Mechanical Core Damage Investigation and Mitigation

Changes in the physical properties of the supercritical CO2 (scCO2) reservoir rock is one of the most important factors in controlling the storage safety at a scCO2 sequestration site. According to recent studies, it is probable that geochemical reactions influence changes in the rock properties after a CO2 injection in the subsurface, but quantitative data that reveal the interrelationship of the factors involved and the parameters needed to evaluate the extent of scCO2-rock-groundwater reactions have not yet been presented. In this study, the potential for employing the surface roughness value (SRRMS) to quantify the extent of the scCO2 involved reaction was evaluated by lab-scale experiments. For a total of 150 days of a simulation of the scCO2-sandstone-groundwater reaction at 100 bar and 50 °C, the trends in changes in the physical rock properties, pH change, and cation concentration change followed similar logarithmic patterns that were significantly correlated with the logarithmic increase in the SRRMS value. These findings suggest that changes in surface roughness can quantify the extent of the geochemical weathering process and can be used to evaluate leakage safety due to the progressive changes in rock properties at scCO2 storage sites.

Permeability within fault zones can vary through time due to repeated deformation events and rock–fluid interactions. Understanding the history of fault zone alteration is critical when building hydrogeologic models and evaluating the risk of mechanical rock failure during subsurface storage. Newly acquired drill core recovered within the fault damage zone and fault core of the Little Grand Wash fault (LGWF) is combined with observations from outcrop, optical petrography, computerized tomography image analysis, and ultrasonic velocity measurements to characterize the rock types and preserved structural deformation features within this fault zone. These data are used to understand the history of mechanical rock failure and mineralization associated with this fluid-charged fault system. We identify multiple structural features and use their cross-cutting relationships to understand the history of deformation and their association with changes in fault zone permeability and rock mechanical properties. At the LGWF zone, structural deformation features vary temporally and are used to recognize a decoupling between fault slip and fluid flow. The formation of most open-mode veins occurred after shear failure within the LGWF zone. Early-developed shear bands are cut by carbonate veins, that are in turn cut by shear fractures, followed by a second phase of vein formation and ultimately by folding in the fault core. This sequence of formation reflects changes in mechanical rock properties due to subsurface rock–fluid interactions and indicates that the alteration of rock matrix by secondary carbonate cement results in increased unconfined compressive strength, decreased permeability, and increased ultrasonic velocity. In this fault zone, and possibly other fault zones, the changes in rock properties associated with deformation can be detected through their geophysical response.

A shaft with an inner diameter of 6 m has been excavated by blasting in Tertiary soft sedimentary rocks and we have estimated the changes of rock properties, such as deformation modulus, permeability and fracture frequency, of the rock around the shaft. We have performed four kinds of in-situ investigations; borehole jack test, cross-hole seismic velocity measurement, water injection test and borehole television survey. We have carried out these investigations before and after the shaft excavation in the same locations, with the same sets of equipment and through the same procedures for accurate estimation of rock property changes induced by the shaft excavation. The measurement locations have been selected in order to estimate the dependency of the excavation disturbance on the distance from the shaft wall and geological conditions such as lithology and faults. Within 0.5 to 1 m from the shaft wall, detectable property changes have been observed. In the rock zone, some kinds of deformation moduli have decreased. On the other hand, the number and apertures of fractures have increased and permeability has also increased by 2 ~ 3 orders of magnitude in the rock zone. Blasting damage is supposed to be the main factor which has induced these rock property changes considering strength and stress condition of the rock. Effectiveness of each investigation method in the estimation of excavation disturbance is also discussed.

SummarySubsurface injection of fluids in a stress‐sensitive naturally fractured rock faces the problem of near‐wellbore fracture evolution and associated changes in rock properties. Numerical modeling of the changes in permeability and poroelastic properties of the near‐wellbore region is challenging due to the coupling between fracture dynamics and poromechanics across multiple length scales of fractures and the host rock. We present a numerical framework to model anisotropic and dynamic evolution in rock properties based on a coupled formulation of fluid flow, rock mechanics, and fracturing, where fracturing‐induced damage is used to update the rock properties. A generalized fixed‐stress method, which accounts for damage‐induced anisotropy in flow and deformation processes, is developed to sequentially solve the equations of flow, mechanics, and fracture evolution. We demonstrate the usefulness of our framework in quantifying the effects of injection rate variability, initial fracture distribution, and in‐situ stress state on the evolution in permeability, elastic stiffness, and the Biot parameters. Our framework does not require the computational mesh to conform to existing or future fractures, allows simultaneous growth of multiple randomly distributed fractures, and can be implemented relatively easily in existing coupled flow‐geomechanics simulators to extend them to model fracturing at the reservoir scale.

