Sand production simulation under true-triaxial stress conditions

Laboratory sanding experiments were carried out under true-triaxial stress conditions. The objective was to investigate the effect of state of stresses and fluid flow on the mechanism of sanding, and the development of the failure zone around the borehole. The experiments were conducted on 100×100×100 mm3 cubic samples of synthetic sandstones. The samples were prepared based on an established procedure developed to produce weakly consolidated sandstone samples with identical physico-mechanical properties. The properties of the synthetic sandstone samples were determined by conducting a series of standard rock mechanics tests on cylindrical plugs. Using a true-triaxial stress cell (TTSC), cubic samples were subjected to true-triaxial stresses and radial fluid flow from the outer boundaries into the borehole. The maximum and intermediate principal stresses were applied laterally in both cases while the effect of changing the lateral stresses on the development of the failure zone around borehole was monitored. It was observed that the geometry (i.e. width and depth) of the failure zone developed around the borehole is a function of the lateral stresses ratio (i.e. lateral stress anisotropy). The experiments were also simulated numerically using ABAQUS in order to validate and interpret the results from the experiments. A good agreement was obtained between the results of both methods, which confirms the importance of lateral stress anisotropy on the evolution of sanding. The observations and results of these experiments and numerical simulations will be presented and discussed

Proposing a sample preparation procedure for sanding experiments

The Authors, during past few years, have performed research on sand production under true triaxial stress conditions. To simulate sanding, 100x100x100 mm3 cubic samples were placed in a true triaxial stress cell (TTSC) and three independent stresses were applied while the pore pressure was increased inside the cell. This resulted in sand grains to be produced through a drilled hole in the sample centre. The experiences obtained through testing several synthetic samples have indicated the significance of sample preparation to obtain valid results. Therefore, in this paper the procedure for preparation of synthetic samples suitable for a sanding experiment is proposed. Also, details of sample preparation for conventional rock mechanical tests to estimate rock physico-mechanical properties including deformability properties, strength parameters and permeability will be presented

Rock engineering systems adopted for sanding prediction in perforation tunnels

Sand production is an important issue in reservoirs with weak or unconsolidated sand formations. Production of sand not only causes several problems in maintaining wellbore integrity but also is a problem during production where damages through the tubing and surface facilities are likely to occur due to the sand grains being transported along this path. The rock engineering systems (RES), initially introduced in mining and civil related geomechanics problems, is one approach to analysing the interrelationship between different parameters involved in a rock engineering project. This is the approach that was adopted in this work to study and predict the sanding potential in perforation tunnels. Sanding mechanism in perforation tunnels during production was reviewed and all effective parameters were identified. An interaction matrix was introduced to study the sanding mechanism through the interrelation between pairs of parameters. The interaction matrix was coded using a semi-quantitative rating approach to determine the interaction between each pair of parameters. The interaction intensity and dominance of each parameter in the system were studied through the cause-effect diagram to classify the parameters. This will assist in finding a better engineering action to mitigate or eliminate instabilities. A sensitivity analysis was conducted on a data set, and major parameters playing in sand production in a perforation tunnel were identified using analytical formulae. The results of sensitivity analysis were compared with the cause-effect diagram derived from the interaction matrix. A good agreement between the two methods was observed. This shows the usefulness of RES for identifying potential sanding solutions through the interaction matrix analysis

A fracture sliding potential index for wellbore stability analysis

Sliding failure along the fractures intersecting a wellbore is one of the major wellbore instability mechanisms. This kind of failure is similar to the slope instabilities, a well-known phenomenon in mining and civil engineering. During drilling operations the drilling fluid can penetrate through fractures and lead to fracture reactivation and wellbore instability. The rock engineering systems (RES), initially introduced in the mining- and civil-related geomechanics problems, is an approach to analyze the interrelationship between the parameters affecting rock engineering activities. In this study, after discussing the sliding mechanism along a fracture in a wellbore during drilling, and identifying all the effective parameters, an interaction matrix is introduced to study the sliding failure mechanism. Thereafter, the interaction intensity and dominance of each parameter in the system is determined to classify these parameters. A systematic approach was used to determine the relative interactive intensity and value of each contributing parameter in the fracture sliding mechanism. As a result, an index is presented to estimate the fracture sliding potential. The results indicate the ability of this method to analyse wellbore instability due to fracture reactivation mechanism. This will assist in finding a better engineering action to mitigate or eliminate potential fracture sliding during drilling. The results show a good agreement with those obtained using Mohr–Coulomb failure analysis and field observations

Experimental sanding analysis: Thick wall cylinder versus true triaxial tests

Using a true triaxial stress cell (TTSC) the authors performed several sanding tests on cubes of synthetically made samples. The samples prepared based on an established procedure developed in the laboratory. Samples, with a dimension of 100×100×100 mm3, were subjected to far-field stresses while increasing the pore pressure inside the cell. Sands were produced from a borehole in the sample centre. An experiment was conducted with anisotropic lateral stress to investigate the effect of stress anisotropy on sand production. By applying uniform lateral stresses, an experiment analogy to TWC was performed for comparison purposes. Comparison of the results of these two experiments demonstrated the importance of considering the intermediate stress component in sanding analysis. The results of these experiments are presented in this paper

The effect of stress anisotropy on sanding: An experimental study

Sand production experiments were carried out under true-triaxial stress conditions. The experiments were conducted on 100×100×100 mm3 cubes of synthetically made samples. The samples were prepared based on an established procedure developed in the laboratory to produce samples with identical physico-mechanical properties and representing weakly consolidated sandstone. Using a true-triaxial stress cell (TTSC), the samples were subjected to 3D boundary stresses and radial fluid flow from the boundaries. The fluid flows through the sample uniformly and discharges from a hole drilled at the center of the sample. The experiment setup and procedure are explained in detail in this paper. The experiments were performed under three different states of stress to study the effect of the intermediate principal stress (in this study, the minimum lateral stress) on the development of the failure zone. The dimension (i.e. width and depth) of the failure zone developed around the borehole were investigated at the end of the experiments. The results of these experiments will be presented and discussed

Numerical simulations of sanding under different stress regimes

Laboratory experiments of sand production conducted under true-triaxial stress conditions were simulated numerically using ABAQUS program. The experiments were performed in a true-triaxial stress cell on 100×100×100 mm3 cubes of synthetic sandstones. Two and three dimensional numerical analyses were conducted to investigate the impact of the magnitude of far-field intermediated principal stress and pore pressure on the failure in the vicinity of a borehole. Different stress boundary conditions were modeled for this purpose. The results provide a better understanding on how the stress anisotropy may have an impact on borehole failure and sand production mechanism. The simulation was used as a tool to optimize and plan the future tests conducted in the laboratory on cube samples. The results of the numerical models will be presented and interpreted