EXTREME OVER-BALANCE PERFORATING / ACID STIMULATION TREATMENT AS A COST EFFECTIVE ALTERNATIVE TO CONVENTIONAL HYDRAULIC FRACTURING ACCELERATES GAS BLOWDOWN OF W31S RESERVOIR, ELK HILLS – KERN COUNTY

The completions in the W31S sand at the Elk Hills field, Kern County, California are optimized to accelerate the blow down of the gas cap. Until recently the preferred completion technique involved a limited entry perforating strategy to enhance conventional hydraulic fracturing to achieve the desired efficiency. This paper will discuss an alternative completion technique that is yielding gas deliverability similar to that achieved with hydraulic fracturing with significantly lower costs.

The alternative completion technique that is employed uses an extreme over-balanced (EOB) perforating technique followed by a nitrified acid stimulation treatment incorporating tubing conveyed perforating (TCP) gun systems with pressurized nitrogen. When the guns are detonated the nitrogen cushion is used to inject into the newly created perforations at pressures well above the fracture gradient. The EOB perforating technique is used to ensure that all perforations are broken down to bypass any near wellbore damage associated with the perforation event or drilling damage effects and then small narrow fractures are propagated away from the wellbore to contact undamaged reservoir rock. Efficient breakdown of the perforation tunnels results in improved injectivity and fluid diversion during the acid stimulation treatment that immediately follows the perforation event. The paper will discuss the results achieved with this perforation technique along with the special tools and precautions that should be followed to safely perform this type of operation.

The EOB perforating technique is effective in cases where mild near wellbore stimulation is needed to improve flow efficiency and is not intended to serve as a replacement for conventional hydraulic fracturing where long propped fractures with enhanced conductivity are required to achieve commercial production rates. The key to using the EOB technique is the identification of reservoir and completion properties that are conducive to applying this technique. Candidate screening criteria - permeability, fluid mobility and open-hole formation test characteristics - are discussed at length. Examples of applying this screening process will be presented along with the results achieved in the W31S reservoir with both the EOB and conventional hydraulic fracturing completion methods.