Production and Geologic Time-Scale Compaction in Rigid Grain-Rich and Ductile Grain-Rich Sands

Hathon, Lori A., Michael T. Myers, Irene S. Espejo, Shell International E & P, Houston, TX

Compressibility of clean, quartzose, unconsolidated sands has been the subject of detailed study since the early deep-water discoveries in the Gulf of Mexico. In this area min­eralogy, texture, and burial and stress histories are very consistent in the Upper Miocene, and Plio-Pleistocene sections. Therefore simple empirical relationships based on thin sec­tion analysis have been constructed and successfully applied to predicting production relat­ed compaction. However, extrapolating these relationships to deeper Gulf of Mexico prospects or to other basins, where the set of controlling parameters is different, becomes difficult or impossible.

In contrast, clean, ductile grain-rich sands and/or highly feldspathic sands, behave dif­ferently even when similar in texture, and subject to similar burial and stress histories as their quartzose equivalents. Sedimentary clasts, volcanic and low-grade metamorphic grains, and feldspar in significant proportions increase the compressibility, and require a dif­ferent set of empirically derived models.

In order to build a widely applicable predictive capability, a series of sand pack experi­ments was carried out to evaluate the influence of grain size, angularity, sorting and compo­sition, by isolating each variable. As a result, predictive models for depletion stress induced compaction have been developed. These models take the form of power law distributions for thresholds and magnitudes of local failures within the sands. This results in skewed com­paction curves with peak compressibility moving towards lower stress as the magnitude of compaction increases. In addition there is an asymptotic behavior at high stress, consistent with the high degree of alteration the sands undergo while compacting.