The development and upgrade of mature facilities is ultimately driven by the significant shift in the composition of the arrival fluids, mainly the increasing water cut and solids production. In many cases, the routing of new well discoveries to these facilities, or the change in characteristics of the crude with the decline in field production, exacerbates this shift.
Consider a typical production profile. It has the characteristic steep increase in production at startup, followed by a maximum production plateau for oil and gas, then a gentle decline to the point of abandonment. In contrast, produced water (and sometimes sand) production generally increases, especially if water injection is installed to enhance oil and gas production. It is this idealized profile that life-of-field operating design cases are based on, following initial reservoir appraisals.
Ian Bedwell is a separation consultant at Maxoil Solutions with more than 25 years of experience covering design, specification, application, installation, upgrading, debottlenecking, commissioning, troubleshooting, and product development. His project experience covers very large onshore facilities, floating, production, storage, and offloading applications and high-performance separation systems.
At first glance, mature field upgrades seem to be a poor value for money compared with early field modifications in which a relatively small increase in production can produce a significant increase in recovery over the life of the field. Mature field upgrades often suffer the disadvantage of causing a disproportionate increase in produced water throughput. For this reason, an operator’s attitude toward mature field management is generally one of minimum investment and breakdown maintenance.
Although late-life oil production is significantly lower than peak production, achieving a modest increase in production can significantly increase revenue, extend economic field life, and improve field recovery. However, achieving such results requires a mind-set that considers the mature facility as an asset that still offers a potential for achieving good economic return on investment, rather than viewing it as a spent asset being run on a shoestring.
While the effects of production profile changes are witnessed across the facility, upstream separation vessels, such as slug catchers and first stage separators, are most significantly affected. As an oil field matures and water cut increases, separation is affected by increasing emulsion viscosity until an emulsion inversion point is passed. For mature field operation and upgrade, water production dominates oil recovery and the separator is needed to operate beyond the limit of its original produced water design. At this point, the handling of increased solids production, changes in fluid properties, and changes in chemical treatment requirements require a new approach.
Consider a scenario in which well fluids are routed directly to the first stage separator, wherein bulk oil, gas, and water separation is achieved. Oil carryover to the second stage separator includes solution gas and entrained water, which are further separated in this unit following heating and pressure reduction. In this instance, the first stage separator throughput reflects the reservoir profile, and is therefore the most affected by the changing characteristics of the wells. It should be noted that the second stage separator’s production profile is not significantly affected by increased water production associated with mature field optimization, assuming good oil/water separation in the first stage separator. In light of this, the implications of mature field production are, therefore, most clearly shown by an inspection of the first stage separator. Although the key aspects of mature field upgrades are highlighted in this way, it must be understood that this is a facility wide issue that must be addressed holistically.