Hundreds of depleted shallow gas wells in the Saco, Montana area of the Northwestern United States have been found to have bypassed pay that until recently was believed uneconomical to recover. Wells in the area were traditionally completed with one production zone and were fractured by conventional means down the casing. The pay zones were then efficiently depleted through the use of siphon strings and flow line compression. Upon review of the well logs, it was determined there may be enough bypassed pay within the existing wellbore to continue producing the well as a multi-zone completion.
This paper outlines the logistics, execution, and evaluation of coiled tubing conveyed fracturing for depleted gas wells. Formations that were stimulated in this study appear to have similar characteristics to the thin producing lenses typically treated in Southeastern Alberta, Canada. The majority of the wells treated received up to six fractures and averaged 100,000 lbs of sand placed into each wellbore. The challenges and results from an initial pilot project are reviewed along with the subsequent multi-well project that followed.
Fracturing through coiled tubing was introduced in 1997 and has become a successful process in stimulating shallow gas wells. The majority of the wells completed with this technology have been recently drilled wells, with some having up to 17 fracture treatments over a 900 foot interval. The standard practice of stimulating a shallow gas well using the coiled tubing conveyed fracturing technique is to start at the deepest perforated interval and proceed upwards.
Some of the benefits in using coiled tubing conveyed fracturing are as follows1,2:
Reduced wellsite visits
Stimulation of bypassed pay
Multiple wells fully completed per day
Protection of casing and/or completion equipment
Increased production/addition of new reserves
Turn around time decreased
Increased flush production
Reduced gas emissions to the atmosphere due to combined flowback
Reduced number of post-fracturing coiled tubing cleanouts performed
By utilizing coiled tubing as a conveyance string on re-completion wells and replacing the existing technique of using work over rigs, rotational packers, jointed tubing and bridge plugs, the treatments can be performed safely and economically on live wells.
The stimulation treatments described in this paper have been performed on existing wells with 4–1/2" production casing and that have been producing for up to eight years. The field's viability in certain areas has been in question for some time as the production rates were close to being uneconomical. Realizing the impact of re-entering existing wellbores and generating increased production, the operator decided to implement a re-completion pilot project. Observing positive results from similar producing type formations that had been achieved in other areas and knowing existing gas was behind casing, coiled tubing fracturing was chosen as the key to making the project a success.