically coupled conditions. This study is expected to provide new insights into the dependence of rock permeability on its microstructures under complex geo-storage conditions.

ABSTRACT: Acoustic properties of dry or saturated porous media can be studied numerically by a combined use of Lattice Spring Models (LSM), Lattice Boltzmann models (LBM), and coupled LBM-LSM models. The methodology is briefly described and it is systematically applied to a series of Fontainebleau sandstones whether they are dry or saturated with incompressible or compressible fluids. The porous structure of these sandstones is measured with Computed Micro Tomography (CMT). The velocities of P-waves and S-waves are determined. While the simulation results are in good agreement compared with the existing methods and experimental data, the velocities are always slightly overestimated; the origin of this bias is tentatively studied.

ABSTRACT: Acidizing and wormholing in carbonate reservoirs is studied through the analysis of the linear acidizing problem theoretically and experimentally. Experimentally, linear acidizing tests were performed in Mons chalk, a high porosity analogue of North Sea reservoir chalk. In the experiments the critical injection velocity for wormhole formation at minimum acid injection was obtained. Theoretically acidizing was analyzed by formulating the problem as reactive moving boundary Stefan type problem. Wormholing was analyzed through a linear stability analysis from the equilibrium state of uniform dissolution. The results of the stability analysis determined the critical injection velocity for wormholing and size effect with respect to the specimen diameter.

ABSTRACT: Understanding the damage mechanism of fractured rock is of great significance to the long-term stability analysis of rock masses. Cyclic uniaxial compression tests were carried out on rock specimens containing pre-existing 3-D surface flaws with different inclination angles. The experiments were performed by varying maximum cyclic stress levels and the test specimens were not completely destroyed to achieve the purpose of controlling rock damage. The rock porosity was measured by nuclear magnetic resonance (NMR) technique before and after experiments. The experimental results show that the higher the maximum cyclic stress level, the greater the axial strain. Different flaw inclination angles have an effect on the stress-strain behavior of rock specimens. And the inclination angle of 43° has the greatest influence on the reduction of specimen strength. Additionally, in terms of porosity, the final porosity is greater than the initial one. Under the experimental conditions of the present paper, when the maximum cyclic stress level is 25.6 MPa, the average value of incremental porosity is the largest, and the average value of irreversible strain is also the largest. This is related to the rate at which the specimen is compacted and the rate of expansion. Therefore, NMR technique is an effective means to quantify rock damage under cyclic uniaxial compressive loading.

ABSTRACT: Instability in geotechnical structures such as dikes or earth dams may be induced by internal erosion. In this phenomenon, finer soil particles are plucked off from the solid matrix by seepage flow and transported through the pore domain over time. Due to the loss of a part of the solid phase, the mechanical properties of the soil are progressively degraded, and, with time, earthen structures are under considerable risk of failure. Research on this topic is still fairly new and much more needs to be understood. In this study, a hydro-mechanical model based on porous continuous medium theory has been proposed to assess how internal erosion impacts the safety of earthen structures. An elasto-plastic constitutive model for sand-silt mixtures has been developed to describe the effect of the evolution of both porosity and fines content induced by internal erosion upon the behavior of the soil skeleton. The model is developed within finite element method framework. It is applied to a dike subjected to internal erosion induced by the presence of a karstic cavity beneath the alluvium layer.

ABSTRACT: Solid particles have been widely used in the petroleum industry for applications including fluid diversion, wellbore strengthening and hydraulic fracturing. The corresponding permeability and fluid flow through packed particles (diverting agents or proppant sands) dictate the effectiveness of such operations. However, to account for the complexity of binary particle mixtures, a more generalized model is required to accurately predict the porosity and permeability, and hence fluid flow.

ABSTRACT: The injection of fluids into a compartmentalized formation induces pore pressure buildup and decreases the local effective stress. Such changes of effective stress may result in re-activation of sealing faults. Variables that affect the amount of pore pressure change include the injected volume and the rock compressibility. This latter has been traditionally measured with isotropic loading compressibility tests. However, reservoir rocks under depletion or injection typically follow a uniaxial strain stress path, rather than an isotropic stress path. Furthermore, injection unloads the reservoir rock rather than loading it as it occurs during depletion. This paper reports experimental measurements of the unloading uniaxial strain compressibility of Frio sand, a member of Tertiary strata in the Gulf of Mexico Basin. The uniaxial strain unloading compressibility of Frio sand is about one third of the uniaxial strain loading compressibility in the range of mean effective stress from 10 to 14 MPa. The uniaxial strain compressibility is expected to be roughly one half of the isotropic compressibility. Reservoir simulation of CO 2 injection in a compartmentalized formation indicate that using isotropic loading compressibility considerably underestimates the expected pore pressure increase.

