Tight sandstone reservoir produces with high water cut

A key field in Serbia has been producing industrial volumes of oil since 1984 from a tight 
(approximately 1- to 10-mD permeability) sandstone formation with narrow net pay (15 to 25 m thick) and weak barriers to nearby water zones. Hydraulic fracturing has been widely used in the field in recent years. On average, the wells produce with 65% water cut before fracturing and 75% water cut afterward, but some wells experience 100% water cut after fracturing.

In 2017, NIS Gazpromneft asked Schlumberger to fracture four wells in the field using the HiWAY technique, which had outperformed conventional fracturing for the operator in other fields by leaving wide channels in the fractures for maximum conductivity. Operations in the first three wells increased oil and water cut, but the fourth resulted in reduced oil production and 98% water cut because of fracture height growth into the lower water zone.

To further assess the potential of the field, NIS Gazpromneft asked Schlumberger to fracture three more wells: two more in the same field and one in a similarly tight field that had no fracturing history because of the high risk of connecting to water through the naturally fractured basement rock.

Stimulation design avoids the water zones

To minimize risks of breaking into the water zones, the Schlumberger engineers designed two stimulation operations using the BroadBand Shield service: one with conventional fracturing and one with the HiWAY technique. BroadBand Shield service uses a fluid system with a proprietary blend of multimodal particles to bridge the fracture tip and prevent excessive fracture growth outside of the designed fracture area. The engineers used an improved fracturing simulator to design optimal fracture operations for each formation and then used an internal expert workflow to optimize the diversion pill to minimize conductivity near the water zones.

The new simulator's algorithm comprises a fine-scale fracture hydrodynamics and in situ kinetics model. In contrast to conventional commercial modeling tools, it has sufficient resolution and other functionality to represent modern stimulation technologies: pulsing injection of proppant; mixtures of fracturing fluids, proppants, and fibers; material degradation; and other features. This simulator accounts for the influence of materials distribution on fracture propagation and calculates fracture conductivity distribution.

Oil production increases with improvement or small effect on water cut

The stimulation operations were pumped as designed in the three wells. After the flowback period, oil production in all three wells was higher (>300%) and water cut was reduced slightly in two wells and increased slightly in the third.

The extremely risky well in the new field saw a slight water cut reduction along with an oil production increase of >3,000%, which enabled it to flow naturally for the first time.

The extremely risky well in the new field saw a slight water cut reduction along with an oil production increase of >3,000%, which enabled it to flow naturally for the first time.