Improved Selection Criteria for Sand Control - When Are "Fines" Fines?

Abstract

Abstract When sand control is required in production or injection wells, one of the key design criteria is that the sand control should allow those "fines" present to pass, whilst retaining "sand". However the definition of "fines" is not universal and some arbitrary size and/or compositional definitions are often used that could potentially lead to inappropriate sand control design. The fines in any rock sample are usually categorized as that portion of the rock which passes through a ~45 micron sieve. This is a convenient cut-off point for laboratory evaluations as it represents the finest commonly used sieve. It has become routine to use the percentage of fines in a rock sample as one of the key parameters for sand control type selection and specific screen selection. In a coarse-grained, well-sorted sand, particles "less than ~45 microns" may well represent that part of the rock that can move through the pore throats. However, significant variation exists in oil and gas reservoirs and this should be considered in order to obtain an appropriate definition of what constitute fines for a specific reservoir. From a reservoir perspective, the preferred definition of "fines" is that part of the rock that can move through the pores of the intact rock. Using pore throat size measurements and estimates, it is possible to develop a much more rigorous mobile fines size for any formation. Coupled with grain size analysis (which should include some laser analysis of the finer fraction) the true "fines" can be quantified for any given sand. The sand control selection is made using this new definition of fines, leading to optimum and tailored selection, based on science rather than convenience or tradition. For sand control performance evaluation it is not only the mobile reservoir fines that are a concern. Should the formation fail into an annulus then the rock components may re-sort in the annulus and bring about high skins and possible screen plugging. Here, the sizing of the fines that are of concern will be different. It has been convenient to adhere to somewhat arbitrary particle size distribution cut-off points when selecting sand control. When informed with an improved understanding of the petrology, particle behaviour, impact of their movement, and the interaction between the formation and the well, while it is being both constructed and produced it is proposed that we can improve the selection process.