Cased-Hole Measurement of Oil in Place, San Joaquin Valley, California

Abstract

Summary A log-inject-log (LIL) method was used to measure residual oil through casing in a reservoir with relatively fresh formation water. Water saturations calculated from pulsed-neutron-capture cross sections logged before and after injection of brine agree within about 10 saturation units (s.u.) with saturations previously measured from cores and from the electromagnetic propagation tool (TM) (EPT) run in open hole. Saturations from LIL and EPT measurements averaged through the interval perforated differ by about one s.u. Oil from cores averaged 0 to 10 s.u. higher than oil from LIL, depending on how core saturations were corrected for the increased porosity typical of cores from unconsolidated sands. Well completion and injection procedures were designed to achieve uniform injection without altering the unconsolidated sand containing liquids at low pressures. Surface filtering of injected liquids was necessary to prevent perforation plugging. Salinity of injected brine was checked by chemical analysis and by thermal-neutron-capture cross sections measured by a portable wellsite device furnished by a service company. Saturations calculated with the near-spaced detector of a pulsed-neutron-capture log were nearly equal to those calculated from the far-spaced detector and from service company correction charts. Residual oil in place (OIP) measured by cores, the EPT, and the LIL method represents saturation after thorough flushing by water. Current O[P can be higher than residual oil by the fraction of oil displaced by mud filtrate or by injected brine. Logged ratios from a carbon/oxygen (TM) (C/O) log run through casing 2 years after casing was set correlated with current OIP calculated from deep resistivity logs run in open hole. The higher oil saturations measured with resistivity logs that investigate beyond the zone swept by mud filtrate or with through-casing logs recorded after filtrate invasion has dissipated include the oil that can be displaced by brine. The silicon/calcium ratio recorded with the C/O log correlates with current OIP. Spectral data recorded with the C/O log and elemental analyses of core samples show that the silicon/calcium ratio is unrelated to the abundance of calcium in the rock in the sandstone logged. Introduction Accurate measurement of residual oil through casing is necessary to assess the potential of tertiary recovery in fields where it is not possible or economically attractive to drill new wells. This paper discusses application oftwo different methods to measure OIP through casing in a well for which previous work demonstrated that OIP can be determined from cores and open hole logs accurately enough to enable confident engineering decisions. Values for current OIP had been established from resistivity logs and the flow properties of cores. Saturations from an experimental nuclear magnetism log were consistent with these values. Residual OIP, the oil that remains after flushing with water, had been measured from core saturations and from the EPT. Results from the open hole studies showed lower- than expected OIP in the zone previously considered to be the most attractive target for a tertiary recovery project. The well was drilled cased in 1976 but was not completed before the work described here. The cased-hole study was proposedto demonstrate the accuracy of the through-casing measurements andto confirm the relatively high values of oil saturation observed in one producing interval above the original target zone. Two years after the well was cased, a C/O log was run to compare lot, response with previously measured OIP. The interval showing high oil saturation then was perforated and an LIL test was conducted to measure oil saturation after the formation was flushed thoroughly by injected brine. Two efforts to measure porosity with similar LIL measurements failed. Several additional C/O log runs were made to investigate changes caused by variations in tool response, effects of fluids in the borehole, and changes caused by brine injection. JPT P. 1295^