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Gravel placement techniques
Gravel packing consists of installing a downhole filter in the well to control the entry of formation material but allow the production of reservoir fluids. The gravel-packed completion is perhaps the most difficult and complex routine completion operation because it consists of many interrelated completion practices.
Objective in gravel packing
There are two primary objectives for gravel packing a well.
- The annulus between the screen and casing must be packed with gravel. Filling the annulus with properly-sized gravel ensures that the formation sand is not produced to the surface.
- Pack each perforation with gravel. Filling the perforations with gravel is the key to obtaining high productivity. In an unconsolidated formation, any perforation that is unfilled with gravel will fill with formation sand and severely restrict productivity from such perforations.
The following discussion deals with filling the annulus. Prepacking perforations with gravel discusses perforation packing.
The crossover circulating technique is the most common method used to place the gravel around the screen. The gravel-pack equipment and service tools allow circulating the gravel down the work string above the packer and into the screen/casing annulus below the packer. The returns flow up the washpipe and cross over into the work string/casing annulus. The fluid used to transport the gravel can either leak off to the formation or be circulated or reversed out of the hole through the washpipe (as illustrated in Fig. 1), depending on the position of the service tools.
A variety of fluids has been used as gravel transport fluids such as:
- Crosslinked gels
- Clarified xanthum gum (XC) gel
- Hydroxyethylcellulose (HEC) gel
The most commonly used fluids have been brine and HEC gel. Gravel packs performed with brine are referred to as water/brine packs or conventional packs. Gravel packs performed with HEC gel transport fluids are referred to as slurry packs, gel, or viscous packs. Table 1 is a comparison of HEC gel and brine characteristics that are important to their use as gravel transport fluids. When using HEC, the gravel is suspended by the gel and settles slowly because of the high fluid viscosity. When using brine as a transport fluid, the gravel settles quickly because of the low viscosity. Hence, higher pump rates may be required to cope with particle settling when brines are used.
Physical model observations
Field-scale model studies with water and gelled transport fluids in a 22-ft-long clear plastic gravel-pack model revealed many significant facts concerning gravel placement. The model simulated a 7-in. casing with a 2 3/8-in. screen that had a perforation shot density from 0 to 12 shots/ft. The model could be rotated to simulate well deviations from 0 to 110° from vertical. The following discussion deals primarily with cased-hole completions. It also applies to openhole completions for gravel packing the annulus between the screen and the open hole.
Brine transport fluids
Simulations with brine transport fluids were performed at deviations from 0 to 110°. The gravel-packing sequence at well deviations from 0 to 45° were highly controlled by gravity and packed from the bottom of the well upwards, as Fig. 2 portrays. As long as finite leakoff occurred through the perforations, they were packed with gravel. The gravel did not begin filling the perforation tunnels until the level of the gravel in the annulus reached the perforation entrance. At this point, the gravel would divert into the perforations (if the perforation was experiencing leakoff) and completely pack the perforation as the annular pack level rose. The result was a tight annular pack that completely prepacked the perforations experiencing leakoff. Well deviations of 45 to 60° from vertical were also completely packed, but the packing began on the low side of the hole and filled the annulus with a series of dunes propagated up and down the length of the model. At about 60° well deviation, the gravel is in transition between falling to the bottom of the interval or remaining at the top of the interval on the low side of the hole. As a consequence, the packing is random, as shown in Fig. 3. The reason for this behavior is that at about 60°, it represents the complement of the angle of repose for gravel that is about 28°, as illustrated in Fig. 4.
Fig. 2—Packing sequence with brine carrier fluids in wells less than 45o.
Fig. 3—Packing sequence with brine carrier fluids in wells at 60o.
Fig. 4—Angle of repose for gravel-pack sand.
As the well deviation exceeds 60°, a gravel dune forms initially at the top of the completion interval and is propagated sequentially downwards from the top to the bottom of the completion interval. This occurs because the angle of repose has been exceeded, and gravity becomes a more dominant force that causes a gravel dune to form in the completion interval. To ensure propagation of the dune, the ratio of the washpipe outside diameter (OD) to the screen inside diameter (ID) must be larger than 0.70. The purpose of the large-diameter washpipe is to divert flow from the annulus between the washpipe and the screen to the annulus outside the screen. Testing and field experience has confirmed that the ideal ratio is probably in the range of 0.70 to 0.80. Additionally, the return flow rate to the cross-sectional area ratio (between the screen and the casing) should be at least 1 ft/sec to supply sufficient transport velocity. This is referred to as the superficial velocity. If the ratio of washpipe OD to screen ID is too small, excess fluid will divert into in the annulus between the screen and the washpipe and the gravel dune will stall high in the completion interval, resulting in a “premature sandout” (see Fig. 5). Fig. 6 shows the effect of washpipe to screen diameter ratios on gravel placement efficiency. If the ratio of washpipe OD to screen ID is too large, sticking the washpipe is a concern, as well as potentially high pump pressures during the final stages of gravel placement. A schematic of the gravel packing process, in wells greater than 60° when a large diameter washpipe is used, is illustrated in Fig. 7. This figure shows the dune deposited and propagated along the low side of the hole (sequences 1 to 10) until it reaches the end of the completion interval (alpha wave). At this point a secondary deposition (beta wave) backfills and packs the volume above the alpha wave to complete the gravel pack.