ABSTRACT: In view of the complicated flow mechanism of shale gas flow, multi-scale media and multiple pressure systems of organic matter/inorganic matrix pores/natural fractures and discrete hydraulic fractures are established to accurately describe the gas flow in shale reservoir. The mathematic model considers mechanisms of non-equilibrium desorption of the adsorbed gas, the viscous flow and Knudsen diffusion of the free gas, and the influence of inorganic matrix pores, natural fractures and hydraulic fractures pressure system on shale deformation. A fully coupled solid-fluid coupling model is established, and it is solved with finite element method. It is found from the results that the pressure drop in natural fractures and matrix pores is the main influence factor that causes stress sensitivity during production, and natural fractures play a more important role. With the increase of stimulated reservoir volume, the stress sensitivity is more significant, especially in the early production stage. The proposed fully coupled solid-fluid model of shale gas flow have the ability to study the influence of solid deformation on different pore pressure systems, which consider the non-linear flow mechanisms. The model is also of great value for predicting the shale gas production more accurate.

ABSTRACT: An approach for constructing a multi-scale parametric Rock-Physics model of effective elastic properties of fractured reservoir rock is presented. The approach provides a link between three scales - core, log and seismic that correspond to conventional works with reservoir rocks in geophysical prospecting practice. At each scale the Rock-Physics model incorporates parameters specifying rock microstructure at the respective scale including shape, orientation, volume concentration and connectivity of voids (pores, fractures, and cavities). The modeling follows the principle “from small to larger heterogeneities” and incorporates elastic anisotropy of any type caused by preferential orientation of fractures. Evidence for fractures at the core and log scales are provided by different techniques commonly used for the analysis including electronic and optical microscopy, and computer tomography for the core scale and results of FMI and UBI for log scale. Fractures at the seismic scales are inserted in the Rock-Physics model based on the results provided by the geomechanical modeling. At each scale a probabilistic solution for the model parameters is obtained.

ABSTRACT: In order to assess the performance of radioactive waste repository in argillite, the investigation of gas migration properties is indispensable. In this work the pore space morphological model is generated through union of excursions of Gaussian random fields with respect to the experimental pore size distributions. Pore space characteristics are investigated through purely geometric analysis using morpho-mathematical operations. This paper firstly gives a brief review on the model generation and the simulation of gas breakthrough process in COx argillite, then focuses on the water imbibition process induced by the gas pressure reduction after breakthrough. Several important parameters such as the gas shut-off pressure, the imbibing water saturation and the residual gas saturation are numerically predicted.

ABSTRACT: Geomechanics provides vital information on the u stresses within a basin, reservoir stability and the impact on fluid flow. This information is pivotal when assessing the viability of a reservoir. This study aims to quantify the geomechanical properties of key reservoir horizons in the Maui-Maari Fields, Taranaki Basin, to refine stress data for modelling, and determine empirical relations, for correlation between laboratory and regional scale. Despite development in the resources hosted in the Maui-Maari Fields, industry currently relies on non-site specific geomechanical properties of reservoir rocks. Initial results indicate significant under-estimation of UCS using existing models, not calibrated to the Taranaki Basin. This has a direct impact on borehole stability and regional stress field calculations, thus downstream implications for any decisions regarding well design and field planning. Laboratory testing is undertaken on outcrop analogues samples to generate the geomechanical data, including porosity, permeability and UCS. Following a practical approach, empirical relations will be determined between the geomechanical properties, specific to the region. In-depth investigation of the reservoir petrophysical properties provides an insight into the role the rock fabric plays on the geomechanical behaviour and the controls on the ability of fluid to flow.

ABSTRACT: Wells used for steam-assisted gravity drainage (SAGD), for cyclic steam stimulation (CSS), for hydrocarbon production in areas with anomalous high geothermal gradient, or for geothermal energy extraction all are operated in high temperature environments where maintaining long term wellbore integrity is one of the key challenges. Changes in cement mineralogy and associated mechanical properties may critically affect the integrity of high temperature wells. In this study, the relation between changes in mineralogy and mechanical properties of API class G cement with 40% silica flour was investigated by exposing samples for 1-4 weeks to temperatures of 60-420°C. The effect of mineralogical changes on mechanical properties was investigated using a combination of chemical and microstructural analysis and triaxial strength tests at confining pressures of 2-15 MPa. The combined effect of changing porosity, microstructure and strength of mineral phases determines changes in mechanical behavior, in particular the formation of stronger mineral phases with higher density affect Young's modulus, failure strength and residual strength. Deformation of the cement is more brittle at low confining pressures and more ductile at high confining pressures with a considerable residual strength after failure and highest reduction in cement stiffness and increase in strength at temperatures of ~250°C.

