A New Method to Identify Proppant Location in Induced Fractures
Authors: Duenckel Robert John , Smith Harry D. , Hao Simon , Gao DaKang , Palisch Terry T. , Han Xiaogang 
Published: March 2015

A new technique is discussed and tested in this work for proppant placement determination. A high thermal neutron capture compound (HTNCC) 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. Compared to the traditional radioactive tracer (RA) techniques, the new detectable proppant is not radioactive so there are no HSE or regulatory issues. Additionally, since the high thermal neutron absorbing compound is placed in the proppant during the process of manufacturing, there is no requirement for special handling or mixing processes at the well site.

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 near-to-far detector count ratio (N/F) 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.

Nuclear Monte Carlo modeling was performed to demonstrate the validation of this technique employing both compensated neutron logging tool (CNT) and pulsed neutron capture (PNC) logging tools. Two field examples from China are illustrated in this paper. The final interpreted locations of proppant are shown in the field examples.

Introduction

Hydraulic fracturing has been frequently used to increase hydrocarbon production especially in formations with low porosity and/or low permeability. 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. In hydraulic fracturing, the high-pressured, sometimes viscous, frac fluids exit the borehole via perforations through casing and cement, and cause the formation to fracture. At the same time, the high-pressured frac fluid carries proppant, such as sand, resin coated sand or ceramic proppant and places them in the fracture. Generally, the induced fracture extends laterally a considerable distance out from the wellbore into the surrounding formations, and extends vertically until the fracture reaches a formation that is not easily fractured above/below the desired frac interval. The direction of the least principal stress within the formation determines the azimuthal orientation of the induced fractures. In the case of viscous gelled frac fluids, gel "breakers" are added in the fluid slurry to reduce the viscosity of frac fluid after a desired time delay. This can enable fracturing fluid to be removed from the fractures and formation during production and leave the proppant in place to keep the induced fractures from closing.