Utilization of Density Logs to Delineate Desaturated Zones and Estimate Porosity in Cased Holes

Abstract

American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc. This paper was prepared for the 41st Annual California Regional Meeting of the Society of Petroleum Engineers of AIME, held in Santa Barbara, Calif., Oct. 28–30, 1970. Permission to copy is restricted to an abstract of not more than 300 words. Illustrations may not be copied. The abstract should contain conspicuous acknowledgment of where and by whom the paper is presented. Publication elsewhere after publication in the JOURNAL OF PETROLEUM TECHNOLOGY or the SOCIETY OF publication in the JOURNAL OF PETROLEUM TECHNOLOGY or the SOCIETY OF PETROLEUM ENGINEERS JOURNAL is usually granted upon request to the Editor PETROLEUM ENGINEERS JOURNAL is usually granted upon request to the Editor of the appropriate journal, provided agreement to give proper credit is made. Discussion of this paper is invited. Three copies of any discussion should be sent to the Society of Petroleum Engineers office. Such discussion may be presented at the above meeting and, with the paper, may be considered for publication in one of the two SPE magazines. Abstract Density and neutron logs have been used in combination to both define and discriminate between desaturated and low porosity zones in cased holes at Coalinga and Mount Poso fields. Preliminary results further indicate that Preliminary results further indicate that cased hole density log readings can be used to estimate open-hole density log readings when favorable casing - cement - hole size condition exist. This technique should have utility in evaluating old wells where porosity control is presently not available and in providing the presently not available and in providing the option to run the formation density log after casing is set when well conditions preclude running the log in the open hole. Introduction Many shallow oil fields in the San Joaquin Valley of California are plagued with extensive reservoir liquid desaturation caused by gravity drainage and/or reservoir cementation. Since both conditions have a profound effect on the planning and ultimate success of supplemental planning and ultimate success of supplemental recovery projects, a considerable amount of engineering effort has been expended to determine the location and degree of reservoir desaturation and cementation. Knowledge concerning the location and extent A desaturated and cemented zones is generally available on only the relatively few wells which have been recently drilled and logged with both resistivity and porosity devices. This unfortunate state of affairs has occurred because: 1. Many wells were drilled before the advent of wireline logging, 2. Many logged wells have only a resistivity record, which alone cannot be used to discriminate between desaturated and cemented zones, and 3. Much of the desaturation occurred after the drilling of the majority of the wells. Many fields have been, or are nearly, fully developed; thus information concerning desaturation and cementation must be derived from the existing cased holes. Cased-hole gamma ray-neutron logs can be successfully used to define sands and shales and to locate porous, liquid-filled zones. However, the neutron log cannot, in general, discriminate between liquid-filled zones of decreasing porosity and porous zones that are desaturated to varying degrees. The problem of discriminating between such zones is problem of discriminating between such zones is solved in the open hole by running a neutron log with either the sonic or density log and crossplotting the apparent log porosities. No such technique has been previously available in cased holes. The density log was developed and has been utilized strictly as an open-hole logging device.