The paper demonstrates a water shutoff technology in naturally fractured reservoir, which is based on simultaneous placement of a polymer gel and foam in fracture, where foam is stabilized by delayed formed gel.
Lab studies were undertaken to develop a gelled foam formulation. A series of fracture blocking tests were performed to evaluate water-blocking capacity of gelled foam in fractured models. The optimized gelled foam has stronger water-blocking capability and greater residual resistant factor than regular gel. The influence of residual oil in fracture to gelled foam is also studied; a pre-treating fluid is used to enhance blockage effectiveness.
The field trial design and execution of gelled foam treatment on an oil well in fractured reservoir are described. The results show that water cut decreases from 90% to around 40%, the oil production rate has been drastically increasing for more than 18 months, indicating longer effective lifetime of gelled foam treatment when compared with other water shutoff trials in this kind of reservoirs.
Water shutoff has drawn special attention in the development of naturally fractured reservoirs because early, excessive water production from production wells.1 Gel placement or gel injection had been proposed and applied to reduce excessive water production in fracture-dominated reservoirs2, 3, 4, 5, 6. Significant reductions in water cut were observed in all treated wells, and the total fluid productivity was typically reduced by a factor of 2. But the positive effects were generally short-lived despite the fact that there existed relatively long treatment lifetime with 1–2 or several years for several gel treatments.. It was also been found that the incremental oil recovery, treatment lifetime, and WOR reduction did not correlate with the volumes and types of treatment, the productivity, the structural position of the completion, completion type, the fluid level before the treatment.
The suggested mechanisms 7 of gel treatment involved:
gellant injection with subsequent flow through the fracture system, accompanied by gellant leakoff through the fracture faces into both the oil and water zones,
shut in to allow gel to form, and
return to production, with the gel substantially retarding water flow from the matrix into the fracture system but not significantly inhibiting oil flow.
The successful treatments depended on the distance of gellant leakoff and the residual resistance factors in the oil and water zones.8
Foam was also used for water shutoff as well as gas shutoff in oil wells producing at excessive gas/oil ratio (GOR).9,10 But the treatment results achieved in most field trials were not favorable with an average success rate of around 50%, and also the lifetime was limited to a few months. The reason is obviously lying in poor stability of foam 11,12, so polymer enhanced foams was introduced for application.13,14, 15 Currently there exists an argument about whether foam could be enhanced by polymer or not, some investigators are convinced that the only effect of polymer addition is to lower oil saturations before and during foam generation and propagation; there is no chemical interactions between polymer, surfactant or thin liquid films.16, 17