Historically, machine learning techniques and particularly neural networks have been applied in the domain of Geophysics to discern rock properties through a quantitative approach from seismic data and associated seismic attributes. However, the success or failure of neural network is dependent on amount of available data. In general, more labelled data during training of the neural network results in accurate predictions. Therefore, seismic based rock property prediction models and their calibration needs to be fed with labelled data coming from well logs in adequate amount. The fact that the number of wells are often limited can be a huge challenge in adopting neural networks predictions. Downton and Hampson (2019) developed a Hybrid Theory-guided Data Science (TGDS) technique to improve the training of the network by augmenting the amount of data used for training. This technique aims at overcoming the issue of scarcity of the well log data. By using the outputs of theory based components like Rock Physics theory as input to the data science component of augmenting the amount of the data. In this method, Rock Physics is applied to model the elastic response due to the changes in rock and fluid properties of the available well control to create a huge dataset of pseudo wells. Then, synthetic seismic gathers are modeled at the pseudo well locations to train a Deep Feed-Forwards Neural Network (DFNN). The parameters estimated during DFNN training with synthetic seismic gathers are then applied on a real seismic dataset. The final predicted P-impedance from this formerly described method was compared to conventional inverted Pre-Stack Elastic Inversion. This paper shows the application of the method of TGDS based synthetic seismic catalog generation followed by DFNN applied on real seismic dataset for a commercially oil producing field in North Sea. The reservoirs are injectite sands which are difficult to image but can be key pay zones with quite high porosity and permeability. Well log curves e.g. P-Impedance, Porosity and Water Saturation serve as guidelines to create the synthetic seismic data. Additionally, with the added advantage of increased synthetic data, pseudo wells were broken into training and validation datasets. So, the DFNN training is carried out on a portion of the training data subset. During the process of training the DFNN, the seismic data remains blind. The estimated DFNN non-linear operator is applied to the seismic data, however, it needs to be scaled to the real data as the training was performed on the synthetic seismic data. The final result of the data science and machine learning approach driven derived P-impedance showed higher frequency and better lateral continuity than the conventional theory based approach of Pre-Stack Elastic Inversion. Augmenting the amount of training data by generating synthetic data based on the statistics of from nearby well control and rock physics relationships is prime differentiator of this workflow. By using rock physics theory a large number of pseudo wells can be generated that gives a range of expected geologic scenarios. Key wells from nearby fields, that do not necessarily tie the seismic under consideration, can be incorporated into the analysis and also, if required additional wells could be incorporated into the analysis as drilled.

Stress, porosity, and failure-dependent compressional and shear velocity ratio and its application to wellbore stability

Weathering Rinds: Formation Processes and Weathering Rates

Weathering rates over 40,000 years based on changes in rock properties of porous rhyolite

Time-lapse seismic is often used for reservoir monitoring. Although some case studies have been reported, especially for offshore oil fields, similar onshore Abu Dhabi studies have been limited. Onshore monitoring challenges are mainly due to hard carbonate reservoir rock, in which fluid displacement invokes only minor changes in rock properties. Moreover, complex near-surface structures create strong surface-related noises such as surface waves and elastic scattering. These noises significantly degrade the repeatability if repeat data are recorded with positioning error of 1 meter. To address the problems above, permanent reservoir monitoring has been spotlighted recently. Although permanent receivers are widely used for seismology, few studies of permanent seismic sources have been reported. In this study, we demonstrate a permanent source, Accurately Controlled Rotational Operated Signal System (ACROSS), which has a fixed position and excites highly repeatable seismic waves by rotating an accurately controlled eccentric mass. We deployed ACROSS at the Aquistore CCS test field onshore in Canada and recorded the wavefield with permanent receivers buried underground. The ACROSS system was operated several times throughout one year. The data acquired in a continuous 45 day operation were analyzed to calculate variation of amplitudes. The result indicated that we had obtained very stable waveforms with less than 3% variation if the data is stacked sufficiently. We also compared the two datasets acquired at different times of the year by calculating the normalized root mean square (NRMS) as a repeatability index. The histogram showed that the peak of NRMS distribution approached ~15% with only front and back mute processing. These results support our conclusion that an ACROSS system has potential to be used for reservoir monitoring in onshore fields by providing highly repeatable data for an extended time period.