Fig. 5—Failed packing sequence with brine carrier fluid in a high-angle well, resulting from a low-rate and small-diameter washpipe.
Fig. 6—.Effect of washpipe OD to screen ID ratios on gravel placement efficiency.
Fig. 7—Packing sequence with brine carrier fluid in a high-angle well using a high-rate and large-diameter washpipe.
Gel transport fluids
Simulations with gel transport fluids were also performed at the same well deviations previously discussed. The packing mechanisms with gel were more complex than with brine because viscous forces were stronger. At 0 to 45°, the high viscosity of the gel allows radial packing around the gravel-pack screen and node buildup at the perforations. At screen connections, voids were commonly observed. But the voids where typically filled by gravel settling after a few hours, provided that the well deviation was less than about 60°. As with brine, perforation packing was complete but occurred only if the perforation experienced fluid leakoff. At deviations greater than 60°, voids persisted in areas where incomplete slurry dehydration occurred (opposite screen joint connections or unperforated sections of the interval). Unlike the lower deviation simulations, gravel pack, settling at deviations greater than 60°, resulted in voids along the top of the gravel pack, as Fig. 8 shows. When the voids occurred opposite the perforations, gravel-pack sand placed in the perforations would be unloaded into the voids when production occurred. Under actual conditions, these phenomena result in either sand production or localized filling of the perforation tunnels with formation sand that will severely restrict productivity. Observations were that gravel packing with brine produces a pack with a porosity of about 37%. The porosity of gel packs is about 42% and can be higher if there are voids.
Fig. 8—Gravel-pack sequence with viscous fluids showing voids.
Based on the results of laboratory testing and field experience, brine exhibits more complete packing of the perforations and annulus under a wide variety of well conditions and is considered by most to be a general-purpose gravel-pack fluid. Gel transport fluids should be limited to use in wells with deviations less than 45° and gross zone lengths less than 50 ft in length.
The main objective of annular gravel placement is to effectively pack the annulus between the screen and the casing or the open hole. For cased-hole completions, an added objective is to pack the perforations with gravel because the latter significantly improves well productivity and longevity. In addition to perforation packing, the quality of the pack in the screen/casing annulus is important, regardless of whether the well is completed cased or openhole. Gravel-pack evaluation logs have demonstrated the superiority of brines over gels in that lower pack porosities are achieved. Brine packs are also more uniform and do not contain voids common with gels that have been verified by post-gravel-pack evaluation logs.
Gravel packing with shunts
Because viscous fluids are still used for gravel packing, particularly in frac-pack applications, there is concern about void formation in the annular gravel pack. A shunt system has been developed that may help solve the problems associated with these high-viscosity fluids (voids). The shunts are actually channels or conduits that are designed to transport gravel through the shunt when bridges are formed in the annulus. Fig. 9 is an example of a shunt activating when a bridge forms in the annulus. Note that the shunt (there can be a single or multiple shunt tubes) is attached to the outside of the screen.
The shunt can be run in either cased- or openhole configurations. For cased-hole applications, the shunt screens are usually run unprotected, but in openhole horizontals, an outer shroud is added to protect the shunts when running in the hole. The shroud may also provide centralization for openhole completions. The horizontal shunt-screen gravel packs are commonly performed in the squeeze mode (no returns), and fracturing is believed to be occurring during the packing process. Reports are that when gravel packing with shunt screens up to 35%, excess gravel is pumped over the hole volume. Whether this means that the excess packed washouts occurred because of fracturing is not clear.
The burden of the additional hardware is increased weight, drag, and dimensional concerns; this limits the diameter of the hole in which it can be run. For example, for a 4-in. pipe-base screen, a 7-in. shroud encases the screen and the shunts. Hence, the minimum hole diameter in which the screen assembly can be run is 8.5 in. For smaller diameters such as 6.125 in., which is probably the most common horizontal openhole diameter, a 4.5- to 5-in. shroud would be required. For this shroud diameter, the screen diameter (pipe base) would probably have no more than 2 in.—meaning the washpipe and shunt dimensions are also reduced. Hence, washpipe and shunt friction pressure limit the length of the lateral that can be gravel packed for the small hole sizes.
- Penberthy Jr., W.L. and Echols, E.E. 1993. Gravel Placement in Wells. J Pet Technol 45 (7): 612-613, 670-674. SPE-22793-PA. http://dx.doi.org/10.2118/22793-PA
- Jones, L.G., Yeh, C.S., Yates, T.J. et al. 1991. Alternate Path Gravel Packing. Presented at the SPE Annual Technical Conference and Exhibition, Dallas, Texas, 6–9 October. SPE-22796-MS. http://dx.doi.org/10.2118/22796-MS
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