Fracking Process: Well Casing & Cementing, Part 2
Column A on Sheet 3 of the attached spreadsheet was filled with the requested information. Packing involves surrounding the well with gravel. Its purpose is to reduce unwanted material but allow the production of the desired fluids.
- 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.
- 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.
- 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.
- 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.