Mathematical simulation of sediment compaction

The Stamp-Load Bearing Strength of Rock An Experimental and Theoretical Investigation Though the stamp-load bearing strength of rock is one of the more relevant rock properties in many mining problems it has been neglected in comparison with the efforts devoted to the strength of rock under uniform compression. As stamp-load bearing experiments offer a simple and effective means for studying the strength of rock under triaxial conditions the bearing strength of the rock has been investigated in detail. Using fundamental equations derived by Love and Sneddon the axisymmetric stress field induced by the indentation of a flat-ended circular stamp into a plane rock surface has been studied. In addition to the normal representation of a stress field by means of stress trajectories and lines of equal principal stresses, potential fracture zones have been calculated for a typical Witwatersrand quartzite. The theoretical analysis of the stampload bearing problem resulted in the acquisition of a deeper knowledge of the complexity of the induced stress field and the fracture mechanism. It has also been shown that there exist serious restrictions of the elastic treatment of this problem. In order to get a more detailed understanding of the actual failure mechanism and to establish the effects of the non-linear behaviour of rock near to failure some rock specimens were instrumented with strain gauges and cantilevers to measure surface strains and deflection outside the contact area of the stamp. From these experiments the conclusion can be drawn that lateral expansion of fractural rock is an integral part of the failure of rock under stamp loads. Based on these measurements a qualitative model of the failure process has been developed. The experimental work on the stamp-load bearing strength of rock has resulted in the following findings: 1. The load-displacement curve of the stamp-loading experiments is more sensitive to changes in rock properties than that obtained from compression tests. 2. The stamp-load bearing strength of rocks has been found to be affected by the size of the stamp. The size-dependent behaviour of rock is more obvious on brittle than on ductile rocks. It can be described by an experimental law of the formσ st =Q a a , withα being between 0 and −0.5 for ductile and brittle rocks respectively. At large stamp diameters the bearing strength tends towards a constant value of approximately four times the uniaxial compressive strength of rock. 3. A direct relation between the exponentα and the inelastic part of the lateral expansion of rock at failure has been found. This enables the determination of the lateral expansion of rock from stamp-loading experiments using different stamp sizes.

Insufficient attention is currently paid to experimental studies aimed to assess the rock strength. Therefore, the objective of this paper is to study the changes in rock properties in the main fracture zone under compression and stretching conditions. The research paper reviews methods to assess fracture activity in mine workings prone to rock bursts. The possibility of detecting near-fracture changes in the rock mass of the mine workings has been determined using acoustic and electromagnetic emission detection devices (equipment) currently available on the market. Work has been carried out to determine acoustic emission (AE) and electromagnetic radiation (EMR) parameters during uniaxial and triaxial loading of rock samples.

P-wave seismic data are routinely employed to map changes in rock properties in the subsurface. Converted-wave (P-SV) data provide additional independent measurements of rock properties which can help the interpretation of subsurface images (Ensley, 1984 ; Miller, 1994).

Geophysics![Figure][1] A small-scale eruption of Mount Aso was also recorded in April 2016. CREDIT: KUZNETSOV ET AL. ; KYODO KYODO/REUTERS PICTURES The dangerous and active Aso volcanic cluster appears to have put an early end to the damaging magnitude 7.1 Kumamoto earthquake that struck Japan in April 2016. Lin et al. found that the fault rupture stopped underneath the Aso caldera. The unzipping of the fault ended where the rocks went from cold and brittle to a more liquid-like magmatic mush. This distinctive example shows how abrupt changes in rock properties can terminate fault rupture and cap the size of an earthquake. Science , this issue p. [869][2] [1]: pending:yes [2]: /lookup/doi/10.1126/science.aah4629

The Anschutz Ranch East Field is a large asymmetric anticlinal trap in the Wyoming-Idaho-Utah thrustbelt of the western USA. The aeolian Jurassic Nugget Sandstone reservoir unit is a layered sequence of dunes and stratigraphically flat interdunes. This interpretation is supported by patterns of reservoir performance in the gently-dipping, nearly planar backlimb of the fold where there is little structural deformation. Structural features which developed during emplacement of the Anschutz Ranch East truncation anticline are partially dependent on the distribution of lithofacies, and the changes in rock properties which result from the deformation are overprinted onto the pre-existing rock properties. An extensive coring programme demonstrates that most of the strain in the Nugget Sandstone is in the crest and forelimb of the fold. Throughout the structure, the deformation is primarily of the compactive (sealing) type, leading to lowered porosities and permeabilities, and thus both to degraded reservoir quality and to flow barriers. Structurally-created low-permeability zones are identified or interpreted at a range of scales, and production histories provide sufficient evidence to establish the role of the larger features as flow partitions.