Elsevier

Measurement

Volume 101, April 2017, Pages 126-137
Measurement

Development of a laboratory-scale numerical model to simulate the mechanical behaviour of deep saline reservoir rocks under varying salinity conditions in uniaxial and triaxial test environments

https://doi.org/10.1016/j.measurement.2017.01.015Get rights and content

Abstract

The maintenance of the long-term mechanical stability of the reservoir rock mass is essential in CO2 sequestration in deep saline aquifers. However, it cannot be confirmed without predicting the worst-case scenarios in saline aquifers, including high salinity conditions and the complexities caused by surrounding factors such as reservoir depth. Laboratory experiments to identify all such situations are difficult due to the advanced facilities required, and the associated cost and time. Therefore, numerical models play an important role in extending laboratory measurements for such complex and extreme situations. Although numerous numerical studies have been performed to date on field-scale conditions in saline aquifers, less consideration has been given to simulating laboratory data, which is important for up-scaling the data to field conditions. This study therefore aims to develop a laboratory-scale numerical model to simulate the mechanical behaviour of brine-saturated reservoir rock under triaxial stress laboratory conditions. The model validation was performed by measuring uniaxial and triaxial laboratory test data under 10–25 MPa confining pressures and the model was then used to investigate the influence of pore fluid salinity percentage on reservoir rock strength by considering various possible salinity levels (5%, 10%, 15%, 20%, 25% and 30% NaCl) and the influence of depth using a range of confining pressures from 10 to 100 MPa.

The proposed numerical model based on the stiffness degradation mechanism of reservoir rock can accurately simulate salinity-dependent stress-strain behaviour under any stress environment (uniaxial/triaxial). According to the model, both pore fluid salinity and confining stress add additional strength to the reservoir rock mass due to NaCl crystallization and pore shrinkage. Importantly, the model clearly shows a reduction of the effect of pore fluid salinity on reservoir rock strength characteristics with increasing reservoir depth or confinement, mostly due to the more significant effective stress at such extreme depths. This provides an important finding on CO2 sequestration in saline aquifers: salinity-dependent strength alteration is not very important for extremely deep aquifers compared to shallow aquifers. Although this model has the capability to simulate the failure of reservoir rock under extreme pressure conditions, the simulation results show a small fluctuation near the post-peak stage due to the complexity of the damage mechanism caused by strain localization.

Introduction

Numerical simulation is essential in addressing issues arising in CO2 sequestration in deep saline aquifers, due to the extensive cost and time required [2], [39], [35], [24], [22]. Generally, most laboratory-scale experiments are limited to low injection pressures, confining pressures and temperature conditions and have limitations in their applicability to real field situations. The development of appropriate laboratory-scale models eliminates such limitations, and models provide the ability to predict hydro-mechanical variations under extreme pressure and temperature conditions. The development of appropriate laboratory-scale models using user-friendly simulators is of utmost importance for research into deep saline sequestration. The present numerical study was motivated by that demand. Although a number of field-scale simulations have been developed to date using different numerical modelling software packages, including TOUGH, COMSOL, FEMLAB, RTAFF2, ECLIPSE and COMET3 [32], [33], [5], [38], [25], [26], very few studies have focused on the small domains available in experimental conditions such as triaxial and uniaxial tests. The impact of CO2 injection on hydro-mechanical behaviour in deep saline aquifers cannot be preciously understood without laboratory-scale models and the combined investigation of laboratory- and field-scale simulations is therefore required to fill the existing knowledge gaps related to the long-term physical and chemical behaviours of deep saline reservoirs.

The COMSOL 5.0 simulator with a user-friendly interface was used in this study to develop a laboratory-scale model to simulate reservoir rock mechanical behaviour in deep saline aquifers in both uniaxial and triaxial stress environments. Although this simulator has been used in the industry for field-scale simulations, to date no study has been reported on the laboratory-scale application of the simulator to salinity-dependent mechanical behaviour in reservoir rock samples. For the purpose of modelling, experimental data obtained from triaxial compression tests on brine-saturated reservoir rock samples were used. Further, three different salinity conditions were selected (0, 10, 20 and 30% NaCl (% by weight)) to investigate the influence of salinity concentration on reservoir rock mass mechanical characteristics in the salinity conditions expected in deep saline aquifers. Here, 0% salinity or water-saturated condition was used for comparison with NaCl-saturated samples to identify the pure salinity effect on the rock mass mechanical properties. This salinity effect on the mechanical behaviour of reservoir rock is important to precisely understand the existing strength characteristics of varyingly-saturated reservoir rocks.

Section snippets

Stiffness degradation mechanism

The analysis of the mechanical behaviour of reservoir rock in deep saline aquifers requires detailed knowledge of its failure characteristics. Sedimentary rocks (soft surrounding rock) in deep saline aquifers often undergo strain hardening/softening under stress application (mechanical loading). Since this salinity-induced mechanical behaviour (strain hardening and softening behaviours) has a significant influence on the reservoir’s capacity, precise understanding of the mechanical property

Experimental data used for model validation

This numerical study is based on the results of two published experimental studies entitled “Salinity-dependent strength and stress-strain behaviour of reservoir rocks in deep saline aquifers: An experimental study” and “Non-linear stress-strain behaviour of reservoir rock under brine saturation: An experimental study” [27], [28]. In these studies, the intact reservoir rock samples (Hawkesbury sandstone) 38 mm in diameter and 76 mm high were saturated with water, 10%, 20% and 30% NaCl

Model geometry, parameters, boundary and reservoir conditions and loading

Since the experimental sample was a cylinder, a 2-D axisymmetric model was selected to simulate the uniaxial and triaxial tests on brine-saturated reservoir rock, and the model was converted to a 3-D model using the results node available under the “Model Builder” interface in the COMSOL simulator. Fig. 5 shows the model geometry and the finite element mesh used in this study. Table 1 presents the meshing parameters used in the model geometry. The input parameters of the model (Table 2) were

Basic assumptions

  • (1)

    The reservoir rock mass with a finite-difference grid-block is homogeneous.

  • (2)

    The reservoir rock sample is at a constant uniform temperature throughout the experiment.

  • (3)

    The reservoir rock mass is free from any matrix-level anomalies.

  • (4)

    The modelled rock domain is bounded by four boundary conditions at top, bottom, right and left boundary.

  • (5)

    All the stress points are on the defined yield surface.

The developed model was mainly used to predict the mechanical behaviour under high confining pressure conditions.

Results and discussion

A comparison of stress-strain behaviours obtained from experiments and numerical modelling under various salinity conditions at 10 MPa confining pressure condition is shown in Fig. 6. As the figure shows, the proposed model successfully reproduces the behaviour of reservoir rock tested in the experimental investigation [28]. However, validation of the proposed model is required before it can be used to predict reservoir rock behaviour under high confining pressure conditions. For this purpose,

Conclusions

Pore fluid salinity and depth influences on the mechanical behaviour of reservoir rock (Hawkesbury sandstone) were numerically simulated following the development of a laboratory-scale triaxial model using the COMSOL Multiphysics simulator. The model validation was performed using measured triaxial laboratory test data under 10–25 MPa confining pressures. The model accuracy was double-checked by converting it to uniaxial test conditions by setting the confining pressure to zero and comparing the

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