The use of through-tubing gravel packs (TTGPs) has been limited in the past to low-flow-rate systems that are not suitable for frac packing or vent-screen systems that use a long annular pack of sand for pressure isolation. Vent-screen installations are increasing rapidly - in both new completions and for remedial work - because of the ability to develop longer fractures and increase permeability with high-rate frac packs. Unfortunately, vent screens have a high rate of failure because of the lack of positive pressure isolation. This paper presents a new re-enterable-screen method that allows a high-rate frac pack with positive pressure isolation either through an inflatable packer or packoff to the tubing.
Geologically young, weakly consolidated formations with little cementation often produce sand as the production rate is increased. Water break-through may also increase sand production substantially. Consequences of sand production may be severe, such as erosion of surface/downhole equipment, loss of productivity due to a sand bridge, and casing damage due to subsidence. Sand control with gravel packing has evolved to address this problem. Properly sized gravel is packed into the perforations and the annulus between the screen and the casing. This allows production fluid flow through the gravel and the screen while preventing formation sand production.
The onset of sand production is often difficult to predict and it is much better to implement sand control before sand production becomes problematic than to address sand in a remedial operation. TTGPs eliminate the necessity of pulling the completion and may be performed at a fraction of the cost of a major workover. There are two major applications for TTGP techniques; marginal wells with limited reserves and new wells specifically designed for high rate TTGP applications. There are two major types of TTGP methods1:
Chemical Methods. Chemical methods involve the injection of resins or chemicals into poorly consolidated formations to provide in-situ grain-to-grain bonding.
Mechanical Methods. In mechanical methods, small-diameter screens are run through tubing and placed inside the tubing, casing, or a previous screen. Gravel is typically used as a filtering medium. To achieve maximum life and productivity when using mechanical methods, it is critical to fill completely the perforations and screen/casing annulus with gravel. Failing to do so results in "hot spots" where formation sand and fluid hit the screen, resulting in screen failure and subsequent sand production for the well (Fig. 1). Selection of the correct gravel size, placement fluid, and pump rate all play a critical role in achieving maximum productivity and life from a completion when using mechanical methods.
The five types of mechanical TTGP methods are discussed and illustrated in more detail in this section.
In the wash-down method, the well is prepacked with gravel by pumping down the production tubing at high pump rates, thus achieving complete coverage of the zone and good packing of the perforations with gravel. Subsequently, a screen is washed down (using a long, small-diameter wash pipe inside the screen) into the gravel pack, with a packoff to the production tubing. This method is typically limited to short screens (less than 100 ft) because of the difficulty in washing down with longer intervals.
In the tubing-packoff method, a screen is run below a gravel-pack crossover tool. Blank pipe is used to space out the packoff to the production tubing. Gravel slurry typically is pumped (by either the circulating or squeeze-pack method) down coiled tubing (CT), which limits the pump rate, making it difficult to achieve a frac pack or even a complete packing of all perforations. The normal applications are with a tubing tail no more than 100 ft above the perforations (because of excessive drawdown and deployment difficulties).
In some cases the tubing packoff method may be used without gravel, it is then a referred to as a "TTGP Screen Hangoff". This is typically a "last ditch" effort to save a marginal well. Since there is no gravel, the screen is subject to erosion in localized "hot spots" and poor production due to an influx of low permeability formation debris.