Tracer Surveys To Identify Channels for Remedial Work Prior to CO2 Injection at MCA Unit, New Mexico

Abstract

A tracer program to identify channels within the Grayburg/San Andres formations prior to CO2 injection has been performed and analyzed at the Maljamar Cooperative Agreement (MCA) Unit. For safety and costs reasons two separate interwell tracer surveys or phases were conducted. In the Phase 1 tests tritium was injected into 62 wells through injection headers to identify field areas where channelling occurs. Within 12 weeks, 25 out of 177 producing wells sampled had a response. Tritium was used because it has a larger acceptable response window between its minimum detectable level and the maximum safe peak response than other available radioactive tracers. This first survey provided important design information for the second survey which included tracers with smaller response windows. The Phase 2 tests included the 19 producers with the larger Phase 1 responses and their 27 active offset injectors. With the use of multiple tracers, the Phase 2 tests identified 14 problem injection wells as channelling sources. Unexpectedly, the source of some channels was farther away than the nearest offset injectors. In addition, the two tracer surveys showed that tracer flow is sensitive to changes in operating conditions. Thus, every reasonable effort should be made to perform tracer surveys under normal operating conditions. Introduction Since EOR injection fluids such as CO2 are expensive, every reasonable effort should be taken to increase their efficiency. Flow channels between injection and production wells can cause inefficient use of injected fluids. Such channels need to be identified early to allow remedial work ore injection wells before the EOR project begins. Well-to-well tracers are an effective way to identify such flow channels and their field wide directional trends. For safety and costs reasons, two separate tracer surveys were conducted at the Maljamar Cooperative (MCA) Unit prior to a proposed CO2 trod of the Grayburg/San Andres formations. In the first survey, tritium was injected into all active injection wells within the proposed CO2 Project Area to identify areas with channeling With tritium, a large area can be surveyed at a relatively low cost. The second survey required the use of three tracers (tritium, carbon-14, and iodine-125) to identify the source injection wells within each early responding area. Besides identifying the channelling areas, the first survey provided important design information for the second survey. Tritium is a relatively safe tracer that has a much higher allowable peak response under Nuclear Regulatory Commission (NRC) guidelines than other alternative radioactive tracers. in addition, tritium is relatively easy and inexpensive to detect. By knowing the amount of tritium causing a given response at a producing well, the required amount of carbon-I 4 or iodine-125 in the second survey could be more reliably estimated. Then, the more stringent NRC response ceilings for carbon-I 4 and iodine-125 could be safely met. At the same time, the designed response had to exceed the detection threshold of available counting instruments. The window between the minimum detectable level and the maximum safe peak response is much wider for tritium. The first survey took advantage of this larger window. MCA UNIT BACKGROUND The 8,040 acre [32 × 106 m2] MCA Unit occupies a large part of the Maljamar Field in western Lea County, New Mexico (Figure 1). The Grayburg-San Andres pool was discovered in 1926, followed by the majority of development drilling in the early 1940s. P. 567^

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