TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractSand retention testing is often required when selecting screen media and media size. It has also been used to define the limitations of retention for different media 1 . However, the results from such tests are very dependent on experimental conditions and prone to artefacts. Small and apparently innocuous changes to the test conditions can cause wide variations in results. Some of the factors affecting test results are discussed. The problems of artefacts can be overcome, at least in part, by considering trends in data rather than absolute results.A previous paper described the performance of Dutch twill weaves (DTW) and identified a selection criteria based on particle size distributions measured by laser light scattering 1 . This paper extends that work to include sintered media from other premium screens and wire wrap screens. The general trend in the data from a number of tests shows good correlations between some aspects of the sand distribution and retention performance of the individual media. From these data, an aperture sizing criteria for sand screens based on laser particle size measurements is proposed, and screen selection parameters considering both retention and "plugging" performance are identified.

One area where water-based muds need improved performance is in shale inhibition. However, before existing mud syste'11s can be improved, the mechanisms by which water invades shales and how present-day inhibitive additives operate must be fully understood. An experimental technique has been developed that uses radioactive tracers to monitor the progress of water and selected dissolved ions through a shale core plug. By varying experimental parameters, such as water composition and applied pressure drop, the dominant mechanisms by which water is transported through shales have been identified. Under conditions of zero applied pressure, diffusion processes control water and ion movement through shales. Concentration gradients are the driving force for mass transfer of ionic species through shales. We observed no evidence to indicate that osmosis caused mass transfer of water. Applied pressure caused an increase in water and ion transport rates. Above a threshold pressure, water and dissolved ions travel at the same rate irrespective of the ion concentration.

This paper describes the development and application of a laboratory procedure for the evaluation of screens for sand control. The driving force for this study was to provide an independent evaluation of all screens on the market, in particular, the new generation of premium screens. The test addresses both aspects of screen performance, namely sand retention efficiency and plugging potential. The difficulties in setting up such a test are discussed, with particular attention paid to the elimination of experimental artefacts. Some of the pitfalls that may be encountered in laboratory evaluation of screens are highlighted. The developed method has been used in screen selection tests for a particular field, and these results are also presented. The data illustrate the sensitivity of the technique for evaluating a range of screens on the same sand, and reasons for the differences in screen performance are explored. Furthermore, it was observed that the method of particle size analysis will affect the apparent particle size distribution of a sand. As a result such parameters as the uniformity coefficient may be completely different for the same sand depending on the method of size measurement.

Sand screens for specific applications are often selected by reference to the results obtained from laboratory sand retention testing. Some recent publications have highlighted the problems of running some types of sand retention tests (slurry tests) at high flow rates, such that the differences between wire wraps screens and metal mesh screens may be exaggerated. With these in mind and also to address some general concerns of the authors ways to reduce flow rates in laboratory slurry tests to more realistic levels have been investigated. This has created some unforeseen effects which are discussed; video has proved invaluable in understanding these unforeseen effects. In addition, an attempt has been made to better define plugging within sand retention tests by relating the pressure build-up gradient from slurry tests to characteristics of the sand itself. Although the pressure gradient generally correlates to certain sand size and sorting parameters the spread in data suggests another factor is important. The purpose of this work is to try and better define the differences in performance between different screen designs, primarily wire wrap and metal mesh screens, in order to better define their application envelopes in terms of sand quality and hence develop more definitive guidelines for screen selection.

Sand retention testing obviously requires sand, and using the reservoir sand is the most straightforward option. However, sometimes reservoir sand is not available in sufficient quantity, and in these instances a particle size distribution matching the reservoir sand is prepared from commercial outcrop sand. The authors were aware that there can be some differences between the results from outcrop and reservoir sands and generally only use reservoir sands, but a recent request for tests using simulated sands gave the impetus for a systematic study of the variations. Retention tests have been performed with different reservoir sands and two versions of their respective simulated sands; one matched to their laser particle size distribution and one matched to the sieve analysis distribution. Sandpack tests and two slurry test methods (xanthan and caesium formate) have been performed with a range of wire wrap and metal mesh screens. There are quite striking differences in the pressure data recorded between the actual reservoir sands and the simulated sands. The trend in the amount of sand produced between reservoir and simulated sand is still uncertain, but the study has shown some instances that could change the screen recommendations. Efforts have been made to understand the differences between reservoir and simulated sands by examining particle shape. The results of this study show that using sands simulated from outcrop rock can give retention test results different from those obtained with reservoir sands. This and other potential test artefacts should hopefully discourage reading too much into retention test data; especially for estimating likely sand production. Predictions of sand screen performance based on lab studies will remain problematic until comparisons with field performance (both failures and successes) can be made.