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Sand control in horizontal and deviated wells
Horizontal well completions have been used in a variety of situations to increase well productivity, improve reservoir management, and access incremental reserves that could not be developed economically with vertical wells. While the first horizontal wells were drilled in competent formations, eventually soft formations were completed horizontally, leading to sand control needs. Practically all horizontal wells drilled in soft formations have been completed openhole. Most of these boreholes did not collapse; however, because many of these completions were in formations where conventional sand-control applications were practiced, slotted liners and wire-wrapped or prepacked screens were run to prevent hole collapse and sand production because horizontal gravel-pack technology was not yet available.
Stand-alone slotted liner and screen completions
The typical procedure for completing horizontal wells with slotted liners and screens includes the following steps:
- Drill the well to the casing seat
- Set casing
- Drill the horizontal section
- Displace the hole
- Run the screen
- Produce the well
It is not always this simple, but the intent should be to follow these guidelines.
Sand control in horizontal wells was originally dealt with by using:
- Stand-alone slotted liners
- Proprietary screens (more recently)
The initial productivity of these completions was usually acceptable, and some were outstanding; however, in most applications, the stand-alone devices either plugged or cut out (eroded) with time. The consequences are either unacceptably low well rates or excessive sand production. The acknowledged stand-alone screen failure rate in the Gulf of Mexico was estimated to be about 25% in 1996. Since that time, numerous additional failures have occurred, and the failure rate has increased substantially. Hence, in most applications, the use of stand-alone screens as retention devices has been disappointing because the stand-alone screen approach forces them to perform as filters (see Fig. 1). Their use in horizontal wells confirmed previous stand-alone experience in vertical wells—they plugged.
Many screen designs were progressively used to determine if particular designs would improve performance:
- Slotted liners
- Wire-wrapped screens
- Prepacked screens
- High inflow-area proprietary screens
As might be expected, these completions experienced a wide range of reservoir situations. Some stand-alone applications have performed exceptionally well. The exceptional wells (mainly in the North Sea) had formation permeabilities in the range of 10 to 12 darcy and were well sorted. Experience in the Gulf of Mexico with stand-alone screen completions has been disappointing; the failure rate from these completions has been well over 70% as of 2000.
In many horizontal wells where stand-alone screens have been used, there are implications that the formation does not collapse around the screen. When this happens, there is an open annulus that serves as a conduit for fluid and particulate transport along the entire length of the screen. There are many examples in which a stand-alone screen completion produced extremely well for a period of time and, then, abruptly lost productivity. The screens appear to be progressively plugging; however, because of the high flow capacity per foot of screen, a short section of unplugged screen can handle enormous flow rates. When the last few increments of screen plug, either production ceases or the screen erodes. Fig. 2 displays an example.
Fig. 2—Schematic of prepack screen plugging.
To combat these problems, technology has been developed to gravel pack horizontal wells because gravel packing can sustain productivity. Gravel packing has always been the state-of-the-art technology for vertical wells. Gravel-pack technology was not available for horizontal wells until 1995, but its acceptance has been steadily increasing and is now the preferred technique. In this service, the screen functions as a gravel-retention device, and the gravel placed around the screen fills and stabilizes the borehole. The streamlines into the screen are now normal because the annulus between the screen and the open hole is filled with gravel, as Fig. 3 suggests. The result is sustained productivity.
Horizontal gravel packing
Gravel packing offers another option for completing a horizontal well when sand production represents a problem. The original perception was that technology was not available for gravel packing long, horizontal completions, and other alternatives, such as stand-alone screens, had to suffice. This is contrary to the fact that the performance of stand-alone screens had been unacceptable in conventional wells.
One of the most disturbing examples, portrayed in Table 1, shows failure statistics and average pressure drops across 43 stand-alone screen completions in horizontal wells in the Gulf of Mexico. Of these, 15 (35%) were classified as failures, but the remaining active wells are producing at an average pressure drop of 545 psi. Taken from the perspective of the flow capacities of screens that were previously discussed, the remaining wells, while still producing and not reported as failures, are also plugged.
