Vertncal Resolution Enhancement Of The Phasor Induction - Applications In Western Canada

Abstract

Abstract A substantial number of wells in western Canada are drilled into thinly bedded reservoirs. Quantitative evaluation of these reservoirs is difficult using the standard induction due to poor verticle resolution and shoulder bed effect. The Phasor induction tool and Phasor processing reduce the shoulder bed effect but do not address the thin bed response. The recent introduction of Enhanced Resolution Phasor processing has made it possible, under favourable conditions to fully resolve the resistivity of a I m bed. This processing has been used successfully in western Canada to give a better resistivity in thinly bedded reservoirs. Typically in thin beds, it yields a higher resistivity. This results in a lower water saturation calculation and a higher reserve estimate. By accurately resolving resistivity the processing gives a better indication of invasion in thin beds. The effect of the processing and its limitations for western Canada are studied using computer modelling and field examples. Introduction Many of the conventional oil and gas wells drilled in western Canada are into reservoirs containing bedding features less than 2 m thick. Quantitative evaluation of these reservoirs is difficult or impossible using the standard induction tool. The limitation of the standard induction measurement prevent it from providing a true resistivity in many situations. The two significant limitations are shoulder bed effect and vertical resolution. At any depth in the wellbore, the deep induction reading is influenced by the formation resistivity for a large distance on either side of the tool. For the common case of a high resistivity pay zone surrounded by lower resistivity shale beds, the shoulder bed effect causes ILD to underestimate the pay zone resistivity. The result is a pessimistic estimation of water saturation and reserves. Charts are available to correct for shoulder bed effect. These charts, however, are only available for a limited number of cases. The design of any logging device involves a compromise between depth of investigation and vertic.al resolution. The relatively deep depth of investigation of ILO occurs at the expense of vertical resolution. The minimum bed thickness that ILO can resolve is approximately 2 m (provided that shoulder bed effects are negligible). This means the peak reading of ILD in a uniform 2 m bed is representative of the resistivity of the entire interval. Further interpretation difficulties arise when ILD is combined with measurements that have superior vertical resolution, such as a porosity device Or the medium induction (ILM). For example, the separation between ILD and ILM is often used to estimate the diameter of mud filtrate invasion. In thin beds the separation between ILD and ILM is often due to the differing vertical resolutions of the two measurements. Recent developments in the application of signal processing techniques to induction measurements have resulted in improved resistivity readings. In 1986, the introduction of the Phasor induction tool and Phasor processing resulted in deep and medium induction logs (IDPH and IMPH) corrected for shoulder bed effects. A more advanced processing technique, Enhanced Resolution Phasor (ERP) processing was recently introduced.