Improved Core Recovery in Laminated Sand and Shale Sequences

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Coring and core analysis are essential to the exploration, development, and production phases of the oil and gas industry. Large-diameter (4-in. (10-cm)) core provides engineers and geologists with direct means to measure physical properties of reservoir rocks at both the microscopic and macroscopic levels. This information provides engineers with clues to improve their understanding of the reservoir and prediction of its performance. If stored properly, core may assist in development of the reservoir many years after the well is drilled. In microlaminated reservoirs, laboratory core analysis is very important because of inherent limitations in wireline log resolution. In these cases, petrophysical information, such as saturation, porosity, and net feet of pay, cannot be calculated from wireline data. Instead, these data must be measured directly from core plugs in the laboratory. Historically, core recovery in these types of reservoirs has not been good (Fig. 1A) using methods designed for firmly consolidated formations. These methods did not achieve satisfactory recovery in unconsolidated sand interbedded with hard shale stringers for two reasons: unconsolidated sand was eroded by mechanical or hydraulic means and shale ''jammed'' in the core barrel, thereby preventing more core from entering. Changes in coring strategies and equipment have nearly eliminatedmore » recovery problems in unconsolidated sand while reducing jams in shale (Fig. 1B). This paper discusses several of these changes and presents ideas for further improvements.« less