Optimizing Longitudinal Fracture Design for Horizontal Well Completions in Laminated Sandstone Reservoirs

Abstract

Abstract Horizontal wells with multi-stage longitudinal fractures have been widely used to develop low-to-moderate permeability sandstone formations in enhanced oil recovery schemes. A long-held misconception exists that high fracture conductivity is not essential to well productivity for longitudinal fractures in openhole completions because the pressure drop through the fracture is small due to the geometry. This study provides workflows to optimize longitudinal fracture design for horizontal well completions on the North Slope of Alaska and presents practical considerations that challenge this misconception. Mechanical properties of core samples across the reservoir interval were evaluated by hardness-based strength calculators and tri-axial compression tests. Based on the rock strength profile, candidate intervals were selected for fracture conductivity measurements. Both summer (low salinity) and winter (high salinity) seawater based fracturing fluids were injected through the propped fracture cell containing both sandstone and mudstone lithologies. Numerical fracture models were built and matched to bottomhole pressures acquired during project appraisal well stimulations. Proppant concentration distribution along the fracture was generated for different design scenarios by varying proppant volume, proppant size, pump schedule, etc. The impact of various design scenarios on well production was also investigated. Full alignment between the fracture plane and the wellbore results in the highest productivity for fractured horizontal wells with openhole completions. However, calculations demonstrate that even a few degrees of misalignment between horizontal well orientation and the maximum horizontal principal stress results in fracture deviation from the open hole. Due to flow along the fracture and convergence from the fracture to the wellbore, fracture conductivity dominates the pressure drop and completion skin factor for this geometry. Since actual fracture conductivities in wells on the North Slope of Alaska are not infinite, it is therefore inappropriate to use "infinitely-acting" fracture assumptions as has often been used historically for longitudinally fractured horizontal wells with openhole completions. Fracture conductivity tests show severe conductivity loss due to gel residue as well as mudstone and seawater interactions. Realistic discount factors for fracture conductivities in the targeted shallow sandstone-mudstone formations were developed for subsequent reservoir studies. Modeling results suggest that larger job sizes and bigger proppant are needed to achieve desirable skin factors and well inflow performance when the fracture becomes misaligned from the wellbore.