Field Application of a New Proppant Detection Technology
Traditional proppant placement evaluation in hydraulically induced fractures has utilized detection of radioactive tracers pumped down hole with the proppant. We are proposing new methods for proppant placement determination. The proppant placement determination method described in this paper eliminates down hole placement of radioactive materials. A thermal neutron absorbing compound is inseparably incorporated into a ceramic proppant during manufacturing in sufficiently low concentration that it does not affect proppant properties. Proppant is detected using standard compensated neutron tools or pulsed neutron tools, with detection based on the high thermal neutron absorptive properties of the compound relative to downhole constituents.
Specifically, proppant is detected using after-frac compensated neutron logs combined with corresponding before-frac logs. Increased thermal neutron absorption by the compound reduces count rates in the near and far detectors, with approximately the same percentage reduction observed in each detector, leaving the N/F ratio unchanged. One detection method utilizes a comparison of before-frac log count rates and after-frac count rates, with reduced after-frac count rates observed in zones containing proppant. A second detection method, especially useful when formation gas saturations change, involves only the after-frac log. Since the near to far detector count ratio is unaffected by proppant, after-frac count rates predicted from the ratio will also be unaffected. These computed/synthetic count rates will be greater than the observed after-frac count rates in intervals containing proppant.
This paper will describe the results of applications of the new detectable proppant in wells in South Texas and Wyoming. The location of the detectable proppant was determined from analysis of before-frac and after-frac compensated neutron logs.
In order to efficiently and economically produce hydrocarbons from subsurface formations and especially in formations with low porosity and/or low permeability, hydraulic fracturing has been a frequently used technique. In hydraulic fracturing, proppant laden fluid is pumped downhole under high pressure, causing the formations to fracture. When pumping ceases, the fractures close on the proppant resulting in high permeability conduits that promote the flow of the hydrocarbons into the well bore. The fracture closure places a compressive "closure?? stress (often exceeding 10,000 psi) on the proppant. This closure stress, as well as expected flow rates and flow conditions will dictate the optimal choice of proppant (uncoated sand, resin coated sand or ceramic). The optimal proppant will be one that maximizes the economic return of the well under realistic conditions [Palisch 2007]. Hydraulic fracturing operations can be conducted in horizontal, deviated, and vertical boreholes, and in open-hole wells or cased wells through perforations. The description below focuses on frac applications in cased boreholes in generally vertical wells, however analogous concepts are generally applicable to most frac situations.
Authors: Robert John Duenckel (CARBO Ceramics Inc.) | Harry D. Smith (CARBO Ceramics Inc.) | Wil Warren (Chesapeake Energy Corp.) | Abram Grae (Shell)