The 1st AAPG/EAGE PNG Geosciences Conference, PNG’s Oil and Gas Industry:
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Determination of Porosity from Mechanical Measurements in Carbonates Affected by Severe Drilling Fluid Invasion


The advent of pressurised mud cap drilling (PMCD), where a light annular mud 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 that high losses of seawater and drilling mud/fluid into the formation makes evaluation of wireline and even logging whilst drilling logs problematic, with difficult determination of porosity and water saturation. Downhole drilling dynamics tools allow for accurate acquisition of drilling parameters close to the bit during drilling. Since the initial point at which the bit penetrates the rock is when the rock has had least exposure of time to invasion from drilling mud/fluid, it is proposed that methods that relate mechanical drilling parameters to rock strength should provide a more accurate indicator of porosity. 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 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. To preserve competitive advantage, details behind the methods used were not published. As logging whilst drilling tools gained in popularity the technique has fallen into disuse, and drilling porosity is no longer used by industry today. An improvement to earlier empirical drilling porosity methods based on the d-exponent is proposed. The method is derived from a first principles approach that honours conservation of energy. The specific energy applied to the drill bit, being the energy necessary to drill a unit volume of formation, is described in terms of the axial, rotational and hydraulic forces applied at the drill bit. This hydromechanical specific energy is equivalent to the rock strength being drilled. By describing the rock strength in terms of its elastic properties, which can be related to mineralogy, fluid content and porosity, it is possible to build a rock model to predict porosity from measured downhole drilling parameters where the mineralogy and fluid content are known. The method is applied to the Pasca A4(AD-1) well which was drilled into a carbonate reef in the Gulf of Papua, offshore Papua New Guinea. This well acquired a full suite of logging while drilling and wireline logs, but interpretation of these logs has proved problematic because of the high losses of drilling fluids into the formation whilst drilling in PMCD mode. This well utilised a downhole drilling dynamics tool to measure drilling parameters at the bit. A porosity log has been calculated using this data and compared to the older empirical d exponent approach and conventional petrophysical wireline log interpretation. Results show that the new porosity method derived from mechanical measurements is accurate and matches conventional wireline log interpretation where invasion can be shown to be minimal (e.g. drilling ahead with no losses and returns to surface) and there is a higher degree of confidence in the electrical measurements. A comparison against laboratory measured porosity from a side wall core sample shows that the new drilling porosity method can be considered more accurate than conventional petrophysical log interpretation which in general underestimates the true porosity under PMCD conditions because of extensive invasion by drilling fluids.