Evaluation of Wireline Down-Log Information and Effects of Increased Log Speeds on Data Quality

Abstract

Wireline logs run in deviated Gulf of Mexico wells using new logging techniques are studied. Formation density and other logs can be recorded logging in or out of a hole at very high speeds and often provide excellent evaluation information. Down logs can be very useful in the event the usual up log becomes impossible. Increased log speeds may be justified in some cases involving hole problems or high rig costs. Introduction A study has been made of openhole wireline logs run in deviated wells in the Gulf of Mexico. Some new logging techniques of interest have been made possible by the introduction of computerized digital possible by the introduction of computerized digital surface logging units. Many of these techniques could not have been used previously due to limitations of the conventional analog surface logging units.Logs run going in the hole (down logs) often can provide valuable formation evaluation information, provide valuable formation evaluation information, particularly porosity data. Sometimes hole conditions particularly porosity data. Sometimes hole conditions are severe enough that the logs run coming up from total depth are not obtainable. Down-log porosity data often can distinguish hydrocarbon-bearing from tight zones and even oil from gas.Of particular interest is the observation that the formation density tool often can provide very good average porosity values when logged going in a deviated hole at a speed of 10,000 ft/hr with the caliper arm closed (Figs. 1 and 2). Down logs using other types of logging tools also have provided critical evaluation information.The accuracy of depth information is a problem, but zone thicknesses often are correct and depth errors can be estimated.The repeatability of various log types when run at speeds higher than normal also has been studied. Formation density logs run in high-porosity formations at 3,000 and 5,000 ft/hr show only minor variations from logs run at the conventional speed of 1,800 - ft/hr (Figs. 3 and 4). This excellent repeatability is due to the high gamma-ray count rates of the density tool in the offshore low-density formations of Pleistocene age. In fact, the variations observable between density logs run over a common interval in these low-density formations may be primarily a function of wall contact effects, not the primarily a function of wall contact effects, not the random statistical count rate variations. Statistics for the compensated neutron and gamma-ray devices relative to logging speed also are studied.Induction-type devices have been run at speeds up to 15,000 ft/hr with excellent repeatability using the computerized surface units.This paper is not intended to be a recommendation for increases in log speeds, particularly for the porosity devices. Increased formation density log porosity devices. Increased formation density log speeds in complex lithologies, for example, could reduce the value of the data greatly. JPT P. 1151