Novel Application to Estimate Cement Quality by Means of Elemental Spectroscopy Data

Abstract

Pulsed neutron log (PNL) is well known for its main application to determine hydrocarbon saturation in cased-hole wells by means of neutron inelastic reaction with carbon and oxygen elements. However, the computed hydrocarbon saturation might be biased by the presence of hydrocarbon in the annulus between casing and formation, in which the cement is not evenly distributed. Cement quality analysis is important to complement the cased-hole analysis, not only to ensure the validity of the computed hydrocarbon saturation but also to provide valuable insights in term of drilling and workover to take necessary action in ensuring zonal isolation between reservoirs. Despite the available technology for cement evaluation, elemental spectroscopy reading from PNL can also be used to give an insight about the cement quality in the annulus since the emitted neutrons from PNL also interact with other elements in the formation and near borehole environment such as silicon, calcium, iron, magnesium, etc. which can be detected and quantified in term of count rate. The quantification of inelastic response between emitted neutron and calcium nuclei from PNL can serve as an indicator to define cement quality in annulus. This paper explains the proposed technique to determine the cement presence by utilizing the elemental spectroscopy reading of calcium since typical elemental composition of cement is dominated by calcium. First, the zones with good cement condition and engaged wellbore need to be defined from CBL and caliper data as reference zones. Then, elemental spectroscopy reading of calcium over these reference zones is used to create the estimated standard calcium spectroscopy response for the whole interval with the same completion feature. Poor cement quality or lack of cement volume is indicated whenever the actual calcium spectroscopy reading is significantly less than the estimated standard calcium spectroscopy response. The result is then validated using the available conventional cement evaluation log such as cement bond log (CBL) and variable density log (VDL). This proposed technique shows satisfactory result in most of the observed intervals. However, it is suggested to perform this technique using multi-arm caliper in order to better simulate the annulus volume for cement identification using down-hole spectroscopy data. It is also advised to utilize the latest generation triple-detector pulsed neutron log to obtain more precise elemental spectroscopy measurement.