This paper describes the field applications of low invasion coring using waterbase coring muds in several Alaskan and lower 48 sandstone reservoirs. This is a companion paper to SPE 20413 which presented the laboratory development of the tow invasion coring method.

High coring rates exceeding 200 feet/hour (61 m/hr) with 100% core recovery were achieved with the new low invasion PDC core bits. In most applications, this technique yields a 2.5 inch (6.35 cm) diameter uninvaded core center in a 4 inch (10.16 cm) diameter core. The invasion of the outer part of the core has been observed in the field as a color change caused by lower oil saturation. Tracers confirm that when coring rate exceeds 80 ft/hr (24.4 m/hr), approximately 80% of the samples taken from the core center have no mud filtrate invasion for permeabilities greater than 50 md.

Connate water saturation measured in offsetting wells by oilbase cores agree with low invasion waterbase cores. This demonstrates the possibility of using low invasion coring for log calibration and other reservoir engineering applications where water saturation is needed. These core samples are also useful for laboratory testing to measure relative permeability and other wettability sensitive parameters.

Field muds with 5 cm3 API fluid-loss may be used in such coring operations as successfully as more costly oilbase coring fluids. The combination of high coring rate (reduced rig time) and use of field muds (reduced mud cost) can have significant impact on coring cost.

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