Revolutionary development of unconventional resources for the last two decades has been largely enabled by horizontal drilling and multistage hydraulic fracturing. With geosteering, horizontal wells as long as 7000 meters can now be placed within a stratigraphic target window as thin as 2 meters, maximizing the wellbore exposure to the reservoir. The philosophy of engineered completions is to place perf clusters in like rock so that all perf clusters fracture simultaneously, and not have one cluster dominate over others. Petrophysical data are needed as a foundation for engineered completion efforts to maximize stimulation effectiveness. Driven by constant effort to reduce rig time, off-line cased-hole logging has been tested as an alternative to open-hole wireline logging along horizontal wells. Acquisition of petrophysical data in cased hole is possible, but there are additional complications that do not exist when logging in open hole, which if not properly accounted for, can result in the logging objectives not being met. The intent of this paper is to communicate those complications and potential pitfalls as well as recommendations to help ensure success.
Here we show a case study of cased-hole formation evaluation along horizontal wells in unconventional reservoirs, and share lessons learned from an operator's perspective. The cased-hole logging suite includes spectral gamma ray, new-generation pulsed-neutron, and monopole and dipole sonic logs, with objectives to evaluate porosity, mineralogy, total organic carbon, Poisson's ratio, and Young's modulus along lateral wells. We emphasize the impact of completion design and hardware on cased-hole measurement, which would require additional casing and cement corrections for pulsed-neutron spectroscopy logs and impose special requirement on sonic tool properties and functionalities. For instance, the iron and calcium elemental measurements from pulsed-neutron spectroscopy logs need to be corrected for completion hardware such as casing clamps and centralizers, and variable cement bond quality along circumference and lateral direction, respectively, before a correct interpretation is possible. A sonic tool used in horizontals needs to be stiffer and shorter than a regular sonic tool and should be able to fire and acquire both monopole (for compressional slowness) and four component cross-dipole (for fast and slow shear slowness) modes. Cased-hole monopole sonic logs can be dominated by the casing signal when pipe is not fully bonded to cement. The significant impact of the cased hole environment can be more efficiently accounted for by integrating azimuthal cement bond data as a quantitative input to the analysis of the nuclear and sonic logs, but advances in workflows and environmental corrections will be required to make this practical.
