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.

For Stand Alone Screen (SAS) completions in open hole oil/gas wells, the sand control screen must retain the formation sand effectively, and provide a conduit for fluids to flow into a producing well. The screen's performance and service life depends on the well's operating conditions. A failure may occur while completing the well, or at some time during the well's operating life. The root cause of the screen failure is often difficult to determine with absolute accuracy. Often, it is determined that screen failure occurs due to a combination of plugging and erosion resulting in the loss of sand control. A comprehensive test program was undertaken over a two year period to evaluate erosion of different screen types for open hole completions. Completions were simulated for wire wrap and metal mesh sand screens commonly used in open hole completions. Configurations of each screen type were installed in a test fixture, and several tests were conducted at different velocities and sand concentrations. The sand was sized to pass through the screen with minimal plugging, which would simulate fines production over time in a producing well. The specific wear rates and per cent of rate loss were determined for each test. Micro photos were used in the analysis of the wear patterns, along with pre and post test measurements of the wire wrap slots or metal mesh pore sizes. This paper will describe the test methods used, the test results and lessons learned, and will describe a model that can be used to predict a screen's service life based on a given set of well conditions in a SAS open hole completion. Introduction Sand screens are often used to provide sand control in open hole completions. A common completion method is a stand alone screen (SAS) where by the screen aperture is sized to directly retain the formation sand. A typical completion is shown in Figure 1.

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.

Dutch twill weaves are a common filter medium for premium and expandable screens. This paper attempts to address the question of which weave aperture size to select for a particular sand. An extensive matrix of sand retention tests using reservoir sands has been performed to gain a better understanding of the retention and plugging performance of dutch twill weaves. Several weaves have been used with different weave aperture sizes as measured by a glass bead test. Sand retention testing is often fraught with experimental artifacts and the results are very dependent on experimental conditions. However, the general trend in the data from a number of tests shows good correlation between some aspects of the sand distribution and the retention performance. In particular the diameter of the largest sand grains for a particular sand (d1, d5 and d10) are shown to be important in determining retention performance. From this data, an aperture sizing criteria for dutch twill weave sand screens is proposed. The plugging performance of the weaves is also examined with respect to weave open area and sand uniformity. Introduction Sizing criteria exist for wire wrapped sand screens1,2 and gravel packs3,4. Some work has been done on screen selection5,6 but little guidance exists for actually sizing premium woven screens. As there as some similarities between weave openings and gravel pore throats, the industry has historically been applying gravel sizing criteria to weave size selection. There is a definite need for some recognized criteria for the selection of premium woven screens and this paper describes an extensive testing programme carried out on dutch twill weaves to assess the performance of this type of media with regard to retention and plugging with a variety of reservoir sands. Special attention has been given to defining the limits in terms of the smallest sand that can be retained by a particular weave. Enough tests have been performed with sufficient sands to define empirical retention curves, which determine the retention performance for each weave. The sand retention tests have been based on the weaves used for the ESS™ expandable sand screen; however the results should also apply to other screens using dutch twills provided the actual aperture size of the weave is known. Particle size measurement using laser diffraction has been used throughout this study. Different particle size measurement methods can give very different distributions and particle size analysis requires an understanding of the limitations of the method used. Sieving has historically been used to measure sand distributions. Screen sizing and gravelpack sizing criteria such as Coberly1 and Saucier3 have been based on sieve distributions. Indeed, some modern slot sizing programs are based on sieve analysis2 and cannot use laser measurements. Laser diffraction is becoming increasingly common as a method of size measurement for reservoir sands and the work described in this paper is based on laser diffraction size distributions. Dutch Twill Weaves Dutch or Hollander twill weaves are used as the filter media for Weatherfords' expandable sand screen, (ESS™) and also other premium screens. Figure 1 shows the weave pattern for a Dutch twill. These weaves do not have an easily identifiable aperture as such, but have a tortuous flow path through the weave. Therefore simple optical techniques for measuring the aperture size of the weave are not appropriate. However, knowledge of the true aperture size of the weave rather than just the nominal size is important when trying to select the appropriate weave size. The aperture sizes of the weaves used in this study have all been measured using a standard glass bead test. In these tests calibration microspheres are sonically sieved through the weave, the amount passing is weighed and referred to the orignal distribution in a calibration graph. Further details of this test can be found in Reference 7. All ESS weaves fall within +/- 5% of the quoted aperture size.