Horizontal gravel-pack technology was developed in the mid-1990s. Studies were performed in a field-scale model that was 1,500 ft long and instrumented with data acquisition; they also contained visual observations of the packing process through high-strength plastic sections in the model. A typical plot of the location of the alpha and beta waves (see the section on gravel placement techniques), as a function of time for a horizontal gravel pack, is illustrated in Fig. 4. The figure demonstrates that the entire 1,500-ft model was packed with gravel. Testing clearly revealed that the height of the alpha wave was not constant with pack length, as had been implied from studies conducted in short models. Instead, the height of the alpha wave was inclined upward from the heel to the toe of the model as Fig. 5 illustrates. The reason for the inclination is a result of fluid loss that reduces the annular flow velocity and increases the gravel concentration, thereby reducing the gravel transport efficiency. The consequence was an increase in the alpha-wave dune height with length.
Having this data provided valuable information for designing horizontal gravel packs. If the top of the borehole interferes with deposition over the top of the alpha wave, deposition stalls, and beta-wave deposition begins at the stall location (Fig. 6). To avoid a premature stall, the superficial annular velocity must be maintained above 1 ft/sec, based on return flow through the washpipe. The superficial velocity is defined as the ratio of the return flow rate through the washpipe to the annular area between the washpipe and the wellbore. Provided that the design of the gravel pack is correct and a superficial velocity of 1 ft/sec is maintained, gravel packing a long horizontal gravel pack can be performed with routine procedures. Gravel deposition (alpha-beta wave) will proceed to the toe of the well, as Fig. 7 shows.
Fig. 4—Gravel dune location (alpha-beta wave).
Fig. 5—Dune height in a wellbore.
Fig. 6—Horizontal gravel pack (partial).
Fig. 7—Horizontal gravel pack (complete pack).
Horizontal gravel designs are available that utilize the concept illustrated in Fig. 8, which shows pressure plotted as a function of pump rate. The fracture pressure is identified, and a treating pressure is shown. Note that at low rates, there is insufficient transport to initiate alpha-wave transport, but at slightly higher pump rates, the alpha wave will prematurely stall. However, at an acceptable pump rate, the entire alpha-beta wave deposition can be performed to complete the gravel pack without fracturing the formation. Fracturing is manifest by a reduction in return through the wash pump and will stall the transport process.
Fig. 8—Horizontal gravel-pack design criteria.
By 2001 more than 400 horizontal gravel packs were performed. Most of these completions had horizontal lengths of 1,500 to 2,000 ft. The longest horizontal gravel pack performed as of mid-2001 was 7,000 ft. The volume of gravel pack in these completions is typically 20 to 30% greater than the theoretical volume, indicating that the annulus is completely filled with gravel. The pack volume being greater than 100% is accounted for by hole irregularities that are larger than the bit diameter. Similar experience has been noted in vertical openhole gravel packs. In several applications of this technology, gravel packs were run behind failed stand-alone screen completions that lost productivity. After the screen was removed and the hole was displaced, new equipment was run, and the well was gravel packed. The performance of the horizontal gravel packs has demonstrated that they maintain productivity compared to the stand-alone screen experience.
- Morales, R.H., Norman, W.D., Ali, S. et al. 1996. Optimum Fractures in High Permeability Formations. Presented at the SPE Annual Technical Conference and Exhibition, Denver, Colorado, 6-9 October 1996. SPE-36417-MS. http://dx.doi.org/10.2118/36417-MS
- Penberthy, W.L.J., Bickham, K.L., Nguyen, H.T. et al. 1997. Gravel Placement in Horizontal Wells. SPE Drill & Compl 12 (2): 85–92. SPE-31147-PA. http://dx.doi.org/10.2118/31147-PA
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