J. Tronvoll

Elasto-plastic modelling of Red Wildmoor sandstone

On the basis of an extensive experimental program of axisymmetric triaxial compression and extension tests, the mechanical behaviour of a weak sandstone (Red Wildmoor sandstone) is analysed and modelled within the frame of the theory of elastoplasticity. Stress-dependent elasticity and damage are incorporated into the model. Pressure sensitivity of the plastic behaviour is described through a non-linear Mohr–Coulomb model with friction hardening and cohesion softening. Emphasis is given on the calibration procedure. Validation of the model is done through back analysis of the experimental tests. Shear-band analysis is performed on the basis of the extension of the constitutive model to non-coaxial plasticity. Copyright

Continuum fracture mechanics of uniaxial compression on brittle materials

Abstract A review of the uniaxial compression test on brittle solids is presented. The discussion focuses on the description of the post-peak behavior, where strength softening and fractures propagating mainly in the axial direction are observed. Based on experimental evidence and dimensional analysis, the post-peak axial stress is seen as a function of the axial displacement normalized by the radius of the specimen, and not of its height. Accordingly, a continuum theory of stress diffusion is developed that is able to account for size effect. As a practical application of the theory, the bearing capacity of a pillar in a deep mine is considered.

Effect of Water Breakthrough on Sand Production: Experimental and Field Evidence

Different field operators have reported that the onset of sand production in a particular field coincided with water breakthrough at the production wells.

Skin Self-Cleaning in High-Rate Oil Wells Using Sand Management

Mechanical skin develops around high-rate oil wells because of mineral salt precipitation in the near-wellbore region. because of fine-grained minerals accumulation that block the near-wellbore pore throats, because of capillary forces development that impair free flow, and because of asphaltene or other organic substance deposition that reduces effective porosity. The removal of mechanical skin is normally carricd out by workover techniques that require wellbore intervention. As early as 1994-95. it was found that for wells in poorly consolidated sandstones many of the problems of mechanical skin could be mitigated by producing the wells at a rate that allowed periodic sand bursts, yet without initiating massive sand influx Also, deliberate sand clean-up activities during well testing could be used to generate skin-removing bursts. The sand bursts are triggered by stress and by the increased hydrodynamic drag associated with steep exit gradients arising from skin, and they have a remarkable clean-up effect on the near-wellbore. bringing flow skin into the range of consistently negative values Wells that are amenable to this production methodology, called Sand Management, benefit from reduced completion costs, increased production rates. lowered intervention frequency, and the development and maintenance of a negative skin. Sand Management, in contrast to sand exclusion, requires coping with higher risks because episodic sand influx events carry with them some danger to integrity of the well and surface facilities. Risk management mplies three actions: high-quality evaluation and analysis before implementation, a commitment to monitoring during production, and a process of continuous re-evaluation of the well during its production life. These risk management activities and protocols form the basis of the Sand Management approach.

Hydrodynamic erosion; A potential mechanism of sand production in weak sandstones

Abstract Fluidized column experiments have been performed, where a 10 cm in diameter and 15 cm high cylindrical sand column was subjected to linear upwards flow of water. The tests were monitored by means of an X-ray CT-scanner. The experiments showed that a change in the flow rate gave rise to a propagating density wave starting upstream at the bottom of the sand body propagating to the top. The experimental observations have been used to calibrate a model which describes erosion processes. Flow rate changes were best described by a temporary change into mildly turbulent flow conditions, which leads to mobilisation of sand particles.

Flow geometry effects on sand production from an oil producing perforation cavity

Abstract Physical model experiments have been performed on perforation cavities in a weak sandstone. The objective of the study has been to investigate the effect of the flow geometry on sand production risk. Two extreme geometries have been tested, one with flow mainly axially towards the tip of the cavity and one with fluid flowing radially towards the side of the cavity. The experiments have been performed in a special pressure vessel for sand production studies. We have recorded the deformation of the cavity, the flow rate through the sample and monitored sand production and acoustic emission activity as functions of the imposed confining pressure and pore pressure drop across the sample. From the tests, we have found initial cavity failure and initial sand production at various combination of confining and pore pressure. The results have also been corroborated by finite element simulations. We have concluded that by directing the flow through the bottom of the cavity sand production is inhibited compared to the case where flow mainly enters through the side of the cavity. This is because the cavity fails along the wall first, while the bottom is intact. As flow promotes the removal of failed material, radial flow increases the risk of sand production.