Taheri, A. (The University of Adelaide)
Rocks in deep underground are subjected to different stresses inherent to natural orogeny, therefore, rock behaviour becomes complicated in response to extraction activities of deep earth resources. Rock behaviour and failure in deep underground is not well understood, leading to flaw engineering designs. In this sense, the complete stress-strain characteristics of intact rock, i.e. the pre-peak and post-peak stress-strain regimes are relevant in the understanding of the total process of rock deformation under in-situ stress. In this study post-peak characteristics of different rocks types, having unconfined compressive strengths of 7 to 123 MPa were investigated in a series of quasi-static monotonic uniaxial and triaxial compression tests. Post-peak stress-strain measurement was achieved by implementing lateral-strain rate feedback method in a closed-loop system to apply axial load to the rocks. In uniaxial and triaxial compressive testing, measurement of complete stress-strain behaviour showed i) either class II behaviour or a combination of class I-II behaviour and ii) a noticeable shifting from class II behaviour toward class I behaviour as the ratio between confining pressure to the unconfined compressive strength increases significantly. The results of the experimental study then were used to develop an index to describe the pre-peak and post-peak behaviour of the rocks, encompassing both confined and unconfined conditions. Reasonable correlation was observed between the new index and confining pressure.
Prediction of rock failure is very important for civil engineering and mining development projects. With this, engineers can investigate the effect of rock behaviour on drilling and excavation performance and rock burst occurrence in deep mining. Some researchers have attempted to develop brittleness indices to describe rock failure behaviour based on rock compressive and tensile strength (Hucka &; Das 1974, Coates 1966, Altindag 2003, Yarali &; Soyer 2011). In addition, several brittleness indices based on the energy of the rock under loading have been proposed (Baron et al. 1962). However, these indices are only considered pre-peak rock behaviour. Munoz et al. (2016a) found that brittleness indices that are developed based on rock strength and pre-peak rock behaviour are unable to predict rock drilling performance. As a result, they developed new brittleness indices to describe rock failure behaviour in uniaxial compressive loading based on pre-peak and post-peak properties (Munoz et al. 2016b). This approach, however, ignores the residual strength of rocks, which might be significant under confined loading conditions. In addition, brittle to ductile transition of rock post failure behaviour is overlooked in the previous work as only uniaxial loading condition were investigated. Therefore, it is required to develop relevant methods able to describe rock pre-peak and post-peak stress-strain behaviour during uniaxial and triaxial loading condition.
Zheng, Jianwei (Tiandi Science & Technology Co., Ltd. / China Coal Research Institute) | Ju, Wenjun (Tiandi Science & Technology Co., Ltd. / China Coal Research Institute) | Fu, Yukai (Tiandi Science & Technology Co., Ltd. / China Coal Research Institute) | Wang, Zhihe (The University of Adelaide) | Pan, Haibing (Tiandi Science & Technology Co., Ltd. / Holsinghe Mine, Shanxi Coal Import & Export Group) | Jiang, Wei (Tiandi Science & Technology Co., Ltd. / China University of Mining and Technology Beijing)
The distribution and evolution characteristic of abutment pressure is a key factor to controlling surrounding rock-mass in underground coal mining. Aiming to analyze the formation mechanism and dynamic evolution characteristic of abutment pressure, different surrounding rock-mass models of stope at corresponding multi-time-space conditions were established based on mechanics of materials. It is demonstrated in this study that the whole life cycle of a fully-mechanized mining face in flat seams, including open-off cut, regular advancing zone and finish line, could be divided into four stages which include: Initial stage (I), Steadily growing stage (II), Dynamic stage (III) and Final stage (IV), where the Final stage (IV) could be further divided into the higher pressure situation and lower pressure situation based on different surrounding rock-mass structures. The open-off cut zone and first collapse of immediate roof are within the Initial stage (I) and Steadily growing stage (II) respectively. The abutment pressure of Initial stage (I) is a static load and the abutment pressure of Steadily growing stage (II) is steadily growing with the advancing of mining face; the value of abutment pressure before periodic weighting is larger than that after periodic weighting when it enters the Dynamic stage (III); the roadways are easily controlled in lower pressure situation of Final stage (IV) and it may avoid the support crushing in higher pressure situation of Final stage (IV). According to the dynamic evolution of abutment pressure featured in life cycle of a mining face, it could make the prediction of high stressed zone, and reinforcement support or pressure-relief methods could be done in advance, thus the occurrence of dynamic hazards of surrounding rock-mass could be effectively prevented.
