FRACTURING-FRAC JOBS. Fracturing is the process of pumping a propping material into a hydraulic pressure created crack in the formation in order to improve well performance. Most of these fractures are vertical and extend out away from the wellbore. In tight formations, this increases the flow area to the well. Various fluids are used, during the pumping operation, as carriers for the proppant, usually sand. Other common proppants are metal or glass beads. The proppant must be as round as possible and contain no fines or clay. Since it must hold the fracture open, its compressive strength is important.
A typical frac job
will normally begin by filling the tubing with saltwater (pre pad). Pump pressure is increased until the formation fracture pressure is reached. A steady injection rate is established and the pad volume is measured. When the pad volume is in the tubing, pumping rate is stabilized and proppant is added downstream of the pump at low concentrations. The amount of proppant is increased until the required amount of fluid and proppant is in the well. When all the frac material has been injected, it is followed with fluid to push it out of the wellbore into the formation. The final amount should be carefully measured to insure that the well is not over-flushed. This is important, because we do not want the fracture to close near the wellbore.
Often radioactive sand is used so the well can be logged to determine the height of the fracture. Care must be taken to prevent exposure to this material while it is on location waiting to be used. Do not handle any sand that may be produced after this operation. Thorough planning is required prior to any frac treatment. Be sure there is adequate pump pressure and hydraulic horsepower (HHP) to initially frac and then to propagate fracture. Meetings should be held to plan the activity. The following items must be discussed.
Hazards and safety precautions.
Each person’s job and location.
Proper testing methods and pressures.
Contingency plans in case of problems.
Personal safety equipment: clothing, ear.
protection, safety glasses, rubber gloves, etc.
Establish good lines of communications.
Placing of no smoking signs and warnings.
Precautions if radioactive material is used.
Emergency handling of personnel in the event of an accident.
- Review of evacuation procedures.
The production of sand with reservoir fluids is a major problem in some areas. It can cut or plug choke and flow lines, cause excessive equipment failure, complicate well clean-out, and cause malfunctions of downhole equipment. Sand disposal can be a problem. Methods to control production of sand include running screens or slotted liners, packing with gravel or sand consolidation with a plastic resin.
Screens are the simplest to install in most cases. The job consists of hanging a slotted liner or wire-wrapped screen opposite sand producing interval. Screen size is effectively too small to allow sand to flow in, while still allowing flow of formation fluids.
Gravel packing (a special sand) is the most common sand control. The well is underreamed or cleaned out and perforated with large holes. Gravel is pumped to hold sand in place. Gravel should be sieved on location and tested for silts, clays and fines.
It should be round and contain only a small percentage of flat or angular grains. Gravel size, based on core or produced sand analysis, is determined by the service company. Plastics are used, some with materials such as ground walnut hulls blended into them. While material may still be permeable, flow capabilities are low enough to prevent any sand material inflow.
These can contain chemicals that are highly irritating to the eyes, lungs and skin. Reservoirs which produce sand usually have little, or poor, consolidating material. The plastic is used as a form of glue around wellbore area to hold sand in place. The best method for a given job depends largely on the formation characteristics, trial and error, experience in an area and coordination with service companies.