The Use of Well Logs To Monitor a Surfactant Flood Pilot Test

Abstract

Summary A logging program has been developed and used successfully to monitor the performance of a surfactant flood pilot test in Loudon field, Fayette performance of a surfactant flood pilot test in Loudon field, Fayette County, IL. This program consisted of induction and carbon/oxygen (C/O) log measurements that were made repeatedly in five observation wells, producing time-lapse records of oil saturation changes. To enable use of producing time-lapse records of oil saturation changes. To enable use of induction logs, the observation wells were completed with fiberglass casing. Special efforts were made to maximize the accuracy of the oil saturations calculated from logs. These efforts included use ofpressure core measurements of residual oil saturation for calibrating log responses,laboratory measurements of the electrical properties of cores, andspecial wellsite calibration and quality control procedures. The log responses in each observation well provided essential data for evaluating pilot performance. The logs measured pre- and postflood oil saturations as well as the arrival time, oil saturation, and duration of the bank of mobilized oil. Vertical stratification effects also were observed. The combination of induction and C/O logs proved an effective means of monitoring a surfactant flood. Saturation measurements from the C/O log complemented the induction log results very well, providing better vertical resolution and a response independent of salinity and formation electrical properties, Introduction A surfactant flood pilot test was conducted recently in a watered-out portion of the Weiler sandstone in Loudon field. Ref. 1 is an over-view of portion of the Weiler sandstone in Loudon field. Ref. 1 is an over-view of this test. The pilot test began May 13, 1980, with the start of microemulsion injection into the four comer wells of a normal five-spot pattern on the L. Ripley (LR) Lease. The test was concluded Oct. 16, 1981, pattern on the L. Ripley (LR) Lease. The test was concluded Oct. 16, 1981, after 2.25 PV total fluid production. As part of a program to evaluate the pilot performance, five observation wells were drilled for making logging pilot performance, five observation wells were drilled for making logging measurements within the five-spot pattern. The objectives of the observation well logging program were to determine the initial porosity and oil saturation distributions and to monitor subsequent changes in oil saturation. Neutron and density logs were run to determine the formation porosity, and induction and C/O logs were used to estimate oil saturations. Core data provided direct measurements of porosity and residual oil saturation (ROS) to calibrate the log responses. ("Residual" oil saturation as used here is not intended to mean true irreducible oil saturation but rather that saturation remaining at the end of water flood when the oil fractional flow is less than 2%.)Both induction and C/O logs were used in the monitoring program to provide independent and complementary saturation estimates. The use of provide independent and complementary saturation estimates. The use of only the induction log was considered insufficient because of its salinity dependence, poor vertical resolution, and dependence on formation electrical properties, which could conceivably change with surfactant- induced changes in wettability. The C/O log did not suffer from these problems but was untested in an application that required the degree of problems but was untested in an application that required the degree of accuracy desired here. For example, the effects of minor lithology variations and a limited depth of investigation on the C/O log response were unknown. During the pilot, it was observed that the different depths of investigation of the tools was an important factor, which resulted in different log responses. To accomplish the monitoring objectives, we selected a logging schedule consisting of repeated time-lapse measurements of oil saturation in the five observation wells. Many logging measurements taken before the stall of injection provided the initial profiles (base response) for the induction and C/O logs in each well. Subsequent logs were run to detect the passage of the mobilized oil bank. From these measurements, several indicators of pilot performance were obtained: vertical profiles of the preflood ROS and postflood final oil saturation, and the arrival time, preflood ROS and postflood final oil saturation, and the arrival time, maximum saturation, and duration of the bank of mobilized oil. Such measurements provided estimates of stratification effects and the overall effectiveness of the surfactant flooding process. Pilot Description Pilot Description Bottomhole locations of the wells forming the pilot test pattern are shown in Fig. 1. These wells make up a 0.68-acre [2752-m2] normal five-spot pattern with five observation wells located within the pattern. All wells pattern with five observation wells located within the pattern. All wells were drilled specifically for the test and were cored during drilling. During the pilot flood, fluids were injected into the four corner wells (LR 10, 11, 14, 18); the central well (LR13) was produced; and oil saturation changes were monitored with logs in the five observation wells (LR 9, 12, 15, 16, 17). The remaining well (LR29) was drilled and cored after the flood to provide an independent measurement of the postflood oil saturation. The typical subsurface lithology in the pilot area is shown in Fig. 2, which presents gamma ray, density, and induction logs from Well LR17. Three lithologic zones are of interest because of their prominence in each of the wells. JPT p. 1379