The Maljamar CO2 Pilot: Review and Results

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Summary.: This paper presents the results of a CO2, flood pilot performed in the Permian Age carbonate rock formations of the Maljamar Cooperative Agreement (MCA) Unit. Field background and pilot development are reviewed. Injection and production history are presented, along with an evaluation of the pilot. A summary of the observation well logging results is also given. Introduction: The 5-acre (20X10-3 -m2) Maljamar tertiary recovery pilot formally ended Jan. 1, 1986. Special features of the inverted five-spot pilot included dual (separate) completions in the Grayburg and San Andres zones and two fiberglass-cased logging observation wells for in-situ monitoring of oil, brine, and CO2 movement. Solutions to operational problems and a determination of the process performance were intermediate objectives of the pilot. The major objective was to provide a basis for commercial scale CO2 floodline economics for, the unit. CO2 retention by the reservoir has prevented major CO2 production. The observation wells indicated that CO2 contacted the entire vertical section. Operating problems and pilot performance have been different for the two zones. Problems, injectivity, oil response, and CO2 production will be discussed by zone. The pilot successfully met its objectives by providing data on field operations during CO2 flooding, as well as the process performance data needed to estimate largescale CO2 flooding results in the field. Operating problems have been largely resolved, incremental oil production has peaked, and performance has been encouraging. An expansion project is being designed. General Information: The 8,040-acre [ 32 × 10 6 -M 2 ] MCA Unit occupies about 20% of the Maljamar field in Western Lea County, NM (Fig. 1). Wells produce from a Grayburg dolomitic sand (Sixth zone) and four San Andres dolomite pay zones (Upper Seventh, Lower Seventh. Upper Ninth, and Ninth Massive) at depths ranging from 3,600 to about 4,100 ft 1100 to about 1250 m], Fig. 2 presents a typical log. Oil gravity is 35 to 37deg. API [0.85 to 0.84 g/CM3]. Reservoir and fluid characteristics are shown in Table 1. The MCA Unit has performed well under waterflood, considering that it is a multizone flood in a heterogeneous reservoir. The better-quality zones. Sixth and Ninth Massive, have performed very well. At the economic limit for the existing waterflood, about 60% of the original oil in place (OOIP) would remain unrecovered. thereby making this a significant target for EOR operations.' The technical feasibility of CO2 flooding was demonstrated experimentally by determination of the minimum miscibility pressure (MMP) in slim-tube floods. At 1,515 psi [10.4 MPa]. the MMP is considerably lower than the average reservoir pressure of about 2.1500 psi [ 17.2 MPa]. Feasibility studies were performed for several different CO2 flood options in the field. Because of the economic risks associated with a commercial-scale project, the decision was made in 1978 to develop a pilot project for the MCA Unit. Information from the pilot would reduce the operational and economic uncertainties of a full-scale project. The pilot was expected to provide information on whether CO2 can mobilize oil in the reservoir, on how much CO2 would be required for a barrel of oil recovered. on how the CO2 injection rates would change with time, and on what values of process parameters should be used in simulations to predict field-scale CO2 flooding operations. Separate floods were conducted in the Sixth (the Grayburg sand interval) and the Ninth Massive zones (representative of the San Andres zones). The two floods were performed simultaneously by use of dual (separate) completions in the pilot wells. Two logging wells, completed with 680 ft [207 m] of fiberglass casing across the pay intervals and with no perforations, were also included. Logging data from these wells would allow determination of vertical variations in horizontal permeability and values for process parameters. An inverted five-spot pattern was selected to decrease the total volume of CO2 that would have to be purchased. At the time, there were indications that decisions about field-scale expansion would have to be made within 2 to 3 years to be confident of securing a CO2 supply. Five acres [20 × 103 M2) was chosen as the largest pilot that could be completed in a reasonably short period of time. Fig. 3 shows the final pilot pattern. Operating Plan: The pilot development and prepilot planning and testing were reported previously. 1 Pilot development and operation consisted of the six phases shown in Fig. 4. JPT P. 1256^

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