Using Foam Treatments to Control Gas-Oil Ratio in Horizontal Producing Wells at Prudhoe Bay

Abstract

Abstract Production at Prudhoe Bay is constrained by gas handling. The objective of this project was to develop a foam injection strategy to reduce gas mobility in producing wells and increase field oil production through reduction of producing gas-oil ratio. Aqueous foam has been extensively studied through laboratory and field experiments for gas mobility control to improve sweep efficiency in gas flooding. However, the potential of foam injection into horizontal producers for mitigating unwanted gas production has not been well understood. In this work, a unique design of laboratory experiments was developed to optimize surfactant formulation for foam generation and stability under the conditions of target gas saturated zones in the Ivishak sandstone reservoir. Gas blocking capacity for different foam placement and flowback strategies in reservoir cores were evaluated to identify important factors for optimized field process design. Based on lab results, five producing wells were selected for repeat injections of brine using varying volumes. Flowback results described the gas-blocking potential and determined optimal foam injection volume for each well. Two of the same wells were then treated with foam and flowed back. Experimental results show that oil tolerance is not a critical surfactant screening criterion for these particular reservoir conditions as the targeted treatment zones are the high permeability channels which have likely experienced a large amount of gas channeling. The threshold surfactant concentration, above which foam blocking capacity did not further improve, was significantly lower than that used in previous lab and field studies. Moreover, foams with an initial apparent viscosity above 50 cP remarkably delayed and reduced gas production rate for over a week in short cores at varying applied pressure gradients. The laboratory observations led to a new foam injection strategy that aims to place surfactant deeper into the gas zone by an optimal foam drive. Field trials demonstrated strong technical success of both brine and foam treatments to block gas production and reduce producing gas-oil-ratio (GOR). Flowback following brine injection demonstrated temporary GOR reduction for a period of about one week. Repeat brine treatments, of varying injection volumes, described the near-wellbore pore space and informed optimal foam treatment volume for each well. Both foam treatments resulted in reduced gas mobility, reduced producing GOR, and longer duration of these effects compared to brine gas blocking. Foam gas blocking effects lasted up to 70+ days, resulting in significant incremental oil production from the field. Foam provides a novel method to decrease producing GOR in horizontal wells in Prudhoe Bay and increase field oil production. Foam treatments are shown to be a cheaper alternative to well interventions, gas handling expansion, or other means of increasing production in a gas constrained system. This work has advanced our understanding of foam potential for gas shut-off in both vertical and horizonal producing wells.