Accurate determination of the propped fracture geometry compared to the stimulation design can help fine-tune geomechanical models and to optimize future hydraulic fracture treatments. Conventional methods to evaluate fracture height with logs use radioactive tracers pumped downhole together with the proppant. Subsequently, natural gamma ray spectroscopy logs are run to detect the tracers; however, safety and environmental concerns, together with regulations regarding transportation, storage and use of radioactive materials, can limit the use of these tracers.

Newly developed proppant techniques allow detection without the need of using radioactive elements. Special compounds coat the substrate or are mixed with the proppant material. These interact with neutron logs, making it possible to evaluate the propped fracture height, and in certain cases, have a qualitative idea of the fracture width. Two approaches used by major proppant manufacturers consist of: (a) compounds that are activated by a neutron source, and (b) compounds that absorb neutrons (high capture cross section) and can be detected using conventional compensated neutron logs (CNL) or pulsed neutron capture (PNC) tools. Both PNC and CNL have a shallow depth of investigation, only a few inches away from the borehole, and therefore they indicate the propped fracture height in the near-wellbore region providing a first estimate where the proppant is placed.

This paper will present the results of the neutron absorbing approach under challenging conditions in dual casing strings in wells located in the northern area of Saudi Arabia. The methodology applied uses a comparison of pre-fracture log count rates and post-fracture count rates, with reduced post-fracture count rates observed in zones containing proppant, using both CNLs and PNC tools. The experience has provided information about the sensitivity of each logging device and allowed the comparison to other methodologies (temperature logs) and the formulation of recommendations.

You can access this article if you purchase or spend a download.