Upper Cretaceous and Paleogene pelagic sediments sampled from the Northern Kerguelen Plateau during cruise MD35 of the Marion Dufresne arc described and correlated with the Late Paleogene sequence drilled at site ODP 737 (Leg 119). Taking into account geophysical data obtained by the cruise MD26, a Lower Cretaceous age is computed for the unsampled acoustic basement. A major tectonic/volcanic event in the Late Paleogene, related to rifting, gave rise to a marked unconformity and hiatus termed the "Acoustic Discordance." Tertiary sediment facies changes were strongly influenced by the evolution of the Antarctic environment.
Production history, fluid pressure and uniaxial compressive strength are basic data to evaluate the risk of sand production. Evaluations of the compressive strength based on logs are illustrated for the example of a well on the Germigny-sousCoulombs structure. Then relationships between compressive strength and porosity are developed using a theoretical approach of grain contacts, the analysis of published rock mechanics data and mechanical measurements on plugs taken from well cores. It is shown that compaction factor can complement porosity in the analysis. A field example illustrates the relationship between a critical differential pressure and compaction factor or porosity.
Production history, fluid pressure and uniaxial compressive strength are basic data to evaluate the risk of sand production. Evaluations of the compressive strength based on logs are illustrated for the example of a well on the Germigny-sous-Coulombs structure. Then relationships between compressive strength and porosity are developed using a theoretical approach of grain contacts, the analysis of published rock mechanics data and mechanical measurements on plugs taken from well cores. It is shown that compaction factor can complement porosity in the analysis. A field example illustrates the relationship between a critical differential pressure and compaction factor or porosity.
Introduction
The general frame of this study is the production of oil and gas from poorly consolidated formations. It is well known that such a production can be hindered by the phenomenon of sand production. Various gravel-pack equipment can be used to prevent sand production but they are costly and generally harmful to well productivity. Therefore the decision to use such equipment has considerable economic impact and requires a clear-sighted evaluation of the risk of sand production.
Tensile rupture and compressive rupture are proposed as possible mechanisms of sand failure. Tensile rupture is possible under two conditions:the fluid pressure gradient at the production face is larger than the gradient of the radial stress,the tangential effective stress does not exceed the level of compressive failure (less than ucs).
As fluid pressure and tangential effective stress are linked through the equilibrium equation of the sand, conditions (a) and (b) impose an upper limitp max to the pressure difference Pd - Pw through the zone drained by a perforation. p max is proportional toucs and various values of the ratio p max/ ucs can be found depending on the drainage geometry (radial, spherical) and on the production history (influence of shut-in periods). Compressive rupture is also possible if, under symmetrical conditions: the pressure gradient remains smaller than the gradient of the radial stress, the effective tangential stress reaches theucs critical level. To these conditions corresponds a maximum of the total depletion ptd which depends linearly on ucs.
The field observations apparently confirm the general trends of these theoretical analyses. For example satisfactory production conditions are obtained if the drawdown pressure Pdd is maintained under 0.5 ucs. Therefore using theoretical modelling and field observations two categories of data appear to be very important: pressure data characterizing the production history and the fluid flow as well as data characterizing the uniaxial compressive strength of the rock.
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