The stability of roadway surrounding rock-mass in underground mining plays a significant role in safety and high production, and can be easily influenced by the distribution and dynamic evolution of abutment pressure (Tan, 2012; Zhang, 2014). Qian et al. (1995, 1996) adopted key stratum theory and “voussior beam” hypothesis to study the interaction between overlying stratums movement and mining induced pressure. Xia et al. (2017) conducted numerical simulation of the whole process of longwall mining on stope pressure in underground mining using FLAC3D. Lai et al. (2014, 2016) studied the transformation mechanics of advanced abutment pressure through physical simulation. Hosseini et al. (2012, 2013) used passive seismic velocity tomography to analyze the variations of abutment pressure around the panel during coal mining. Ren et al. (2014) studied the dynamic feature of advanced abutment pressure in shallow mining through physical simulation，numerical simulation (FLAC3D) and in-situ observations. Xia et al. (2011) analyzed the wave form and inversion of the in-situ micro-seismic data obtained from a fixed mining face to determine the interaction between micro-seismic activities and advanced abutment pressure. Zhu et al. (2016) adopted micro-seismic monitoring technical for abutment pressure monitoring in coal mining. Jiang et al. (2002, 2006) analyzed the peak value of abutment pressure in coal mining through mechanical model on stope. Zhao et al. (2006) adopted ADINA finite element to discuss the range and peak value point of abutment pressure. Li et al. (2005) hold the view that the abutment pressure was consisted of static pressure and dynamic pressure. Zhou et al. (2016) used hollow inclusion strain cells measurement technique to get the evolution law of abutment pressure with mining face advancing. Zhang et al. (1994) studied the distribution law of abutment pressure with advanced support resistance. Pan et al. (2014, 2015) took the coal seam and immediate roof in front the mining face as elastic basement firstly, then used mechanical model to analyze the mechanical behavior of roof before periodic weighting.
Many flow models have been developed for rock fractures with complex void geometries. These models are mostly based on the Cubic Law assumption that the flow rate is linearly related to the pressure gradient and the cube of fracture aperture. Very few models account for the non-linearity in flow that occurs when the Reynolds number exceeds its critical value. This paper presents a non-linear model for flow in two-dimensional rock fractures based on a second-order perturbation solution to the two-dimensional Navier-Stokes equations under the pressure boundary condition. The effects of fracture geometrical properties and the Reynolds number are both considered in the model. The proposed flow model was validated using a two-dimensional fracture by comparing the results with those obtained by directly solving the Navier-Stokes equations in COMSOL. The results from this study show that the proposed model can capture well the nonlinear flow behaviour for Reynolds numbers ranging from 0.1 to 100.
Fluid flow in rock fractures is an important process in many underground geotechnical applications including in-situ mineral recovery, underground water systems and hazardous waste disposal (Brown 1987; Zimmerman and Yeo 2000; Berkowitz 2002; Hunt and Sahimi 2017). Numerous fracture flow models have been proposed to describe the flow behaviour in complex fracture void geometries (e.g. Zimmerman et al. 1991; Ge 1997; Brush and Thomson 2003; Konzuk and Kueper 2004; Wang et al. 2018a). These models are mostly developed on the basis of the assumption that the Cubic Law (CL) (Lomize 1951) is valid either throughout the entire fracture or within a specified scale. In these models, modifications to the aperture field are incorporated by considering the flow tortuosity and local roughness. Although flow predictions may be improved, most of the existing models only provide reasonable estimations when flow is predominately linear, i.e., when the Reynolds number (Re) is less than 10. However, the flow rate tends to be non-linearly related to the given pressure gradient for Re>10, and the non-linearity of the flow is widely reported from both flow experiments and numerical simulations (e.g. Tan et al. 2004; Zimmerman et al. 2004; Javadi et al. 2010; Zou et al. 2015).
The aim of this study is to present a non-linear flow model which accounts not only for the geometrical effects but also the influence of Re on the behaviour of flow through rock fractures. The proposed model is validated against the numerical solution of Navier-Stokes equations (NSE) in a two-dimensional fracture for Re from 0.1 to 100. Results from this study show that our proposed model captures well the non-linear flow behaviour for the range of Re examined.
