During drilling, openhole logs provide valuable data to identify the location of hydrocarbon resources, characterize the host formation, and quantify the asset in terms of size and producibility. Such data acquisition during field appraisal plays a critical part in the preparation of the field development plan. However, openhole logs may not be available or may not be possible due to various constraints. In some cases, data that is essential for petrophysical evaluation has been overlooked during the planning of the openhole log acquisition. In other cases, wellbore conditions while drilling may preclude openhole logging in favor of securing the well. This latter scenario can often occur in appraisal wells when knowledge of the pressure regimes in the different formations in the field is still incomplete.
Several logging services are available for acquiring formation information through casing. This capability can provide an excellent option to acquire the necessary formation evaluation data through casing, thereby completing the input to the decision-making process.
We present three separate examples in which casedhole formation evaluation was used to augment data acquired in the open hole for an improved formation evaluation. The applications range from providing reliable density and sonic data for seismic tie-in to providing more accurate estimates of porosity and saturation for selection of test intervals or for input to the static geological model. The examples demonstrate the use of various through-casing formation evaluation technologies including density, neutron, resistivity, acoustic compressional and shear slowness, and pulsed neutron capture and inelastic spectroscopy logs. We show that through a judicious combination of logs the primary evaluation objectives can be fulfilled.
We discuss the challenges in data acquisition, the steps for quality assessment of the data acquired through casing, and interpretation procedures to integrate all the data in an answer product. The substantial benefit of such an option to the operator is also discussed.