Porothermoelastic Response and Damage Potential of Tripping Unconventional Cores from Six Different Shale Plays

Abstract

Coring operations in the petroleum industry are generally performed to study geological, petrophysical, geomechanical, and drilling properties of reservoir formations and characteristics of reservoir fluids. Some of these studies need intact cores for performing mechanical tests or for measuring reservoir fluid properties. Therefore, retrieval of intact cores is crucial to coring jobs. During the coring operation and tripping a core out of a well, it experiences a sudden or gradual decrease in confining stress, pore pressure, and temperature (time-dependent boundary conditions) depending on the mud pressure, in situ stress, reservoir temperature, and reservoir pore pressure. Some porothermoelastic (PTE) properties of the retrieved core also change due to the change in reservoir conditions at or near the surface. Specifically, changes in the core properties depend on the in situ conditions, mud pressure, and the tripping velocity. These conditions or phenomena, which occur simultaneously, induce mechanical, poroelastic (PE), and thermal stresses in the core, which may cause failure. In this paper, the PTE behavior of unconventional tripping of cores is studied with the goal of identifying the most influential variables and the risk of core damage. First, the PTE and in situ properties of six different shale plays located in the US are collected from the literature. The Marcellus shale data are then used to identify the following three most important controlling factors and their effect on the core damage: use of PE and/or PTE approaches, use of constant or variable properties, and change in tripping velocity. Second, the gradual change in stress, pore pressure, and temperature during core tripping is examined in the Marcellus shale. Third, PTE behavior and damage potential of the selected six shale plays during tripping are investigated, and the results among these reservoirs are compared. Fourth, a comprehensive sensitivity analysis is performed for each of these shale plays via changing the input variables by selected percentages and observing the change in the effective stress field. Finally, some guidelines are presented, along with an optimization procedure for tripping velocity, for efficient coring operations. A commonly used software, FLAC3D, is used for all numerical analyses because of its capability and features of simulating coupled problems as a PTE medium with variable properties and time-dependent boundary conditions.