The conventional dual laterolog (DLL) instrument, providing valuable information on formation resistivities, has been used in the petroleum industry for many years. However, in many situations, physical limitations of the DLL (e.g., limited vertical resolution, poor radial resolution in the presence of deep invasion, incorrect readings due to the Groningen effect, artifacts occurring in deviated wells) do not allow the interpreter to obtain the required information. Moreover, DLL data interpretation is normally accomplished on a level-by-level basis (1-D), which may result in overlooking hydrocarbon-bearing formations, underestimation of reserves, and low confidence in the interpretation results.
Currently, when oil production originates in thinly laminated reservoirs penetrated by vertical or highly deviated wells, the DLL tool alone cannot satisfy industry requirements for accurately predicting formation resistivity. The new High-Definition Lateral Log (HDLL) tool was developed to provide high-resolution array resistivity measurements and overcome most of these shortcomings.
The HDLL array tool performs a detailed radial sounding of the formation to evaluate the drilling fluid invasion profile. The tool provides a high vertical resolution detecting thin beds up to a 9-in. thickness. The data does not suffer from the Groningen effect and borehole dip artifacts. The resistivity image of the formation around the borehole, delivered at the well site, provides information necessary to delineate permeable zones and supports immediate operational decisions. Application of 2-D/3-D inversion-based interpretation allows the interpreter to recover the true formation resistivity and thus more accurately delineate and estimate the hydrocarbons.
The scope of the paper is to briefly introduce the HDLL technology and present quantitative results of petrophysical interpretations derived by the application of conventional and array-type resistivity measurements.
Two case studies for vertical and highly deviated wells from Oman and the North Sea demonstrate the added value provided by the HDLL technology. In the presented cases, HDLL-based interpretations show not only improved delineation of the known reservoirs but also extra pay intervals overlooked by DLL-based interpretations. The hydrocarbon saturation derived with the application of HDLL data is higher than the hydrocarbon saturation derived by the application of the conventional petrophysical interpretation. Use of the entire data suite of array data provides a much higher level of confidence in the presented hydrocarbon estimates.
Introduction
The resolution of existing laterolog-type devices can be insufficient in complex hydrocarbon reservoirs. The High-Definition Lateral Log (HDLL) provides formation resistivities at multiple depths of investigation in conductive, water-based drilling mud systems. The HDLL tool addresses three primary limitations of the conventional dual laterolog (DLL) systems that measure formation resistivitiesin thinly bedded hydrocarbon-bearing reservoirs;in the presence of deep drilling fluid invasion; andin highly deviated and horizontal boreholes.
In order to extract the information fully from the HDLL array data, special interpretation techniques are required. Modeling and inversion are such techniques, which allow complex geological scenarios to be incorporated into the interpretation process. Geophysical inversion, utilizing forward modeling for different types of logging data, provides the most accurate and reliable set of physical formation properties. Using inverted HDLL data results in a better reservoir description, a detailed evaluation of the drilling fluid invasion profile, and a more accurate water saturation (Sw) determination. This information increases the reliability of oil-in-place calculations.
