The advent of pressurised mud cap drilling (PMCD), where an underbalanced mud (“light annualar mud”, LAM”) is continually pumped down the annulus into a reservoir whilst drilling, allows for safe drilling of highly porous and permeable formations such as carbonate reefs. It has been applied successfully to wells drilled onshore and offshore in Papua New Guinea. The downside of the PMCD technique is high losses of seawater, drilling mud/fluid and cuttings into the formation leading to both porosity occlusion and an altered borehole environment inconsistent with assumptions used in classical log analysis. This makes evaluation of wireline and even logging whilst drilling logs problematic, with difficult determination of porosity and water saturation (Kyi, Han, Lee, Roberts, & Maeso, 2015). Since the initial point at which the bit penetrates the rock is when the rock has had least exposure of time to any invasion, it is proposed that the problems faced by conventional interpretation of electrical logs can be resolved through application of an alternative method that measures mechanical rock properties at the bit as the rock is drilled.
This insight is not new. The term “drilling porosity” refers to techniques by which porosity of a formation being drilled could be determined solely from mechanical measurements such as rate of penetration, weight on bit and rotational speed without the need for any separate electrical logging tool. It is no coincidence that the introduction of drilling porosity methods coincided with the advent of computerised mud logging units in the late 1970s which allowed digital recording of surface drilling parameters for the first time. Accurate monitoring and collection of surface drilling data was a necessary component for development of drilling porosity logs. Mud logging companies promoted the use of drilling porosity as a real-time indication of reservoir quality and pore pressure during drilling (Zoeller, 1972). However, details behind the methods used were not published, presumably to preserve competitive advantage. As logging whilst drilling tools gained in popularity the technique has fallen into disuse, and drilling porosity is seldom used by industry today.
Peter Kirkham is the Project Engineering and Commercialisation Manager at Twinza Oil where he helped to rediscover the Pasca A field and shape its development concept over the past five years. He holds degrees in in Engineering from the University of Cambridge and Petroleum Engineering from the University of NSW. Peter specialises in integration of knowledge and data across different technical and commercial disciplines to develop a comprehensive asset understanding.