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Improper formation sand sampling techniques can lead to gravel packs which fail due to plugging of the gravel pack or the production of sand. Because the formation sand size is so important, the technique used to obtain a formation sample is also important. With knowledge of the different sampling techniques, compensations can be made in the gravel pack sand size selection if necessary.

Produced Samples.

In a well producing sand, a sample of the formation sand is easily obtained at the surface. Although such a sample can be analyzed and used for gravel pack sand size determination, produced samples will probably indicate a smaller median grain size than the formation sand. The well’s flow rate, produced fluid characteristics and completion tubular design will influence whether a particular size of formation sand grain is produced to surface or settles to the bottom of the well. In many cases, the larger sand grains settle to the bottom, so that a sample that is produced to the surface has a higher proportion of the smaller size of sand grains.

This means that the surface sample probably is not a good representation of the various sizes of formation sand which are present. Also, the transport of a sand grain through the production tubing and surface flow lines may result in small corners being broken from the sand grains, causing the presence of more fines and smaller grains. This is sometimes called grain shattering.

Grain shattering also reduces the quantity of larger formation sand grains, giving the impression of a smaller median grain size than the formation sand actually has. The use of produced sand samples may result in the use of smaller gravel pack sand than required.

Bailed Samples.

Samples collected from the bottom of a well using wireline bailers are also relatively easy to obtain, but these also are probably not representative of the actual formation sand. Bailed samples will generally consist of the larger size sand grains, assuming that more of the smaller grains are produced to surface. Bailed samples may also be misleading in terms of grain size distribution. When closing the well in to obtain a sample, the larger sand grains will settle to the bottom of the well first, and the smaller sand grains will fall on top of the larger ones. This results in a sorting of the formation sand grains into a sample which does not representative the formation sand. The use of bailed samples may result in the design of larger than required gravel pack sand which can result in sand production (small formation particles passing through the gravel pack) or plugging of the gravel pack (small formation particles filling the spaces between the gravel pack sand grains).

Sidewall Core Samples.

Sidewall core samples are obtained by shooting hollow projectiles from a gun lowered into the well on an electric line to the desired depth. The projectiles remain attached to the gun via steel cables, so that when pulling the gun out of the well, the projectiles are retrieved with a small formation sample inside. Taking sidewall core samples is generally included in the evaluation stages of wells in unconsolidated formations and these are the most widely used sample type for gravel pack sand design. Although more representative than produced or bailed samples, sidewall core samples can also give misleading results. When the projectiles strike the face of the formation, localized crushing of the sand grains occurs, producing broken sand grains and generating more fine particles. The core sample may also contain drilling mud solids that can be mistaken for formation material. Experienced lab analysts can separate the effects of crushing and mud solids to some degree prior to evaluating the sample, thus improving the quality of the results.

Conventional Core Samples.

The most representative formation sample is obtained from conventional cores. In the case of unconsolidated formations, rubber sleeve conventional cores may be required to assure sample recovery. Although conventional cores are the most desirable formation sample, they are not readily available in most cases due to the cost of coring operations.

If available, small plugs can be taken under controlled circumstances at various sections of the core for a complete and accurate median formation grain size and grain size distribution determination.

Gravel Pack and Sand Design - Formation Sand Sampling

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