Wide permeability range challenges stimulation economics

For a long, inclined condensate producer well in Kazakhstan, an operator needed an economical stimulation treatment to maximize production. But the challenging reservoir conditions, including high temperature (93 degC [200 degF]) and wide permeability heterogeneity, limited the options.

The main concern from a stimulation standpoint was a thief zone identified during drilling as a sequence of natural fractures. Stimulation fluids would most likely flow into the fractured zone, precluding thorough stimulation of the full length of the wellbore.

The operator considered mechanical diversion to segment the lateral and isolate the thief zone for separate treatment, but it was not technically or economically feasible in the well because of the 4 ½-in completion string and inability of the straddle packers to hold the required differential pressure in the 7-in casing.

Acid and diversion technologies optimize stimulation

To distribute matrix stimulation across the full lateral, Schlumberger recommended a tailored engineered diversion program with OpenPath stimulation services as the foundation.

OpenPath Reach extended-contact stimulation service maximizes matrix or fracture stimulation penetration with engineered acid system selection to ensure compatibility with reservoir conditions and chemistry. The main acid system chosen for this well was the single-phase retarded acid system, which penetrates deeply into the reservoir before beginning to spend, even in high-temperature limestone reservoirs.

To eliminate the problems posed by the natural fractures, the engineers designed the operation to begin with MaxCO₃ Acid degradable diversion acid system, which diverts fluids temporarily from thief zones, controlling leakoff with wall building while minimizing the volume of fluid required.

To ensure distribution of the acid systems across the long lateral, the engineers turned to the OpenPath Sequence diversion stimulation service, which uses engineered composite pill systems to temporarily block high-permeability areas and divert fluid into areas of lower permeability. The composite pill chosen for this well comprises unimodal particles in a carrier fluid that degrades in the presence of hydrocarbon. It is designed to pass through the completion and accumulate in a perforation or along an open hole to address medium permeability contrasts within an order of magnitude.

For smaller permeability differences, the engineers added sequences of VDA acid. This fluid system has low viscosity when pumped but gains viscosity as it spends, making it an effective and economical diverter for areas with similar permeability.

Treatment improves production up to threefold

The treatment was pumped in three stages with two diversion pills to maximize stimulation across the lateral and limit leakoff to meet economic targets. Pressure response indicated the diversion was successful.

Well production before the treatment was 5 to 8 m³/d [31 to 50 bbl/d] of condensate with gas lift. After stimulation, production increased to an initial rate of 50 m³/d [314 bbl/d] with gas lift for the first week and then stabilized at 20 to 23 m³/d [126 to 145 bbl/d] with gas lift.