When oil or gas is produced from poorly consolidated strata, co-production of solids may occur. Even though both Chalk and coal reservoirs are susceptible to this phenomenon, it is more acute in weakly or non-cemented sandstones and is then known as sand production. Sanding problems have been reported in many petroleum regions: the Gulf Coast (USA), Alberta (CANADA), the Gulf of Guinea, the North Sea and many sand control techniques have been developed (Gay, 1978 or Summan et al., 1985). However, despite constant improvements, they generally reduce well productivity. Until recently, high oil prices permitted companies to adopt an ultra-safe policy and to use exclusion techniques as soon as hints of sanding problems were detected. For example, Summan et al. (1985) report use of sand control techniques in a newly discovered reservoir because neighbouring reservoirs were equipped with such devices. However, low oil prices make productivity loss a crucial factor. Field cases have shown at reservoirs presumed to be affected could be produced without exclusion devices and without sand production. This demonstrates the need to isolate the mechanisms behind sand production in order to develop robust prediction techniques. After summarizing mechanisms considered responsible for sand production, the paper will address current predictive techniques. Two major approaches exist, log and core analysis, both with limitations. A study to characterize the sandstones of a deep oil reservoir in the North Sea will be presented. This study will be used as a test for the various predictive approaches. It will be shown that a mixed approach combining both log and core analysis is superior for new field development planning.
Historically, the most important contribution to understanding sand production was made by Bratli and Risnes (1981) and Bratli et al. (1983). They accounted for inward fluid flow and modelled the stresses around the cavity -i.e. the well itself in the case of an open hole or the perforation in the case of a cemented and perforated hole (Fig. 1). They developed closed-form solutions for cylindrical (open hole) and spherical (perforation) cavities in a linear elastic, perfectly plastic, Mohr-Coulomb material under hydrostatic loading and inward fluid flow. They identified two types of failure.
1. The first type is failure by excess deviatoric stress. They show that the major parameter governing this type of instability is the drawdown pressure (δP) -
i.e. the difference between formation and bottom-hole pressures. This type of instability is known as unstable sand production, as it is thought that it cannot stop by itself. For particular conditions, this type of instability can be controlled by reducing the drawdown pressure. The value of δP is of extreme economic importance, as it controls the rate of production. The aim of most predictive techniques is to maximise δP while minimising sand production.
ii. The second type is failure by excess tensile effective radial stress. This occurs when the pore pressure gradient at the wall is larger than the total radial stress gradient, and the effective radial stress becomes tensile.
