Acid Fracturing Treatment Design Optimization in North Sea Chalk Wells in Norway
Authors: Pandey Vibhas J. , Ruysschaert Rodrigo , Prosvirnov Maxim , Ferris Matthew M. 
Published: January 2025

Acid fracturing is a common stimulation technique employed in enhancing hydrocarbon production from the chalk formations in the North Sea sector of Norway. The efficacy of this stimulation method relies on several factors, some of which are well defined and understood whereas the others become obvious only after detailed analysis of multiple treatments which could prolong the optimization process. This study highlights the improvements in stimulation design over the past decade that resulted in optimal placement and subsequent improvement in well performance.

The Paleocene Ekofisk reservoir of Cenozoic era trends along a north-south anticline with a characteristic central graben from normal faulting, that bears an inverted tear drop shape in subsurface structural maps. The formations exhibit porosities in the range of 30 to 40% and are known to be primarily composed of chalks that are fine grained limestone mostly consisting of the skeletal remains of coccolithophores. The trap was plenished by underlying Kimmeridgian shales of Jurassic era.

The limestone rich formation enables effective use of hydrochloric acid of various concentrations, which usually depends on formation temperatures, expected reservoir pressures and the anticipated acid-mineral reaction from rock minerology. However, the high formation temperature (250 °F) requires limiting the acid/rock exposure time especially for uninhibited acids to prevent closed-fracture type conditions, induced because of stress-relief from rock-acid reaction, early in the treatment. The enhanced fluid leak-off resulting from acid reaction requires employing appropriate gel type and volumes in the pad stages; also, the injection rates must be designed to promote uniform fluid distribution across the multiple sleeves in each zone.

The improvement in treatment design process and its optimization across several fields and formations was a direct outcome of detailed treatment modeling prior to the execution, followed by real time analysis of bottomhole injection pressures, and using the observations to modify the future treatment designs. The analysis and continuous improvement process resulted in a comprehensive overhaul of treatment design approach including changes to pad acid volume design, injection rates selection as a function of formation characteristics and stresses, fracture growth desired and exposure time limits, optimization of pad and acid cycles to maximize the stimulation impact and production, and initiation of a shift from using legacy pad fluid to more effective low gel loading borate cross-linked fluid with superior rheology and leak-off control capabilities. Over the years, several fold improvements in fracture conductivity were realized resulting in improved performance.

The acid fracturing treatment designs resulting from analysis and continuous improvement over several years resulted in formulations that are practical and easily implementable in the field. The approach discussed in the study can be readily adopted worldwide in other chalk reservoirs of similar settings.