Lithology Determination Using Downhole Bit Mechanics Data

Abstract

Abstract A technique has been developed which uses downhole weight on bit (DWOB) and downhole torque (DTOR) measurements to determine the occurrence and thickness of cemented zones in the pay sands of the Kuparuk River oil field. The DWOB/DTOR measurements are made by the MWD tool which is located above the bit and is normally not affected by frictional losses along the drill string. The cemented zones, containing siderite in the upper sand and ankerite in the lower, are high density streaks that reduce production or in the case of Ankerite are non productive. Using these measurements the cemented zones can be detected, in real time, when the bit first contacts the zone. Because the downhole sensors respond immediately to the changes in lithology the vertical resolution is one ft. which is better than the open hole logs commonly obtained in the field. From comparisons with density-neutron log data it was demonstrated that the new technique reliably discriminated between cemented and non cemented zones while drilling. Because of the reliability and the high vertical resolution of the technique, the mapping of the cemented zones and the resulting oil in place estimates have improved. The lithology information obtained from the DWOB/DTOR measurements compliment the triple combo data. By comparing the drilling mechanics data with the density and neutron logs suspect data can be highlighted. In areas where washouts have occurred the wireline data may be adversely affected but the drilling mechanics arc not. The acquisition of downhole bit mechanics data is a useful addition to the logging program and can be effectively utilized in combination with a full log suite or as a replacement for the density-neutron tools as an ankerite or siderite locator.