Correlation between Natural Gamma Radiation and Mechanical Stratigraphy in the Cretaceous Eagle Ford/Boquillas Formation of South-Central and West Texas, 20-R8257
Principal Investigators
Alan P. Morris
Ronald N. McGinnis
Kevin J. Smart
David A. Ferrill
Mary Katherine Todt
Inclusive Dates: 10/01/11 – 01/01/12
Background — The Cretaceous Eagle Ford/Boquillas (EFB) Formation in south-central and west Texas consists of interbedded shale and limestone layers, and is one of the largest unconventional hydrocarbon plays in the United States. The success of this play is dependent on the ability to "geo-steer" sub-horizontal well legs into and through sections of the formation that are both hydrocarbon rich and intrinsically able to support hydraulic fracturing. EFB lithologies have low intrinsic permeability, and effective production requires induced fractures. Hydraulic fractures induced during well completion must be appropriately developed and must remain open after completion. Strong (more brittle) rock layers such as limestone are generally easier to hydraulically fracture, and these fractures tend to remain open; however, strong layers may have lower hydrocarbon content than weaker shale layers. Weaker (less brittle) rock layers are generally more difficult to fracture and induced fractures in such rocks often close following completion, rendering them useless in terms of production.
Geo-steering is most often accomplished by monitoring natural gamma radiation during drilling and comparing the results to a reference stratigraphic section. There is no existing direct correlation between gamma emissions and rock strength. Gamma emissions are generally high in rocks with high clay mineral content (shales) and low in rocks with little clay mineral content (limestones). Although it is generally recognized that shales are weaker than limestones, data to define a robust correlation between rock strength and gamma emissions are lacking. This data gap limits industry's ability to easily predict rock strength characteristics, needed for planning and executing hydraulic fracturing, from their navigational tool of choice (natural gamma radiation).
Approach — The objective of this project is to answer the question: What is the correlation between natural gamma radiation and rock strength in the EFB of south-central and west Texas? To answer this question, SwRI researchers established a mechanical stratigraphic section through an outcrop exposure of the EFB and matched it in detail with a natural gamma radiation profile through the same stratigraphic sequence. A hand-held radiation detector and a Schmidt Rebound Hammer were used on well-exposed sections of the EFB.
Accomplishments — A correlation was established that can be used to predict rock strength from gamma log character. There is a broad, negative correlation between R and y. Low values of R correlate with high y values, and vice versa, as illustrated in the figures.
