Bennett M. Richard, James M. Montagna, W.L. Penberthy Jr.
Baker Oil Tools
Houston
Some stand-alone screen designs resist plugging and tolerate erosion substantially more than others. These improved designs will usually enhance well performance for open hole horizontal wells.
This second in a three-part series on completing open hole horizontal wells discusses screen designs for preventing sand entry. The first part was published in OGJ July 21, p. 71.
Wire-wrapped screens are gravel retention devices when used in a gravel pack. However, when screens are used as the sole means (stand alone) of sand control, they function as a filter.
In stand-alone service, conventional and prepacked screens have over a 25% failure rate in the Gulf of Mexico and elsewhere. The failure rate is increasing. Some projects have experienced a 100% failure rate.
Sand entry
The entry of formation sand into a horizontal well presents the same problems as in vertical wells, such as lost productivity, erosion, and/or sand fill in processing facilities. Downhole is the most desirable location to restrict the entry of formation material into the well.However, in spite of the advantages of downhole sand exclusion, sometimes well productivity is also restricted, particularly when stand-alone screens are the sole means for controlling sand entry.
To prevent formation sand entry into horizontal wells, operators initially installed mechanical devices such as slotted liners and wire-wrapped or prepacked screens. In some respects, using this technology was surprising because stand-alone screen performance in vertical wells was disappointing and has experienced low productivity because of plugging.1
In many cases, a screen as the sole means of controlling formation sand downhole was effective for sand-control, but it also caused serious or total loss in well productivity. On the other hand, gravel packing technology was not initially thought to be available for long, horizontal wells, and the stand-alone screen probably was deemed the only alternative.
The initial productivity of many slotted liner/screen installations in horizontal wells has been surprisingly good to impressive as compared to vertical wells. However, subsequent losses in productivity and eventual sand production have been experienced in many wells, particularly in the Gulf of Mexico.2 3 Failures in the North Sea are becoming common.
Prepacked screens have performed better than slotted liners or wire-wrapped screens. The degree to which productivity is maintained seems to be related to the permeability, formation grain size, the amount of clay and shale exposed to the horizontal section, and whether a clean, stable hole was available for the completion.
Homogeneous formations with high permeabilities and large sand grains seem to maintain productivity much better (such as those more common in the North Sea) than dirty, fine grained, low-permeability sands (typical in some Gulf of Mexico formations).
Studies conducted in the Gulf of Mexico have reported a stand-alone screen failure rate of over 25%.2 North Sea experience appears to be somewhat better, but failures have been documented there also. However, an even higher percentage of wells are producing at restricted rates as a consequence of plugging or sand production that have not yet been classified as failures.
The pressure drop across many stand-alone screen completions in the Gulf of Mexico averages over 700 psi which is a clear indication of a flow restriction because the pressure drop across the sand face is low. The other implication of the high pressure loss across these screens is that plugging or erosional failures are imminent.
The fact that the pressure drop across the screens averages about 700 psi leaves little doubt that while the screens are not completely plugged, flow is significantly restricted. Well productivity losses occur either initially, gradually, or when the production history changes.
Screens
The use of stand-alone screens may in some respects be viewed as retrofit completions because screens were not designed or intended to be used as the sole sand-control technique in oil and gas wells. They were designed as gravel retention devices to be used in conjunction with a gravel pack.In this service, the slot dimension is typically about half the gravel size which causes the gravel to bridge on the slot.
Stand-alone screen installations require the screen to function as a downhole filter rather than a gravel retention device. While the screen slot widths are typically equal to the ten percentile diameter of a typical sieve analysis, the smaller formation particles bridge in the slot and enhance their plugging tendencies. This is the probable reason that screens perform better in clean, large grained than dirty formations.
Fig. 1 shows a gravel pack and a stand-alone screen completion from the perspective of the screen function.
After installing a stand-alone screen completion, many procedures require the screen to be washed, circulated, or acidized to promote productivity. The acid soaks are sometimes lengthy and may have negative effects on screen integrity. These operations may contribute to plugging or sand production.
The rate at which the screen plugs seems to be related to the installation procedure, slot width, solids concentration in the fluid, flow rate, and time when water production starts.
The primary reason that screens perform as well as they have in horizontal wells is believed to be related to the high screen inflow area and the low flow rates per foot of screen, which is typically less than 5 bbl/day/ft (some are less than l bbl/day/ft). However, a few high-volume wells produce at rates of 30 bbl/day/ft or higher from reservoirs with permeabilities in the multi-Darcy range.
Implications are that the annulus between the screen and the borehole does not always collapse, which is contrary to intuition. Given a choice, a collapse of the formation around the screen is preferred to prevent annular flow; however, because of the low pressure drop across the sand face this usually does not occur.
An open annulus probably creates a worst-case condition because formation material entrained in the flow stream can travel in an open annulus and cause the screens to progressively plug, and, as a consequence, high flow rates eventually occur over the remaining open screen area (hot spots).
Fig. 2 illustrates progressive screen plugging and hot spot development. Hot spots are probably exacerbated by irregularities, cracks, or damage to the prepack layer in the screen. The consequence is either screen erosion or plugging, the eventual loss in productivity, or mechanical failure. Hence, it appears that the same plugging phenomenon noted in vertical wells is also occurring in horizontal wells. It just takes longer.
