Abstract

This paper describes the mechanical behavior of unsupported open-holes (such as wellbores, perforations and cavities) in selected chalk samples from a European North Sea hydrocarbon reservoir.

The main objective was to characterize cavity failure as a function of stress and fluid saturation. A series of compaction tests were performed on Thick-Wall Cylinders (TWC) of deeply buried North Sea Chalks, complemented with Uniaxial Compression Strength (UCS) tests. Comparison of oil vs. brine saturated specimens demonstrated water-induced "softening", drastic weakening, and then collapse of the cylindrical cavity. Images from X-Rays and thin-sections depict fracture-bands extending from the pilot-hole. These near-wellbore shear bands are interpreted to be pathways for enhanced permeability.

The results and graphic images allow improved understanding of rock failure processes in chalks, as influenced by the varying degrees of water saturation and loading conditions. A poly-axial shear failure criterion is shown to predict TWC onset of instability. The knowledge gained on the failure mechanisms provides the exploitation engineer with scenarios on how flow-paths and well-inflows are affected by drawdown and water-floods. They also give diagnostics for declining productivity, which could lead to casing damage and wellbore failure. A field example is in wellbore acid fracturing, some of which lead to excessive liner deformations and even wellbore failures. Such downhole deformations are thought to be similar to those observed in these laboratory tests, mechanisms related to pore pressure depletion and/or stimulation. Acid-fracturing plans and models could be revised with the aid of processes, laboratory data, failure criteria, and results described here.

Introduction

Several oil fields in the North Sea have undergone periodic acid stimulation campaigns over the years. Typically, the results obtained after each acid fracturing treatment are excellent; several wells reported productivity index (PI) improvements ranging between 40 and 70. However, it has been commonly found that these results are short lived; the wells experience a severe decline in PI, and return to pretreatment production rates in as short as 1 year. Other problems have been observed in these fields; excessive casing deformation has been reported to occur in numerous wellbores. Previous field investigations have shown a correlation between casing deformation and perforation location. Casing failure was observed very near perforations subjected to acid-frac treatments. This suggests that large rock deformation, which causes casing failure, is a consequence of the acid stimulation process itself. The TWC laboratory program performed in this study provides some insight on the behavior of boreholes in chalks under stress.

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