The operating envelope and environmental demands for hydraulic fracturing and stimulation treatments in the North Sea are increasingly larger and more complex. As a result, a generation of vessels designed and built in the 1980s are increasingly unable to achieve operational, economic and/or environmental goals. Thus, a new generation of vessels must be designed specifically to meet these European requirements.The design characteristics of a new vessel can be split into five categories: (i) stimulation plant (ii) marine systems (iii) environmental protection (iv) welfare of the crew and (v) regulatory compliance. All five categories are critically important to achieving the stated goals for the new generation of European vessels, and all require careful integration of current cuttingedge technology with forward-thinking design concepts that will enable technical evolution over the expected 30-year lifetime of the new vessels. This paper will outline the most significant problems faced by the current generation of stimulation vessels, compare current technological solutions to those problems, and demonstrate that integrating those technologies in a new vessel can achieve performance and flexibility far beyond what is possible by retrofitting them on an older vessel or in a modular system on the deck of a supply boat.Tail-end production will have an increased importance in the North Sea and in all offshore production as a large number of producing fields near the end of their life. The effective deployment of production enhancement with stimulation vessels has a major part to play in extending field life.

Oil exploitation in the Bachaquero field in east Maracaibo Lake has been occurring for more than 50 years. Sandstone is the primary formation type, and nonconsolidated and poorly consolidated sands are common in this field. Complex mineralogy and fines migration have become root causes of production decline and formation damage. This paper describes a comprehensive approach to reservoir characterization that has contributed to the successful stimulation of the sandstone formations in the field. Chemical stimulation, specifically matrix acidizing with hydrofluoric (HF) acid systems that are customized and tailored to reservoir characteristics, has proven to be effective at enhancing production in this field. The types of clays that are present include kaolinite, illite, smectite, chlorite, and mixed-layer clays; feldspars are also present. An adequate analysis of each well helps to ensure that HF acid dissolves the clays to restore permeability without promoting nonsoluble fluorosilicates precipitation through reactions with aluminosilicates. Variations in mineralogy determine fluid performance and make customized fluid selection necessary. The high presence of feldspars requires more conservative treatments to avoid undesirable precipitations. Reservoir characterization and fluid tailoring has helped ensure treatment success, but other good practices also have been applied to help achieve production goals. The stimulation treatment design includes pumping formation-conditioning fluids before and after the main acid; using different types of organic solvents to dissolve asphaltene deposits in the well; performing near-wellbore (NWB), hydrochloric (HCl) acid, and HCl/organic acid blend preflushes and post-flushes to treat calcium carbonate and control the pH and iron precipitation in the reservoir; achieving short-term clay inhibition and long-term clay stabilization; and using other fluids, such as relative permeability modifiers (RPMs) for water-control applications and diversion of treatment in laminar reservoirs with petrophysical heterogeneities. All of these combined practices have resulted in successful stimulation of the field. This paper discusses in detail this comprehensive approach to reservoir characterization applied successfully in wells in the Bachaquero field. The workflow includes candidate analysis, from reservoir description and mineralogy and formation damage mechanism identification to stimulation treatment design, laboratory fluid systems tailoring, placement and diversion techniques, pretreatment operational task fulfillment, field execution, quality control, and post-job evaluation through analysis of records and statistics.

The Bachaquero field is located on the east side of Lake Maracaibo, where oil exploitation has been occurring for more than 50 years. Primarily composed of sandstone, most of the producing reservoirs in this formation are from the Tertiary period and can be found in the Miocene Epoch. Nonconsolidated and poorly consolidated sands are also common in this field. Complex mineralogy has been identified as the primary cause of production decline for wells in this field, with fines migration being the principal mechanism of formation damage. Other factors also influence the decline in production rate, including high-permeability formations, salinity, pH changes, and drag forces caused by fluid-flow velocity, multiphase flow, turbulence, and fluid viscosity. Chemical stimulation has become a useful technique for enhancing production, and matrix acidizing with hydrofluoric-acid (HF) systems has proven to be very effective in this field. Matrix stimulation is a technique that has been used extensively since the 1930s to improve production from oil and gas wells and to improve injection into injection wells. Matrix stimulation is accomplished by injecting a fluid to dissolve and/or disperse materials that impair well production in sandstones or to create new, unimpaired flow channels between the wellbore and a carbonate formation. In matrix stimulation, fluids are injected below the fracturing pressure of the formation. Substantial production improvements can be achieved with matrix stimulation if treatments are engineered properly. It is well known that HF reacts with clays present in the reservoir to dissolve them and restore original permeability, but some of those reactions are not always desired. Secondary and tertiary reactions of HF with aluminosilicates can promote nonsoluble fluorosilicates precipitation, which requires that fluids be tailored for compatibility with the formation's mineralogy. Variations in mineralogy determine which fluid performs better, and a high presence of feldspars requires more conservative treatments to avoid undesirable precipitations. A stronger retarded HF (RHF) has also been used to treat wells that are deeper in the formation. Other good practices in addition to primary acid selection are also applied to help ensure treatment success. The stimulation treatment design includes pumping formation conditioning fluids before and after the main acid; using different types of organic solvents to dissolve asphaltene deposits in the well; performing NWB, HCl, and HCl/organic acid blend preflushes and post flushes to treat calcium deposits and control pH and iron precipitation in the reservoir; achieving short-term clay inhibition and long-term clay stabilization; and using other fluids, such as relative permeability modifiers (RPMs) for water-control applications and diversion of treatment in laminar reservoirs with petrophysical heterogeneities. Each of these combined practices have resulted in successful stimulation of the field. This paper discusses a comprehensive approach that has been successfully applied in wells located in the Bachaquero field in the Maracaibo basin. The workflow includes a candidate analysis, from the reservoir description, mineralogy, and identification of the formation damage mechanism, to stimulation treatment design, laboratory fluid-systems tailoring, placement and diversion techniques, pretreatment operational task fulfillment, field execution, quality control, and post-job evaluation through analysis of records and statistics.