Reconstruction of Lithology Profile in Absence of Formation Cuttings and Logging While Drilling in an Overpressured Sequence: A Fit for Purpose Approach

ABSTRACT Lithology identification during drilling is the first step in formation evaluation for oil and gas exploration. In exceptional cases, both formation cuttings and logging while drilling data may be unavailable for a section of the drilled area due to various unavoidable reasons. Essentially, the rate of penetration serves as a preliminary approximation for distinguishing between drilled lithologies; however, it is ineffective when controlled drilling occurs in an overpressured section. Variations in drilling parameters will affect mechanical logs by lowering the drilling exponent and sigmalog in coarser clastics compared to finer clastics. It is recognized that as pressures increase, lower mechanical log values will be evident in finer clastics than in the underlying coarser clastics. In a deep onshore exploration, a high pressure, high temperature (HPHT) well located in the Coastal Swamp depositional belt of the Niger Delta in West Africa had to employ slim hole drilling techniques in the deeper Agbada Formation. The Logging while drilling (LWD) tools were positioned significantly offset from the bit. A kick occurred in the overpressured section, and well control activities prevented the normal circulation of cuttings to the shale shaker, rendering cuttings unavailable for analysis. Real-time pore pressure prediction using drilling-based techniques was inverted to interpret lithology. Mechanical log parameters were calculated and calibrated with the immediately overlying drilled section, where cuttings and LWD logs were available. The integration of Pore Pressure and Lithology Prediction was pursued by employing a variant of mechanical specific energy, the drilling rate to applied weight on bit ratio (ROP/WOB), for coherence and quality checks. The challenge of lithology identification was successfully addressed, enabling the creation of an inferred lithology profile for the unlogged section that also lacked drill cuttings, clarifying the kick zone lithology. This interpreted lithology is a crucial aspect of characterizing the formations. It offers a clear understanding of sedimentation patterns and related seismic responses for future exploration in the depositional belt. The cost-effective fit-for-purpose approach is a robust exploration practice. Key words: Lithology, HPHT, Exploration, Kick, LWD, Dc exponent, Sigmalog, MSE, Fit for purpose, Geological model, Exploration practice.