Gravel Placement Through Perforations and Perforation Cleaning for Gravel Packing
Authors: Penberthy W.L. 
Published: December 2007

Gravel Placement Through Perforations and Perforation Cleaning Perforations and Perforation Cleaning for Gravel Packing

Summary.
In large-scale model testing, the effectiveness of gravel prepacking under four different conditions was studied: when there had been prepacking under four different conditions was studied: when there had been no previous sand production, after sand production had occurred, after perforation washing, and after perforation surging. In the case of no perforation washing, and after perforation surging. In the case of no previous sand production, pressure parting occurred in the unconsolidated previous sand production, pressure parting occurred in the unconsolidated formation when a critical injection pressure level was exceeded. The orientation was normal to the least principal stress.
In those cases where earlier fluid production caused the removal of formation sand. prepacking produced multiple pressure parts in the low-stress-state sand in the vicinity of the perforation because stable cavities had not tonned. The orientation of these pressure parts was also normal to the least principal stress. pressure parts was also normal to the least principal stress. Gravel prepacking after perforation washing and surging resulted in gravel-placement geometries that were controlled by the stress state and geometry of the formation sand around the perforation after these operations. Prepacking after perforation washing generally resulted in placement that resembled the geometry of the washed region, which was usually a void. Prepacking after surging resulted in pressure parting and considerable mixing of the prepack gravel with the formation sand because stable cavities had not formed.
Perforation washing results demonstrated that washing should be conducted at the maximum practical pump rate with water used as the wash fluid. Perforation surging tests indicated that the amount of formation sand removed was proportional to the surge pressure. pressure. Introduction
Gravel packing has proved to be a viable well-completion technique to exclude formation sand from the produced fluids. The state of the art has steadily improved during the past 50 years to the point where, in certain oil- and gas-producing region, the majority of wells are gravel packed. Although gravel packing can be performed with either openhole or cased-hole techniques, cased-hole gravel packs are more commonly used. Reasons for this choice include packs are more commonly used. Reasons for this choice include fewer complications with drilling and completion operations, as well as reservoir and workover considerations. Openhole gravel packs are usually selected when completion and reservoir conditions are ideal. Because such conditions seldom prevail, cased-hole gravel packs have been the usual choice. packs have been the usual choice. Yet laboratory and field data have shown that cased-hole gravel packs generally have lower productivity indices than openhole gravel packs generally have lower productivity indices than openhole gravel packs. This fact relates to the entry of fluid through packs. This fact relates to the entry of fluid through perforations, which exposes a small fraction of the reservoir sand to the perforations, which exposes a small fraction of the reservoir sand to the well. Added to this problem is the fact that about two-thirds of a perforation's cross-sectional area is filled with either gravel or the perforation's cross-sectional area is filled with either gravel or the reservoir sand. Lower productivity can result if proper completion procedures are not implemented. Research and field experience procedures are not implemented. Research and field experience have shown that the best plan is to perforate the well properly and to prepack the perforations with gravel.
Failure to take these steps will lessen fluid inflow because the perforations tend to fill with reservoir sand. Not only will perforations tend to fill with reservoir sand. Not only will productivity be low, but high completion pressure drawdown and short productivity be low, but high completion pressure drawdown and short completion life will result as well. To improve cased-hole gravel pack productivity, large-diameter, high-density perforating pack productivity, large-diameter, high-density perforating programs have been initiated. In addition, such perforation cleaning programs have been initiated. In addition, such perforation cleaning methods as washing or surging have been used to enhance well productivity. These measures are designed to remove plugging productivity. These measures are designed to remove plugging material and formation sand from the perforations so that highpermeability gravel can subsequently be packed into the perforations- The benefits of this practice are higher productivity, lower perforations- The benefits of this practice are higher productivity, lower pressure drawdown, and longer completion life. pressure drawdown, and longer completion life. When no perforation cleaning has been conducted, prepacking the perforation tunnels is theoretically the optimum procedure Possible without resorting to high pump rates and pressures. On the Possible without resorting to high pump rates and pressures. On the other hand, should workover operations be conducted to gravel pack, a well that was initially completed by perforating only, any prepacking operations would have to be Performed through the prepacking operations would have to be Performed through the perforations from which sand production has occurred. Here the prepack perforations from which sand production has occurred. Here the prepack geometry should depend on the size and shape of any void around the perforation, as well as the stress state of the formation sand.

The Model
To study the effects of perforation washing, surging, and prepacking under various conditions, a physical model was designed and constricted. The model consisted of a thick-walled 42-in. 11.07-m] -ID pressure shell rated at 500 psi [3.4 MPa] and an inner pressure shell rated at 500 psi [3.4 MPa] and an inner formation-sand container with a 36-in. [91 -cm] ID and 42-in. 11.07-m] length. The container was a wire-wrapped screen (slot width of 0.002 in. [0.05 mm]). Because the screen was used as a container. it was cut longitudinally and reverse-rolled so that the "keystones" were along the ID of the screen rather than in the normal position, toward the outside. A simulated wellbore was at one end of the model. It was arranged so that the size and number of perforations could be adjusted to meet test conditions. A floating piston was positioned at the opposite end of the container to relieve internal forces when gravel was pumped into the model.
The laboratory model was designed to approximate the boundary conditions that exist around a well-, a photograph is shown in Fig. 1. Fig. 2 illustrates the interior of the model and shows the wire-wrapped screen/sand container and the wellbore section. Fig. 3 is a schematic of the model packed with sand. The design of this particular model permitted three-dimensional leakoff through permeable sands. Also, the end effects were sufficiently removed from the vicinity of the perforations that their effects on gravel-placement geometry were minimal. perforations that their effects on gravel-placement geometry were minimal. It was believed that this model could simulate field conditions to the extent that a reasonable understanding of near-wellbore placement of gravel was possible.
With a pneumatic tamper. the model was packed in a vertical position with 13 ft3 [0.37 M3) of Brazos River sand to a packed position with 13 ft3 [0.37 M3) of Brazos River sand to a packed height of- 18 in. [46 cm). Sands ranged in permeability from 140 md to 3.7 darcies and contained about 6% clay. The average grain diameter of the sands at the 50 percentile point was 0.0042 in. [0.11 mm). The uniformity coefficient averaged about 2.3.
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