2.1 Governing equations for fracture flow
In general, fluid flow in rock fractures can be considered as a single phase, incompressible, steady state laminar flow, which is governed by the NSE (Zimmerman and Bodvarsson 1996; Crandall et al. 2010) as follows:
where ρ is the density, u is the velocity vector, μ is the viscosity and P is the reduced pressure. The left-hand side terms in Eq. (1) are the advective acceleration terms and the two terms on the right-hand side of the equation are, respectively, the pressure gradient and the viscous force terms. For incompressible fluid, such as water, mass conservation is assumed throughout the fracture, which gives:
Under the creeping flow condition, where the advective acceleration terms are negligible compared with the viscous force terms (Brush and Thomson 2003; Zimmerman 2005), the NSE can be reduced to the Stokes equations:
In this case, the flow rate would be linearly proportional to the pressure drop along the flow direction. If at a relatively small scale, the upper and lower surfaces of the fracture are represented by smooth plates, Eq. (3) can be integrated along the local aperture by incorporating the non-slip boundary condition, yielding the Local Cubic Law (LCL) (Brush and Thomson 2003) in the form:
Relative permeability curves in coalbed methane reservoirs (CBM), acquired by analysis of production data, can differ from laboratory-measured curves due to complications such as stress-desorption dependent permeability and cross-formational flow. This paper aims to derive relative permeability curves for coalbed methane reservoirs using production data analysis, as well as discuss curve characteristics and shapes. Field examples from the San Juan Basin in the US and the Qinshui and Ordos Basins in China are presented to provide a worldwide view of relative permeability curve shapes. These field examples are analysed using a tank type model, a common production data analysis tool, and the influential factors on curve shapes are discussed. The results and analysis indicate that permeability enhancement during the life of the well, and cross-formational flow between the coal seam and adjacent formations, can strongly control curve shapes. These effects, when not detected, can result in irregular relative permeability curve shapes obtained by analysis of production data. Direct measurement of permeability enhancement requires time-lapse production tests while investigation of cross-formational flow of water into coal seams requires hydraulic connectivity assessment, which are time consuming and expensive to conduct. The signatures of relative permeability curves presented in this study allow indirect determination of permeability enhancement and cross-formational flow in coal seam gas reservoirs.
ABSTRACT: Considerable stability problems, such as weakening of the rock formation and borehole convergence, can occur when drilling into areas comprising a poorly cemented formation. In this study, a laboratory-scale convergence measuring device (CMD) has been developed based on the local deformation transducer (LDT) concept, and successfully applied in a series of laboratory tests to continuously monitor borehole deformation in synthetic thick-walled hollow cylinder (TWHC) specimens with various cement contents. Displacement measurements obtained from the CMD were calibrated against a digital micrometer. Calibration results showed that the performance of the CMD and the digital micrometer matched with high accuracy. TWHC test results showed that an increase in the cement content resulted in an increase in the deviatoric stress required to initiate a borehole convergence. Moreover, it was observed that the increment in the level of the deviatoric stress required for the borehole convergence initiation decreased, as the confining pressure increased. These findings can help design more appropriate support systems for drilling operations involving exploration boreholes, as well as oil and gas wellbores to improve the drilling performance in poorly cemented formations.
Borehole instability involving convergence can result in the borehole collapse in weak underground formations. The majority of oil and gas reservoirs are located in weak sedimentary formations where particles are not strongly bonded, resulting in weakening of the rock formation in the vicinity of the borehole and often leads to a borehole convergence. This causes the drilling operations to stop in extreme cases. Monitoring the borehole condition and gaining a better understanding of it is essential for early detection of instability issues and minimizing borehole failure. While various convergence measuring devices such as caliper log, borehole extensometer, and convergence monitor are being used in the field, relatively few efforts have been made to accurately measure the borehole convergence in laboratory experiments for weak rocks. The cell of Bonnechere, the USBM gage and the CSIRO HI cell have been proposed for laboratory-based tests to measure the borehole deformation (Amadei and Stephansson, 1997). However, their application requires an extensive installation procedure and is limited to a specific borehole diameter size matching the size of the monitoring device, i.e. the diameter must be more than 30 mm. In the current study a cost-effective, versatile and reliable convergence measuring device with a quick and easy installation procedure was developed based on the concept of the LDT proposed by Goto et al. (1991) to evaluate the borehole deformation in thick-walled hollow cylinder specimens. These new CMDs have been calibrated and tested in laboratory conditions in order to verify their performance under different stress paths for poorly cemented sands.
Compressibility needs to be accounted for when estimating productivity decline in closed gas and oil reservoirs, and in closed aquifers. Previous works derived an analytical model and well index for inflow performance accompanied by fines migration and consequent permeability damage for incompressible flow towards well. In the present work, we account for fluid and rock compressibility. The problem with given and constant well production rate is investigated. Mathematical model is developed, which provides well productivity index decline with time. Under this model, the solution of damage-free compressible flow in a closed reservoir is matched with the impedance growth formulae for incompressible flow in the well vicinity. The well production data have been successfully matched by the model; the tuning parameters have the common values. It allows indicating the fines mobilization, migration and straining as possible well impairment mechanism in wells under investigation.
