Packer Usage

• Gravel packs are an effective well construction method that allows oil companies to achieve maximum output with minimal sand production. They are most often used in "unconsolidated sand aquifers, such as most of the coastal plains areas of the world, or in unconsolidated alluvial formations. Often, these aquifers are made up of poorly graded and poorly cemented sands, and the wells will produce sand during pumping."
• Gravel packing involves the installation of a downhole filter in the well. The purpose is "to control the entry of formation material but allow the production of reservoir fluids."
•  Steps in the process are detailed below.

Clean Components

Casing

•  Reverse circulation is the most common method of flow for cleaning the casing.
• Reverse circulation is used because it is more effective than conventional circulating. Gravity assists in moving the material down the hole where it is "more efficiently circulated to the surface because of higher velocities in the work string than in the annulus."
• To clean the casing, the following types of cleaning agents should be employed: mechanical, hydraulic, and chemical.
• Mechanical agents are most often in the form of casing scrapers; hydraulic agents are primarily push pills and filtered brine. Casing sweeps involve a chemical wash designed to clean off polymers, oil, and solids that adhere to the casing wall.

Open Hole

• Reverse circulation is also the preferred method of cleaning for an open hole.
• Using this process, wellbore losses and instability can be detected and repaired. The method also pushes any unrecovered material to the bottom, where it is out of the way.

Work String

• The work string should be sized to allow reverse circulation, and should be "run open-ended to minimize back pressure on the formation."
• The work string has the same types of dirt found in the casing. Unlike casing, "both the inner and outer surfaces of the work string must be clean because completion fluid is circulated along both surfaces."
• The work string should be visually inspected before it is put into the well to ensure cleanliness and good mechanical condition.

Filter Completion Fluids

• Whether water or oil-based, gravel-pack completion fluids must be clean so that suspended particles do not clog or reduce the "permeability of the formation, perforations, or gravel-pack sand."
• Completion fluids are "filtered to 2 or 10 microns, but in some cases, they are filtered to 1 micron."
• The fluid can be filtered in one of two ways, either by a diatomaceous earth (DE) filter upstream combined with a cartridge filter unit downstream or with a cartridge filter unit alone. A graphic of the filtration system is shown on page two here. 

Perforation Process

• Once the well components have been cleaned, and the completion fluids are filtered, the perforation process begins.

Perforation Systems

• There are three perforation systems used in the oil and gas industry. They are casing guns, through tubing guns and tubing conveyed guns.
• Decision making for gravel packing is primarily about "selecting the perforating gun and charge configuration that will provide adequate inflow from the reservoir."

Gun Size

• The process of perforation is done by lowering a perforating gun into the well to a planned depth and safely firing the explosive.
• When perforating a well, shape charges are fired, and then energy from the explosion creates tunnels through casing and cement and then into a reservoir.
• If the through tubing gun is used, the "gun size is smaller than 2-1/8."

Lowering Procedures

• The desired interval length is the primary determinant of the type of perforation systems.
• Wireline guns are usually favored for short, one-gun perforations. 
• Tubing conveyed guns are traditionally used when intervals greater than 30 ft are required "because the entire interval can be perforation underbalanced with a single run of the perforating assembly."

Penetration

• The diameter and depth of a perforation hole are dependent on the objectives.
• Typically, gravel-pack charges have penetrations of 8 to 10 inches. 

Pack Gravel

• Filling the perforation tunnels with gravel can be done either before the well gravel packing is done or at the same time. Which technique to use at each site is driven by the well parameters
• A crossover circulating technique is most commonly used to drive the completion fluid and the gravel. The well is filled from the bottom up. The goal is to pack the annulus between the screen and the casing.
• If the completion fluid is viscous (oil-based), it may be necessary to insert a shunt. See graphic number two here.