Gravel Pack Evaluation

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Neal, Marvin R., SPE, Schlumberger Offshore Services Summary Many wells in the Gulf of Mexico are completed with internal gravel pack assemblies to prevent sand production. A reliable method for evaluating the pack is needed to reduce the chances that a work over will be required later. Field logs made with tools chosen from test-well experiments confirm the application of wireline logging techniques to provide a qualitative evaluation of gravel pack quality. pack quality. Introduction Many logging tools have potential for investigating the annular space between screen and casing in an internal gravel pack. Among these are neutron porosity, acoustic velocity, density, and pulsed neutron decay time tools. The choice of tools used for the examples in this paper was based on previous experience and the known factors of depth of investigation and response to metal. The tools chosen werea compensated neutron tool, 3 3/8-in. diameter,a nuclear fluid density meter, 1 11/16-in. diameter, anda dual-spacing gamma ray tool, 1 11/16-in. diameter. Measurements were made in two test wells equipped with standard gravel pack assemblies in 9 5/8-in. and 7-in. casing. Response to various problem gravel pack situations was studied by making successive runs in partially packed sections. partially packed sections. The compensated neutron, nuclear fluid density meter, and dual-spacing gamma ray tools all gave equally good results, and each responded well to changes in the density of the material in the annulus between screen and casing. This paper describes the test wells and the logs that were obtained from various gravel pack situations. Actual field logs that further validate the effectiveness of the technique are shown. Theory Compensated Neutron Tool The compensated neutron tool uses a neutron source and two detectors (near and far). In its conventional usage the count rates from the two detectors are used to compute a ratio from which porosity can be determined. The compensated neutron tool measures the hydrogen index of a formation by emitting fast neutrons into the formation and detecting those that are slowed to the thermal energy level. This is possible because hydrogen has the greatest slowing effect on neutrons. The fluids that are contained in the pore space are rich in hydrogen atoms, whereas the formation rock contains little or no hydrogen. Therefore, the compensated neutron log reflects primarily the fluid-filled part of the formation. Two detectors are used to eliminate part of the effect of the surrounding borehole. The difference in the spacing causes one detector to be more affected by the borehole than the other and allows computation of a ratio of the two that is relatively free of borehole effects. Gravel pack logging utilizes the fact that the detectors are affected differently by the area between the tool and the formation. When the space between the screen and casing is filled with fluid, the tool is looking at a zone that has an apparent 100% porosity. When that fluid is displaced with gravel, the apparent porosity is greatly reduced. Proper calibration and scaling of the near and far count rates cause them to read on top of each other in zones that are 100% packed and to separate in zones of less than 100% pack. The greatest separation is in zones that have no pack at all. Nuclear Fluid Density Meter Tool The nuclear fluid density meter uses a gamma ray source and a single detector. The source emits gamma rays into the borehole and the surrounding area. Some of the gamma rays are scattered back to the detector. The number of gamma rays returning to the detector is an indication of the density of the material through which they traveled. JPT p. 1611