# Introduction

3DEC offers a comprehensive set of capabilities for modeling fluid flow and the effect of fluid pressures on rock/soil. It is well known that fluid pressure in rock or soil reduces the effective stress, thereby increasing the probability of failure — either slip on joints or plastic flow of solid material. It is therefore critically important to be able to consider the effect of fluid in mechanical stability analyses.

3DEC offers different levels of sophistication for modeling fluid. The user can simply specify water pressure everywhere in the model and these pressures are used to calculate effective stresses. Effective stresses are then used in the calculations to determine if there is failure of solid material or slip on joints.

A more sophisticated analysis can be done in which fluid flow is calculated based on specified material properties and fluid boundary conditions (pressure or discharge). The fluid flow calculation can be performed on its own (uncoupled) or coupled to mechanical calculations. Fluid flow calculations can be done on the joints and/or the matrix material (blocks between the joints). Fluid pressures are continuous between the joints and the matrix such that “leak-off” from the joints into the matrix can be simulated.

3DEC also offers the ability to model the flow and mechanical effect of proppant. Proppant is assumed to be composed of small particles that are transported in the fluid with the purpose of propping open fractures at the end of injection operations. Proppant flow is calculated by assuming the proppant and fluid is a mixture with some concentration. Proppant concentration changes as a result of advection. If the concentration is high enough, then the proppant will start carrying load and will effectively prop open the host fracture. Other proppant effects considered in 3DEC include gravity-induced settling, bridging and convection.

The user is strongly encouraged to become familiar with the operation of 3DEC for simple mechanical problems before attempting to solve problems in which flow and mechanical effects are both important. Coupled flow and mechanical behavior are often very complicated, and require a good deal of insight to interpret correctly. Before starting a big project, it is very important to spend time experimenting with a small-grid version of the proposed simulation to try out various boundary conditions and modeling strategies. The time “wasted” on these experiments will be amply repaid in terms of an overall reduction in both staff time and execution time.

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