Completions in the Weber formation have historically been perforated over balanced and then acid stimulated. Flow rates before acidizing were virtually 0 Mcf/D. Acid was an important step in the completion process. The Weber gas reservoir has low permeability (0.5 – 1.5 mD) and is now low pressure (2800 to 2300 psi at 14,000 ft). Typical flowing rates for these wells range from 1 to 5 MMcf/D. A new perforating technique utilizing the dynamic under balanced method was implemented in two wells, Brady 38W and Brady 56W. The wells were perforated with tubing-conveyed guns with initial over balances of 3250 and 3750 psi. The dynamic under balanced method creates an instantaneous under balance even though the annular space between the gun and the casing is initially over balanced. This perforating method also leaves significantly less perforation "skin." The proof is in the success of the two wells, which have rates equal to or in excess of the existing wells and yet required no acid stimulation. From historical comparisons of 18 prior completions, these wells should have had nearly zero flow without acid when perforating using an initial over balance of up to 3750 psi in the annulus. This paper discusses the new dynamic under balanced perforating design and its application to low-permeability, under-pressured reservoirs.
The Brady Field was discovered in 1973 with the completion of the Brady 1W well in the sour Weber formation at a depth of 14,000'. The original reservoir pressure was 5700 psi or 0.41 psi/ft. 14 additional wells were drilled from 1973 until 1975 and completed in the same Weber formation. A plant was constructed to treat and recycle the gas through the formation. One additional well was drilled in 1982 and then another in 1985. The field extracted liquids from 1973 until 1998 when the recycle process was discontinued and the field went into blowdown phase. The Weber formation consists of approximately 600' of interbedded sandstone with multiple shale and dolomite stringers. The completion practices for this formation typically consisted of selectively perforating 1 SPF over several intervals with casing guns under a full column of fluid. After perforating, the tubing and packer were run and the wellbore fluid was displaced with nitrogen. A flow test was then performed. The usual poor performance of the wells required an HCL/HF acid treatment that generally produced much more favorable results. Three of the 18 wells had to be hydraulically fractured to improve performance.
In 2002, a recompletion in the Brady 38W to the upper section of a producing Weber well was attempted using dynamic under balanced perforating. This process was chosen due to the complexity of the wellbore. Figure 1 illustrates how the presence of some old squeeze perfs above the zone of interest complicated the ability to do a remedial acid treatment. The existence of squeeze perfs also created complexity when considering the need for a possible fracture stimulation if perforating alone did not create the desired results. It was determined that dynamic under balanced perforating provided the best possible chance for a non-stimulated completion. The greatest challenge in applying this technique was recognizing and accepting that the initial static conditions just before perforating would actually be 3250 psi over balanced. Without creating a dynamic condition, this well would surely need additional stimulation.
Later in the same year, Anadarko decided to drill a new well in the Weber formation. This was the first new Weber well drilled in over 17 years. This new well provided the option to choose from any perforating and stimulation combination desired. The results of the dynamic under balanced perforating technique on the Brady 38W were strong enough that the same perforating technique was chosen for this newly drilled well, the Brady 56W. In this case, the pre-perforating static conditions would be even higher than the Brady 38W at 3750 psi over balanced. Again, based on history, this would have certainly been a candidate for additional stimulation unless we were able to create a dynamic under balanced condition. The results of this completion were likewise very successful and created enough interest to warrant investigating other applications of this perforating technique in other fields around the world.