Abstract

Seawater-based fracturing fluids are favorable in offshore locations because of the readily available seawater. This can minimize or even eliminate costly vessel trips necessary to transport fresh water to rig sites, while also reducing rig downtime. This paper presents the development of a low-residue fracturing fluid that uses seawater as a base fluid and also presents the results of successful field applications.

This seawater-based fracturing fluid uses a low-residue polymer crosslinked with zirconium that has a pH of less than 10 to minimize damage and residue encountered using other fluids. The fluid was tested for rheological properties, proppant-carrying capacity, retained permeability, and compatibility with formation fluid. To improve timing and efficiency of operations, a multistage fracturing completion was used wherein each fracture sleeve was opened by dissolvable balls. Three wells were treated in the offshore Romanian Lebada fields. Well A comprised high sandstone content. Wells B and C comprised a higher limestone content but contained too much clay and sandstone for an acid fracturing treatment. Both wells exhibited a moderate-to-high number of natural fractures. Because of moderate permeabilities in the range of 0.1 to 2 md, crosslinked fluid was used in the pad and subsequent proppant-laden stages.

During development evaluations, the fluid remained stable for approximately 1 hour at 325°F with a gel loading of 6 kg/m3 (50 lbm/1000 gal) The thermal stability of the fluid system was improved when compared to the alternative hydroxypropyl guar (HPG) fluid. A delayed crosslinker was used to maintain low-friction pressure during the treatment. No scale formation was observed. The target reservoir temperature was 199°F. Additional testing optimized gel loading at 3.6 kg/m3 (30 lbm/1000 gal). The fluid suspended the proppant for approximately 2 hours. Less than 1% residue was formed after fluid breakdown, which was much lower than the residue generated by the HPG fluid. A regained permeability of 92% was obtained from a sandstone core, demonstrating the low-damaging nature of the fluid. The broken fluid was fully compatible with crude oil and completion brine.

Twenty total stages of hydraulic fracturing operations were designed and executed successfully in the three horizontal wells. A 16/20-mesh resin-coated proppant (RCP) was used at a maximum concentration of 720 kg/m3 (6 lbm/gal) during the tail stage. A total of 1533 tonnes (~3.38 million lbm) of proppant was pumped in 5915 m3 (~1.56 million gal) of crosslinked fluid.

The seawater-based fluid properties present an innovative approach for addressing the water requirement issue for offshore stimulation operations. This fluid is an excellent candidate for fracturing operations and can help operators maintain low costs per barrel of oil equivalent (BOE).

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