Induced fractures during coring and core retrieval are common and widely observed occurrence in unconventional core. In unconventional reservoirs, the mechanism generating these fractures result from complex interactions between the layered and heterogeneous rock fabric, their contrasting properties between layers, their low permeability, and the loadings imposed during coring and retrieval. In this paper, we analyze the dominant mechanisms inducing fractures during coring and core retrieval in unconventional reservoir rocks, using finite element numerical analysis that couples fluid flow and mechanical deformation via standard poro-elastic mechanics. Results of failure analysis during coring indicates that centerline fractures in layered mudstones are promoted by the presence of rock anisotropy and layers with strong contrasting properties (hard/soft couplets). Results of failure analysis during core retrieval indicates that the risk of core damage during tripping is higher for overpressure reservoirs than for normally pressured reservoirs. Results also show that this risk can be minimized by reducing the core retrieval rate (500 to 1500 ft/hr based on the conditions used in this paper) and by reducing the overbalance. To facilitate the use of these results to coring operations, we define dimensionless equations that incorporates multiple modelling parameters, to define the critical retrieval speed above which the core will be damaged. We also provide field examples to compare them with the simulation results.

1. INTRODUCTION

Induced fractures during coring and core retrieval are common and widely observed occurrence in unconventional cores. There are many researchers that investigated the occurrence of these fractures, as detailed in Kulander and Dean, 2003, and Jaeger and Cook, 1963. The most common coring-induced fractures can be classified, based on their shapes, as disking, petal and centerline fractures (Lorenz and Cooper, 2018). In unconventional reservoirs, additional coring-induced mechanisms result from the complex interactions between the thinly layered rock fabric, the strong contrast in properties between layers, and the overall heterogeneous nature of unconventional rocks, including their varying properties, their low permeability, and the varying loadings imposed during coring and retrieval.

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