Induced Fractures in Core
Fractures induced in core are fractures that develop during drilling, coring, or subsequent handling. Such fracture scan be grouped into three distinct categories. Drillinginduced fractures propagate in the rock ahead of the bit during drilling. Because they may extend laterally beyond the column to be drilled they may be present in the borehole wall. Coring-induced fractures can develop anywhere within the core barrel. However, they generally form near the bit and before, or at, the scribe knives. Coring-induced fractures that form at specific locations within the barrel can possess distinctive characteristics. Handling-induced fractures form primarily during removal of the core from the core barrel, or during the plugging and slabbing process. Such fractures also may be induced when core is transported or when core lengths are sized to permit boxing for shipment and storage. Handling-induced fractures include desiccation cracks that may develop during core storage.
Drilling- and coring-induced fractures are initiated by the coupling of stress states related to drilling, burial, or insitu stresses within the undisturbed rock, as well as stress imbalances caused by removal of overburden pressure. Handling-induced fractures generally are attributed to stresses caused by three-point bending and impact. In all cases, development of induced fractures may also be influenced by preexisting mechanical anisotropies such as microfractures, natural fractures, faults, bedding planes, cross-bedding, solution cleavage, and residual stresses. In short, induced fractures can be attributed to unique stress states that may develop before, at, or behind the bit, at the scribe knives, within the core barrel, or
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The characterization of naturally fractured reservoirs should include core analyses that encompass interpretation of natural and induced fractures. Unfortunately, to date, the differentiation of induced fractures from natural ones in core has been somewhat speculative and often is based on improper techniques. Consequently, bad interpretations have been made and useful information contained in both natural and induced fractures is overlooked. This book addresses the problem of distinguishing natural fractures from induced fractures in both oriented and unoriented core. Natural fractures include any cored fracture that existed in a volume of rock prior to initiation of drilling or coring-related stresses. Induced fractures in core are those that develop during drilling, coring, and subsequent handling. Many of the procedures for distinguishing between the two are based primarily on recognition of fracture surface structures and fracture traces that differ between natural fractures and induced fractures.