During drilling it is important for the driller to know the properties of the formation being penetrated. Mud weight, the necessity for wiper trips and gas sand detection are some of the routine drilling operations that depend on the type of rock being drilled. To properly perform these operations, an analysis of the formation lithology on a real-time basis can be very important. Recent developments is MWD technology coupled with sophisticated computer processing at the rig site now provide unprecedented real time analysis of the formation being drilled. This formation analysis greatly facilitates decisions about safe mud weights, correlation and marker identification, and safe and efficient drilling. Also, the formation analysis can allow the geologist to perform a preliminary evaluation in order to plan the wireline logging program.

The term "formation analysis" as it is used here includes properties commonly estimated by oilfield geologists and petrophysicists. In addition to distinctions based on rock type (sandstone, shale, anhydrite…) and mineralogical composition, it incorporates porosity and fluid saturations since these properties influence drilling operations and are essential to computations of pore pressure.

In this paper we present the process of formation analysis while drilling. Starting at bit depth the mechanical measurements of downhole weight and torque provide an indica-tion of lithology and early warning of possible pore pressure changes. The interpretation is refined as subsequent measurements of spectral gamma ray, resistivity, neutron porosity, bulk density and Pe become available as drilling proceeds and the various MWD subs pass a particular horizon.

The interpretation of lithology is performed in a number of ways depending on which data are available at a particular depth. At bit depth the mechanical data allow distinction between major lithology types. Once the gamma ray and resistivity measurements are available a volumetric analysis is possible which provides information about sand and clay volumes as well as porosity. This volumetric analysis can subsequently incorporate the full suite of measurements. Combining gamma ray, neutron porosity and density also enables classification of lithology by reference to a prebuilt data base of lithologies. This analysis can be further refined by the inclusion of cuttings data. The analysis of lithology is extended to provide qualitative detection and description of thin beds through the supplemental use of curve shape analysis. Porosity and fluid types axe evaluated using a standard log analysis method. In this way an increasingly refined specification of formation properties becomes available as drilling progresses.

We will describe the identification of different lithologies, including the identification of thin beds, the identification of hazardous gas sands and the estimation of pore pressure in real time, which enables us to substantially reduce the risk of kicks and differential sticking.

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