Since the late 1980s when Maersk published their work on multiple fracturing of horizontal wells in the Dan field, the use of transverse multiple-fractured horizontal wells has become the completion of choice and the “industry standard” for unconventional and tight-oil and tight-gas reservoirs. Today, approximately 60% of all wells drilled in the United States are drilled horizontally, and nearly all are multiple-fractured. However, little work has been performed to address and understand the relationship between the principal stresses and the lateral direction. This paper has as its goal to fundamentally address the questions: In which direction should I drill my lateral? Do I drill it in the direction of the maximum horizontal stress (longitudinal), or do I drill it in the direction of the minimum horizontal stress (transverse)?
This work focuses on how the horizontal well’s lateral direction (longitudinal or transverse fracture orientation) influences productivity, reserves, and economics of horizontal wells. Optimization studies, with a single-phase fully 3D numerical simulator including convergent non-Darcy flow, were used to highlight the importance of lateral direction as a function of reservoir permeability. The simulations, conducted for both oil and gas formations over a wide range of reservoir permeability (50 nd–5 md), compare and contrast the performance of transversely multiple-fractured horizontal wells with longitudinally fractured horizontal wells in terms of rate, recovery, and economics. This work also includes a series of field case studies to illustrate actual field comparisons of longitudinal vs. transverse horizontal well performance in both oil and gas reservoirs, and to tie these field examples to the numerical-simulation study. Further, the effects of lateral length, fracture half-length, and fracture conductivity were investigated to see how these parameters affect the decision of lateral direction in both oil and gas reservoirs. In addition, this study seeks to address how completion style (openhole or cased-hole completion) affects the selection of lateral direction.
The results show the existence of a critical reservoir permeability, above which longitudinal fractured horizontal wells outperform transverse fractured horizontal wells. With openhole completions, the critical permeability is 0.04 md for gas reservoirs and 0.4 md for oil reservoirs. With cased-hole completions, longitudinal horizontal wells are preferred at a reservoir permeability above 1.5 md in gas reservoirs, and transverse horizontal wells are preferable over the entire permeability range of this study (50 nd–5 md) in oil reservoirs. These are new findings. Previous work generally suggested that longitudinal horizontal wells are a better option for gas reservoirs with permeability over 0.5 md, and for oil reservoirs with permeability over 10 md.
This work extends prior study to include unconventional reservoir permeabilities. It provides critical permeability values for both gas and oil reservoirs, which are validated by the good compliance between actual field-case history and simulation results. This work also demonstrates a larger impact of completion method over fracture design. These findings could guide field operations and serve as a reference for similar studies.