Abstract

Field production data from unconventional reservoirs including Eagle Ford, Marcellus, Wolfcamp, Utica, and Vaca Muerta formations indicate that the landing location of horizontal wellbores is a critical control variable to well productivity, and that small changes in the landing depth (e.g., from 15 to 30 ft.) may result in significant changes in well performance. The reasons for these changes are not yet well understood. In this paper, we show that hydraulic fracture height growth is less extensive than anticipated from hydraulic fracturing modeling or microseismic monitoring, and that proper landing depth selection helps improve the final propped and connected fracture height. We also show that a primary source of height growth suppression is the pervasive rock layering in mudstones and hybrid reservoir systems, which often exhibit strongly contrasting properties between layers and weak interfaces at their contacts. In some reservoirs, bed parallel ash beds, mineralized veins, and slickensides are also present, and all of these interfaces can be weakened further by tectonic deformation. We studied rock layering and various types of weak interfaces in outcrops and cores, including their geologic and stratigraphic context. We also studied the effect of these on hydraulic fracture height growth, using large-scale laboratory hydraulic-fracturing testing, and through field diagnosis of hydraulic fracture height growth in vertical pilot wells. These evidences indicate that the pervasive layering and weak interfaces induce step-overs and branching during hydraulic fracturing, which close after pumping and truncate the initial hydraulic connectivity of the fracture. We observe that during fracturing, the distance from the wellbore to the weak interfaces is a critical measure that controls whether or not these can be overcome without developing step-overs. Proper selection of the lateral landing depth thus depends on understanding rock layering and the distribution of weak interfaces in the section to be fractured. It depends on anticipating critical step-overs and truncation points, and reducing their effect on the hydraulic fracture by adjusting their distance from the wellbore. It depends on choosing the optimal wellbore location that will extend the propped and connected surface area and will maximize economic well production. Using this methodology we have recommended lateral landing depths on multiple wells in the Wolfcamp, Eagle Ford, Utica, Velkerri, and Vaca Muerta plays. This was done while maintaining the same completion and fracture design, so as to evaluate the effect of lateral landing depth alone. Results show a 22% to 46% increase in well production and greater consistency in production from well to well. Some of these results are publicly available.

Introduction

Field production data from unconventional reservoirs including Eagle Ford, Marcellus, Wolfcamp, Utica, and Vaca Muerta formations indicate substantial variability in well productivity, which is incommensurate with the variability in reservoir properties. Despite our industry's improved understanding of reservoir quality, reservoir thickness and reservoir pressure, we still observe that a substantial number of the wells drilled are uneconomical or marginal, while others perform well, and few outperform all expectations (Hodenfield, 2012). Given our understanding of reservoir quality, what then drives the large variability in well production?

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