Time Dependent Conductivity of In-Basin Regional Sands

Abstract Baseline conductivity of a proppant pack is to be measured and reported at 50 hours at constant stress according to procedures set out in API RP 19D. However, it has long been recognized that conductivity at stress continues to decline beyond 50 hours - 50 hours being a rather arbitrary timeframe. Thus, conductivity at a given stress is time dependent. This Time Dependent Conductivity (TDC) should be determined and included in predictions of fractured well performance. Studies of the decline of proppant conductivity with time at stress have been published by numerous authors over the last few decades. These studies however were confined to traditional proppants: i.e., ceramic proppants of variable densities, northern white sands and Brady (brown) sands. These studies preceded the advent of the utilization of regional sands now dominating the selection of proppants for fracturing in North America. A methodology for predicting conductivity as a function of time and stress for the traditional proppants had previously been proposed by the Stim-Lab Proppant Consortium. A series of conductivity tests have been conducted on numerous regional sands to obtain the required data and extend this methodology to these materials - and thus compare to the traditional northern white and Brady (brown) sands. The result of this study shows that while the 50-hour conductivities at stress (per API RP 19D) of regional sands are often reasonably acceptable, in general the rate of conductivity decline over time for the tested regional sands is nearly always greater, and in some cases significantly greater, than that of northern white and Brady sands. Further, it was observed that the rate of conductivity decline is a function of the material properties such as shape and crush resistance and not related to sieve distribution or concentration. Also, since the minerologies of the tested regional sands are similar to the northern white and Brady sands with silica dominating, the difference in conductivity performance is concluded to be the result of the differing crystalline structures, shapes and/or impurities of the silica grains. Recognition and definition of the degree to which conductivity decreases as a function of time may aid in proppant selection and in propped fracture performance predictions.