Use of Well Logs To Characterize Fluid Flow in the Maljamar CO2 Pilot

Abstract

Summary The Maliamar CO2 pilot in Lea County, NM is a 5-acre [2 x 104 -M 2 ] inverted five-spot pattern. Two zones are being flooded: a Grayburg dolomitic sand at 3,700 ft 11 130 m] and a San Andres dolomite at 4,050 ft [1230 m]. Two logging observation wells, completed with fiberglass casing through the section of interest, are located in line with the center injector and one of the corner producers. Nine months of freshwater injection in the center well was followed by 9 months of brine injection. A series of induction logs monitored the passing of the freshwater/brine interface, providing data for a preliminary passing of the freshwater/brine interface, providing data for a preliminary characterization of flow in the zones. The brine also established a uniform salinity at the observation wells for saturation determination. Gamma-emitting tracers were injected into each zone of the center well as part of a well-to-well tracer study. Frequent gamma ray logs were run in the observation wells to see whether the movement of the tracers could be detected and used to characterize water movement. The results were very encouraging and provided better vertical and time resolution than the induction logs. The numerous responding layers in each zone could be classified by tracer arrival times into only a few basic types. Injection Of CO2 and follow-up brine has been monitored with a series of induction and neutron logs to follow the changes in water and CO2 saturation as the flood progressed. Introduction The 5-acre [2 ×104-M2] CO2 pilot in the Maljamar field is now in the post-CO2/brine-flush phase. The Maljamar field, located in the northwest comer of Lea County, NM, produces from several Grayburg dolomitic sands and San Andres dolomite zones at depths ranging from about 3,600 to 4,100 ft [1,100 to 1,250 m]. The field has been under waterflood since 1963. Fig. 1 shows the part of the field where the pilot is located. The contours are on the top of the San Andres formation. The pilot, as shown in Fig. 2, is an inverted five-spot pattern with a center injection and four corner pattern with a center injection and four corner production wells. In addition, there are four outside production wells. In addition, there are four outside water-injection wells for pressure confinement. The center injection well and four producers are dually completed in the Grayburg Sixth zone at a depth of 3,700 ft [ 1130 m] and the San Andres Upper Ninth Massive zone at 4,050 ft [1230 m). As mentioned above, the Grayburg Sixth zone is a dolomitic sand. Its top is about 100 ft [30 m] above the top of the San Andres. The Upper Ninth Massive zone, referred to as the Ninth zone, is a dolomite. Its top is about 220 ft [70 m] below the top of the San Andres. Two logging observation wells were drilled on a line between the center injection well, MCA358, and the northeast production well, MCA359. Their surface locations from MCA358 are 100 and 150 ft [30 and 46 mi for MCA363 and MCA364. respectively. The corresponding distances at formation depth are approximately 90 and 140 m P7 and 43 m]. They were completed with 680 ft [207 m] of 516-in. 114-cm] fiberglass casing through the zones of interest to allow the use of an induction log to measure formation resistivity. They have not been perforated. perforated. The pilot is operated as a standard production pilot and also as a logging observation pilot. The use of logs to characterize fluid flow complements production data in several ways: logsprovide vertical definition that is not possible otherwise;provide an earlier interpretation of possible otherwise; (2) provide an earlier interpretation of the process performance;can provide vertical sweep data; andwill provide an in-situ estimate of residual oil saturation after CO2 flooding that is independent of sweep-efficiency estimates or of material balance based on an estimate of initial oil saturation The injection schedule for MCA358 is shown in Fig. As can be seen, fresh water was injected from Aug. 1981 until May 1982, when a switch was made to brine with a salinity of about 170 g/L, which is similar to that of the original formation water. In Oct. 1982, slugs of cobalt-60 and cobalt-58 were injected into the Sixth and Ninth zones, respectively. Brine injection was shut in in Feb. 1983, and after some startup difficulties, CO2 injection started in May. In Dec. 1983, injection was switched back to brine. We are still in the Post-CO2/brine phase of the pilot. phase of the pilot. Logging Objectives The objectives set out for the logging observation wells were (1) to identify layers responding to injected fluid, (2) to determine arrival times for each fluid in each layer, (3) to test our ability to observe gamma-ray emitting tracers as they pass the observations wells (4) to monitor changes in water, oil, and CO2 saturations in each layer, (5) to provide data on vertical sweep efficiency, and (6) to evaluate pilot performance. JPT p. 883