ABSTRACT: In view of the physical–chemical properties of CO 2 , this paper studied the impact of CO 2 –brine–rock interaction on rock properties and fracture propagation during supercritical CO 2 fracturing in Chang-7 tight sandstone. A series of static soaking experiments and true triaxial fracturing experiments were performed to investigate the influence of CO 2 –brine–rock interaction on rock properties and fracture propagation during supercritical CO 2 fracturing. Experimental results showed that calcite, dolomite, K-feldspar, and albite, were variably dissolved during the static soaking experiment. With the increase of reaction time, the number of dissolution pores increased and the pore size enlarged, which caused the enhancement in porosity and permeability (up to one order of magnitude) and the decrement of tensile strength (up to 47%). Compared with slickwater fracturing, supercritical CO 2 fracturing reduced the breakdown pressure by 15% and increased the number of fractures. The soaking treatment of the open-hole section of fracturing specimen with CO 2 -saturated brine reduced the breakdown pressure by 21% and improved the fracture complexity conspicuously in the case of supercritical CO 2 fracturing. The obtained results indicate that the physical–chemical properties of CO 2 can improve the fracture complexity effectively during supercritical CO 2 fracturing in Chang-7 tight sandstone formation.

ABSTRACT: Carbon dioxide (CO 2 ) sequestration is one of the emerging strategies to contain anthropogenic carbon. Successful application of CO 2 based Enhanced Oil recovery (EOR) techniques and its storage in unconventional reservoirs requires comprehensive understanding of CO 2 -fluid-rock interaction. The aim of this study is to characterize the microstructural evolution and chemo-mechanical interaction of shales with CO 2 rich brine. Shale assessment, in this case, is focused on the characterization of its constituent phases such as clay, quartz or calcite. Various types of Eagle Ford shales with different compositions have been considered. First, samples are exposed to CO 2 -rich brine at controlled temperature and pressure conditions. Nano-indentation and Energy Dispersive X-Ray Spectrometry (EDS/EDX) are then used to quantitatively and semi-qualitatively assess the changes in phases and in mechanical properties of the rock samples. Micro-computed tomography (micro-CT) is concurrently used for analyzing the spatial distribution and textural alteration of constituents of shales after reaction. Results have revealed the existence of three regions of starkly different behaviors after CO 2 exposure: a dissolution zone, a precipitation zone adjacent to the previous zone followed by a non-reacted zone. In each of these zones, phases present different mechanical properties, and their thickness and location vary with chemical composition and porosity.

ABSTRACT: Very soft carbonate rocks belonging to the Calcarenite di Gravina Formation (upper Pliocene-lower Pleistocene) crop out extensively in the Apulia region (Southern Italy) and, as such, are involved in a huge amount of engineering boundary value problems regarding building foundations, underground caves and sea cliffs. These calcarenites are poorly graded and dense, irregularly cemented, light yellow coloured and mainly composed of bioclasts. Throughout the region, these rock materials exhibit spatial (both horizontal and vertical) variability as a consequence of a complex depositional mechanisms so that their overall behaviour is strongly conditioned by depositional fabric and diagenetic processes. At the rock mass scale, post-depositional tectonic episodes and groundwater circulation have to be considered. In order to explore all the factors influencing the physical and mechanical response of these materials at the sample scale, a large set of data on different facies was collected and compared among them. Therefore, petrographic analysis on thin sections and, water imbibition, uniaxial compression and indirect tension (Brazilian) tests were carried out on samples retrieved from several sites of the region. In particular, the mechanical behaviour of these calcarenites results to be strongly affected by the presence of water in the pores, with strength and stiffness that significantly reduce in the transition from dry to saturated conditions; times and modes of saturation processes are strictly linked to the fabric features of the different facies. The post-peak behaviour of the rock samples was investigated by using a servo-controlled testing machine and a clear brittleness was generally observed for most of the samples, even though strength decay heavily reduces under saturation conditions especially for those samples characterized by an extremely variable distribution of cement.