A Method for Detection of Untapped Intervals in a Complex Lithology System

Abstract

This paper is a case study calibrating well log derived lithology indicators with productivity attributes from modern production logs. More than thirty wells drilled thirty years ago into the South Ellwood Field were completed based on limited understanding of the well log responses in the complex Monterey Formation. From a retrospective study of well productivity, Wylie, Ershaghi and Christensen1 presented a pattern recognition technique for identifying productive intervals. Availability of recently obtained state-of-the-art production logs has provided invaluable information about the relevance of the well log derived potential zones to positively tested productive and undamaged intervals. This paper includes an analysis of the information processing from the Flow View and Gas Holdup Sensor Tool. A calibration of production log data and well log lithology pattern studies is the basis of designing a re-development strategy for untapped intervals not perforated in the initial completion work. Furthermore, the calibration methodology is extended to a nearby field with similar geology for reassessment of reserves potential. Introduction With the limitation imposed by regulatory agencies, the prospects of obtaining permits for re-completion rather than new drills are becoming of significant interest to California offshore producers. A major geological horizon with substantial potential in productivity is the extensive Miocene Monterey Formation. The geologic characteristics of this formation have been discussed by researchers such as Issacs2, MacKinnon3 and Belfield4. Recent reservoir studies by our team and others have pointed out the compartmentalization of Monterey horizons separated by major and minor faults. The compact laminations of various lithologies leaves very little communication path along the bedding planes. It is recognized that regional stresses are the main cause for creating a series of highly permeable brecciated and faulted intervals. As such, wells completed with course direction parallel to the brecciated intervals would turn out to be poor producers. A re-drill of such wells into the neighboring fractured intervals can change the productivity characteristics of the well. The Monterey Formation has provided substantive oil production for the State of California since its discovery in the 1970's. Its complex geological fabric consists of a mixed lithology characterized by folds, faults and fractures. Well completion in fractured intervals accounts for a large share of the oil produced from the Monterey. Identification of lithology intervals with production potential (IWPP) in the formation would serve not only to increase production, but also extend reservoir life. The major problem in deciding to re-drill or re-complete an existing well is the unavailability of complete suite of logs. From an earlier study by Wylie, Ershaghi and Christensen1, a procedure was devised where IWPP could be mapped along the well course from radar diagrams incorporating signals from seven logs. This methodology was substantiated from the correlation data observed between the historical cumulative productivity of the wells and the density of the occurrence of IWPP. With the advent of a new production logging tool (GHOST®), an opportunity developed to test the concept of proposed zonal detection method vs. the information obtained from such a modern approach to interval testing. This paper includes an analysis of the IWPP detector as calibrated vs. the GHOST® tool with the potential extrapolated to detect untapped zones behind the pipe.