Can Rare-Earth Elements be Useful Geochemical Tracers for Gaining an Insight into a Hydraulic Fracturing Event?

Sam Chaudhuri¹, M. Totten¹, J. Miesse², and G. Riepl^3
1Department of Geology, Kansas State University, Manhattan, Kansas 66506
²Pablo Energy II LLC, 801 S. Filmore, Suite 139, Amarillo, Texas 79101
³Petroleum Geologist, 119 N. Robinson, Suite #530, Oklahoma City, Oklahoma 73102

To determine whether or not analyses of rare-earth elements in fluids related to hydraulic fracturing of low permeability hydrocarbon-bearing formations could provide valuable information concerning a hydraulic fracturing event, an investigation was made of both pre-frac injection fluid and flow-back fluids, collected over a span of nine days from a well that was drilled into the Devonian Woodford Shale in the western Arkoma Basin in Coal County, Oklahoma. While the total REE content of the pre-frac injection fluid was about 6.29 ppb, the contents of the flow-back fluids ranged from about 4.46 ppb to about 3.80 ppb. With the exception of the flow-back fluids collected on the second day after the day of fracturing, all other flow-back fluids had nearly identical pre-frac fluid normalized relative rare-earth element distribution patterns, which were characterized by a Ce negative anomaly with a trend of progressive relative enrichment from Pr to Yb. The relative rare-earth element distribution pattern for the second day flow-back fluid differed markedly from that of the other flow-back fluids. The second day flow-back fluid, unlike the other flow-back fluids, had a prominent positive Eu anomaly and also a slightly positive Ce anomaly, but without the accompaniment of any relative enrichment in the relative distribution pattern from Pr to Yb. The contrast that was seen in the relative rare-earth element distribution pattern between the second day flow-back fluid and the other flow-back fluids makes a strong suggestion that that this hydraulic fracturing event created some fractures that went across the production zone. The Ce negative anomalies that were evident for most of the flow-back fluids may be interpreted in terms of precipitation of oxide or oxy-hydroxide phases produced by fluid-rock and fluid-fluid interactions. Relatively low Al contents of the flow-back fluids, ranging from 0.13 ppb to 0.38 ppb, as compared to the pre-frac (or injection) fluid whose Al content was about 0.86 ppb, suggest to a possibility of precipitation of an oxy-hydroxide phase. Progressive increase in the relative distribution patterns of the rare-earth elements from Pr to Yb for all but the second day flow-back fluid suggests the possibility of a carbonate dissolution phenomenon. This is supported by the Sr concentrations of the flow-back fluids which ranged from 2.76 ppb to 5.07 ppb, as compared to the pre-frac fluid with a Sr concentration of about 0.27 ppb. Knowledge of potential intra-formation reactions following a hydraulic fracturing, as may be traced from the rare-earth element signatures of flow-back fluids, may be used to find an appropriate chelating agent which may be added to the pre-frac fluid before it can be used to another hydraulic fracturing event in an adjacent area so that gains may be made in the production yield.

AAPG Search and Discovery Article #90152©2012 AAPG Southwest Section Meeting, Fort Worth, Texas, 19-22 May 2012