Identification of Payzones from Log Measurements in Casing in Difficult Drilling Environment

Abstract

Open hole log evaluation has been traditionally used for formation evaluation purposes and is commonly used for reservoir characterization. Adverse hole conditions precluded use of these services. Under these circumstances, it becomes necessary to acquire the necessary log data after the well is cased for reservoir evaluation. Cased hole logs provide answers with acceptable uncertainty in characterized formations. However, in cased wells, which are not well characterized, the results tend to have a higher uncertainty. In one of the wells in Egypt the operator could not acquire the data in the open hole section because of adverse hole conditions. High quality cased hole data was acquired in the interesting section even in difficult borehole condition (heavy wash outs etc). The measurements are compared with the available open hole data and the responses explained. All the data which could have been acquired in the open hole was acquired in the cased hole with the available cased hole sensors.. Based on the data from the cased hole the bore hole condition at the time of casing the well was predicted. We also demonstrate that gaseous hydrocarbon can be identified from the cased hole epithermal neutron measurements which compared well with the resistivity and the density neutron separation. Reliable matrix density values have been estimated from the cased hole spectroscopy providing a reliable porosity values from the cased hole density. Reservoir parameters have been estimated in the cased hole section. It was possible to devise cost effective planning for completing the well based on these parameters. The ability to provide cased hole measurements helped the operator in avoiding a costly side track. Introduction Open hole formation evaluation from open hole logs has been the standard in formation evaluation for many years. During the last five years we have seen a trend in using some of the open hole services in cased hole for making a meaningful formation evaluation. These measurements have also been strengthened with the addition of tools exclusively for cased hole services. E & P companies always prefer to have the open hole logs for estimating the reservoir parameters. On numerous occasions, the risk of open hole logging is high. It makes economic sense to conduct logging operations after the well has been cased under those conditions. This paper describes the results of the analysis behind casing carried out for one of the operators in Egypt. The operator could not record open hole logs in the zone of interest because of bad bore hole conditions. The alternative was to side track the well so that necessary data can be acquired in the zone of interest. This was not a viable alternative since there was no guarantee that the well condition would improve while drilling the side track. After carrying out a detailed techno economic evaluation it was decided to case the well and run the fairly comphrensice suite of analysis behind casing services (ABC) for detailed formation evaluation. The basic services consist of the cased hole formation resistivty (CHFR(*), cased hole formation density (CHFD*), cased hole formation porosity (CHFP*). In addition to these basic services the client also reorded the DSI* and the ECS* for estimating the sonic slownesses and derivingt the lithology from the spectral measurements respectively. In this article we discuss the comparison of the basic open hole and the cased hole logs are discussed. The petrophysical results from the open and the cased hole are analyzed. A "synthetic density" log was also computed from the continuous grain density estimated from the spectroscopy measurements. There was a good correspondence of the match between the synthetic density and the cased hole density. The CHFP was also used for identification of gas bearing zone from zones and were confirmed by testing results.