Stand-alone screens are probably acceptable for re-entries to access low reserve situations. But for long-term productivity, this completion option is considered to be risky, except in ideal conditions. This is because the completion may lack the productivity to deplete the reservoir in a reasonable length of time without a workover or re-entry.
Screen testing
- Small-scale Lab Test
- Full-scale Tests
- Erosion in 8 gauge Wire-wrapped Screen
- Erosion Test Fixture
- Screen Erosion
After a sufficient number of completions have been performed, trends in performance usually can be established. Unfortunately, field experiences are not commonly well documented and often are taken out of context. Good comparative examples are rare because engineers do not always document results when they occur. Later, the details become misstated, lost, or confused. Hence, good documented field case histories are rare.
The question that is invariably raised is: Do stand-alone screens plug or erode, and is there documented testing and field results that support the reply to this question? The answer to both parts of the question is yes.
One way of avoiding confusion is to perform comparative testing where equipment is tested under the same conditions and comparisons made after testing has been completed.
Fig. 3 shows the flow capacities of 1-ft long lengths of conventional wire-wrapped screens and slotted liners gravel packed with 20/40 U.S. mesh gravel using water as the flow medium.1 Note that under these pristine conditions the flow rates are high and stated in units of bbl/day/ft of screen, yet the pressure drops are extremely low.
The higher pressure drop associated with the slotted liners are a consequence of their low inflow area; however, the incremental pressure drop is still low. The implications of these data are that screens or slotted liners do not create a significant restriction to flow when used in conjunction with a gravel pack.
Field results in open hole gravel packs support this conclusion and have shown that screens in gravel packs do not progressively plug with formation material provided that they are placed on production initially in an unimpaired condition. The reason is that the gravel pack maintains a finite stress against the formation sand face and minimizes particle migration.
Experience with stand-alone screens in vertical wells has been that they plug, sometimes within less than 1 hr of service.
Comparative testing has been performed to document and evaluate the plugging and erosion resistance of commercial screen designs such as conventional wire-wrapped screens, prepacked screens, and proprietary screen designs.4 Testing was performed on laboratory and field-scale equipment.
Laboratory samples consisted of flow testing through 2-in. diameter screen sections while field scale samples used 4-ft long sections of commercially available screen, as shown in Fig. 4.
In both tests, the susceptibility to plugging and/or erosion was evaluated for each screen design by flowing through it a 50/50 mixture of AC coarse test dust and 70/140 U.S. mesh gravel. The laboratory-scale tests were concluded when the pressure drop across the screen sample reached 100 psi, while the field scale testing was suspended when the pressure drop was 1,500 psi.
Fig. 5 shows that the laboratory-scale tests observed plugging for all screens evaluated, but various screen designs had significantly different plugging resistance. Field-scale testing confirmed the laboratory-scale test results.5
Fig. 6a portrays the pressure drop as a function of particulate loading per unit area of screen. Fig. 6b compares the plugging resistance of wire-wrapped and prepacked screens and shows substantial differences in performance.
Note in Fig. 6b that the 4-gauge screen plugged quickly followed by the prepacked screens with 12/20, 16/20, and 20/40 U.S. mesh gravel.
The 8-gauge screen appeared to perform much better and suggested that for these test conditions it may have been the screen of choice. However, further investigations of the screen showed that it had eroded (Fig. 7).
Evidence of erosion can be observed in Fig. 6b where the pressure stabilized as a consequence of the erosion; however, the onset of the screen erosion actually appeared to have occurred about halfway through the test and became progressively worse.
Erosion testing of screens to simulate gas service has also been conducted (Fig. 8). The test consisted of flowing 60 psi air at a rate of 50 cu ft/min containing 30/100-mesh blast sand conveyed at a rate of 25 lb/min against a test sample. The test flow rate equals a flow rate of 4.3 MMscfd/ft of screen.
The high rate was not meant to replicate particular conditions, but allowed quick comparisons in erosion resistance. The erosion observed after testing (Fig. 9) clearly shows that there are significant differences in the erosion resistance of the various screen designs. Some were eroded to failure in less than 2 min under the test conditions.
The shrouded vector weave design is clearly more erosion resistant than other designs and demonstrated no noticeable metal loss during the same test period as other designs.
References
1. Penberthy, W.L. Jr., and Shaughnessy, C.M., Sand Control, SPE Series on Special Topics, Vol. 1, 1992.2. Perdue, J.M., "Completion Experts Study Gulf of Mexico Horizontal Screen Failures," Petroleum Engineer International, June 1996, pp. 31-32.
3. Park, E., and Procyk, A., "Workover of a Failed Prepacked Screen in a Horizontal Well," Offshore, May 1996, p. 92.
4. Penberthy, W.L. Jr., Richard, B.M., and Montagna, J.M., "A Review of Oil-Well Screen Designs and Performance," Baker-Hughes Inteq internal report, April 1996.
5. Ali, S., and Dearing, H.L., "Sand Control Screens Exhibit Degrees of Plugging," Petroleum Engineer International, July 1996, pp. 36-41.
The Authors
Copyright 1997 Oil & Gas Journal. All Rights Reserved.
