Unconventional organic-rich carbonaceous shale (OCS) in North Kuwait has unique characteristics due to its high total organic content (12–14%), its complex mineralogy (blend of carbonates and clastics) and Jurassic/post-Jurassic complex burial and tectonic history. The OCS is under/over-laid by tight carbonates, where the system permeability is in the range of nano to micro-Darcy. Conventional techniques to initiate flow from this complex reservoir have proven ineffective, which led the appraisal team to develop novel and extensive workflows to characterize the formation properties for successful hydraulic fracturing and to determine the commercial flow potential of hydrocarbons.
Three vertical wells were identified as the high potential candidates in the appraisal program, prior to the completion of horizontals in this complex formation. A comprehensive logging suite was acquired which included spectroscopy, sonic and image logs. Fracture pressure analysis was performed on an offset well followed by exhaustive diagnostic tests using both non-viscous and viscous fluids. Bottom hole gauges were hanged in the well during the diagnostic tests as well as temperature logging was performed after the diagnostic pumping to define fracture height growth/containment. Proppant fracturing in these wells are planned using nonradioactive tracer proppant.
Maximum clay content in the OCS is estimated close to 40% in addition to the high organic content. Anisotropic mechanical earth model was built using sonic log response for fracture modelling in planar 3-D simulators. Some open fractures were observed from the image logs. Fracture pressure analysis on the offset well confirmed “fraccability” of the OCS within the allowable pressure limits. Analysis of the diagnostic tests confirmed significant impact of natural fracture on hydraulic fracture propagation. Nolte-Smith plot using actual bottom-hole pressure revealed very high net pressure slope during viscous fluid stages that raised significant concerns on possibility of placing proppant. Interventions after the diagnostic tests showed flowback of formation material in the form of “formation creep” into the wellbore and that lead to concerns on formation integrity and risks in proppant placement.
This paper discusses various methods used to understand the complex OCS formation and integrates extensive analysis achieved through different methods. Interpretation of this data-set and building a model which will be used for development of future horizontal wells will also be presented. It is believed that the inherent challenges presented by the OCS are unique without any true global analogues in the unconventional gas and oil programs.