Data Integration Case Study: Using Cased Hole Saturation Measurements And Sub-Layer Pressures to Find Bypassed Oil In The Shallow Oil Zone, Elk Hills Field, CA
Authors: DeRose William E. , Katragadda Dave P. 
Published: December 2007

The Shallow Oil Zone (SOZ) is the largest oil reservoir within the Elk Hills Field. Discovered in the early 1900's, it has been produced extensively by gravity drainage to very low (5 to 15 psig) gas cap pressures in the upper, Eastern SOZ Sands. The reservoir is a series of sands interbedded with shales that are aerially continuous, but highly compartmentalized by extensive faulting. This paper presents a case study on the search for bypassed oil that is trapped along these faults and in individual layers. The approach includes the use of both cased hole and open hole logs. Saturation log data is integrated with sub-layer pressure and permeability data taken from formation tests of new and recompleted wells.

Over twenty cased hole pulsed neutron spectroscopy and capture logs have been run. Recompletions in intervals showing promising oil saturations were monitored closely to determine the relationship of oil productivity and log-derived oil saturations as measured through the casing. The objective of this process is to find the moveable oil in each sand member or lobe, in wells that are thought to be watered or gassed out, and are uneconomic to produce. Reliability of cased hole logs to indicate remaining oil is also discussed. Well selection criteria, data acquisition, analysis, and results are presented.

Pressure and permeability data acquired from the individual sand members of new wells before casing is run provides additional information on fluid contacts in each sand layer. Forty wells have been pressure tested with a formation tester over the last four years. Cross plots of this combined pressure data versus corrected vertical subsea depth allows gradient curve-fitting from well to well. Fluid gradient analysis allows an estimate of where oil-water and gas-oil contacts lie between well locations, and helps confirm the presence or absence of sealing faults between wells. This information is incorporated into pore volume maps for each sand layer to allow for accurate estimation of remaining oil reserves and hopefully to show where bypassed oil can likely be found.

Results of the logs are shown in terms of oil production from several wells that were perforated based upon the log information. This points out the need for sub-layer management in an effort to maximize oil recovery.

Introduction

The SOZ at Elk Hills Field (Navel Petroleum Reserve in California - NPRC) is severely pressure depleted in the eastern portion of the field. Water is encroaching on the north and east flanks in the SubScalez (SS) and Mulinia (M) Sands. There is also limited water encroachment along the south flank in the SS and M sands. The 76 year productive life of the reservoir has resulted in large gas caps with pressures less than 10 psig and oil bands with less than 50 psig. Gas is now being injected to maintain the gas cap pressure at 45 psig and some dewatering wells are employed to stabilize the water influx along the north flank. A "zero psi" gas collection system helps to maximize the differential pressure between the reservoir and the wellbore.

The SS and M Sands are interbedded with shales that are areally continuous, but segmented by faulting. Permeability variations between sand layers have contributed to separate gas-oil and water-oil Contacts. This type of reservoir lends itself to problems with bypassed oil in individual layers.

Early wells were completed with slotted liners, resulting in commingled sand layers. Thus, wells that appear to be no longer economic due to water or gas breakthrough may have potential if gas and oil layers can be identified in shut-in areas of the field.

This paper identifies, discusses and shows results of logs that can be used to determine which sand layers contain bypassed oil. Example wells are existing solid and liner completions with non-perforated sands behind pipe and new wells drilled in areas thought to contain additional reserves.

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