Injectivity decline by fines migration with two-phase flow is important in low-salinity and smart waterflooding in oilfields. The complexity of detachment of the natural reservoir fines, their mobilization, migration and straining in two-phase environment preclude simple formulae for injectivity decline prediction. The objective of the present study is to derive of the semi-analytical model for two-phase axisymmetric flow with variation of injected salinity, fines migration, and consequent permeability damage. A simple and robust model allows investigating the effects of injection rate, injected salinity, oil viscosity, relative permeability, and kaolinite content in the rock on skin-factor growth.
Borazjani, Sara (The University of Adelaide) | Behr, Aron (Wintershall Holding GmbH) | Genolet, Luis Carlos (Wintershall Holding GmbH) | Kowollik, Patrick (Wintershall Holding GmbH) | Zeinijahromi, Abbas (The University of Adelaide) | Bedrikovetsky, Pavel (The University of Adelaide)
We derive a general system of equations accounting for two-phase fines migration with fines mobilization by injected water with different salinity, rock plugging by the migrating fines and consequent permeability damage in the swept reservoir zones. The analytical model derived contains explicit formulae for watersaturation and ion-concentration fronts along with pressure drop and water-cut in production wells. The model developed is applied to the cases of heavy oils, in low consolidated rocks with different clay composition and different injected and formation water compositions. We show that non-equilibrium effects of the delayed fines release highly affect incremental oil during injection of different-salinity water. The oil-recovery is maximum for fast fines release. For slow fines release, the recovery tends to that of "normal" waterflooding. The fines-migration-assisted smart waterflood is successful in reservoirs with a high content of finesgenerating clays in the rocks (kaolinite, illite, and chlorite). A novel analytical model presented in the paper allows predicting reservoir behavior and incremental oil for different compositions of injected water and clay contents in the rock. It permits recommending ioniccomposition for the injected water.
Russell, Thomas (The University of Adelaide) | Chequer, Larissa (The University of Adelaide) | Badalyan, Alexander (The University of Adelaide) | Behr, Aron (Wintershall Holding GmbH) | Genolet, Luis (Wintershall Holding GmbH) | Kowollik, Patrick (Wintershall Holding GmbH) | Zeinijahromi, Abbas (The University of Adelaide) | Bedrikovetsky, Pavel (The University of Adelaide)
The main objective of this work is to characterize the formation damage induced by fines migration in reservoir rocks with different kaolinite contents. The problem is particularly important for water production during oil and gas well operations, and for injectivity and sweep during low-salinity waterflooding.
We perform laboratory corefloods using aqueous solutions with different salinities in engineered rocks with different kaolinite content, yielding fines migration and permeability alteration. A novel methodology of preparing artificial sand-packs with a given kaolinite fraction has been established. Sequential injections of aqueous solutions in order of decreasing salinity were performed in five sand-packs with different kaolinite fractions varying from 1 to 10 weight percentage. Severe permeability decline was observed when deionized water was injected into the cores.
A new analytical model that captures the effects of fines release with delay and their re-entrapment by the rock has been developed. The new model allows for explicit expressions for the attached, suspended, and strained particle concentrations, as well as the pressure drop across the core. The analytical model shows good agreement with the laboratory-observed phenomena across a wide range of kaolinite concentrations. The model constants are presented for each of the five cores and lie within typically reported values.
The laboratory protocol and mathematical model allows for reliable prediction of fines-migration related formation-damage during waterflood, EOR, and commingled production of low-salinity water with oil or gas.
Borazjani, S. (The University of Adelaide) | Behr, A. (Wintershall Holding GmbH) | Genolet, L. (Wintershall Holding GmbH) | Kowollik, P. (Wintershall Holding GmbH) | You, Z. (The University of Adelaide) | Bedrikovetsky, P. (The University of Adelaide)
The aim of this work is the development of mathematical models for oil–water flow with varying injected water compositions in oil reservoirs. The model accounts for two main mechanisms of low salinity waterflooding (LSW): wettability alteration and salinity-variation-induced fines migration. Analytical and numerical models are developed and applied to low salinity fines-assisted core flood tests.
In large reservoir scale, the general system of equations permits for an analytical solution. The solution is obtained by recently developed splitting technique, where the stream function is used as a variable instead of time. Large-scale approximation assumes that six dimensionless groups for dissipative and non-equilibrium effects are negligibly small. Numerical results are obtained for the general system, which accounts for dissipative and non-equilibrium effects.
The effects of fines migration and wettability variation on oil recovery are discovered as two separate mechanisms in LSW. The significant EOR-effects of both mechanisms are observed under the typical oil reservoir conditions.