Horizontal, Near-Wellbore Stress Effects on Fracture Initiation

Abstract

Throughout the past decade, the number of horizontal wells drilled in the United States has continued to increase. Several fields currently being developed are only economical with the application of horizontal wells. One such area where this is the case is in the Bakken formation in the Williston Basin. In the Bakken, operators have used a variety of different completion methods ranging from multilateral, openhole wellbores to single lateral cemented liners. Regardless of the completion method, hydraulic fracturing is required for increased oil recovery. In the past two years, most operators have started to converge on running uncemented liners with external packers as the preferred completion method. The purpose of the external packers is to compartmentalize the wellbore for more efficient fracture stimulation, as demonstrated by Miller (2008).

As this type of completion becomes more common, three key questions should be addressed: (1) how does the fracture initiate in an open-annulus wellbore (transverse or longitudinal), (2) what is the affect of the packer type on the stress on the wellbore surrounding the packer, and (3) can any of this be used to help with fracture design, or does it even matter?

Finite-element modeling provides several advantages for modeling the different physical characteristics and responses for a system with very different sensitivities across the scale of the model. In the cases considered in this paper, the edge of the wellbore must be finely modeled to take into account the hoop stresses developed as a result of removing the rock volume from the original stress state. This is further complicated by the effects of orientation of the wellbore relative to the maximum and minimum horizontal stress and of different packers placed in the wellbore for isolation between stages.

Conclusions on fracture-initiation behavior based on typical wellbore geometries, physical-rock properties, in-situ stress scenarios, and common packers are presented in this paper.

Introduction

The Bakken formation is present in only the subsurface of the Williston basin, which underlies much of North Dakota (ND), eastern Montana (MT), and northwestern South Dakota (SD), extending into Canada (Fig. 1). The Upper-Devonian Lower- Mississippian Bakken formation is composed of two organic-rich shale members that encase the clastic/carbonate Middle Member and overlay the Devonian Sanish/Three Forks formations (Fig. 2). Presently, there have been more than 650 horizontal wells drilled in MT, and nearly 600 wells drilled in ND. The majority of these wells were drilled in the Middle Member of the Bakken. Some wells are now being drilled in the Sanish/Three Forks, but the majority drilled continue to be in the Middle Member of the Bakken. According to a 2008 U.S. geological survey (USGS), there are an estimated 3.0 to 4.3 billion barrels of undiscovered, technically recoverable oil in the Bakken formation. This makes the Bakken formation the largest current USGS-oil assessment in the lower 48 states. It is also the largest "continuous?? oil accumulation ever assessed by the USGS. Advances in drilling and completing long horizontal laterals have been the key to this large, technically recoverable oil volume.

The Middle Member is a relatively thin section (10 to 30 ft) when compared to other formations being completed with horizontal wells, some hundreds of ft in height. Fracture gradients in the Middle Member range from 0.70 psi/ft in MT, to more than 0.85 psi/ft in areas of ND. The pore pressure also varies across the basin, ranging from 0.50 psi/ft in MT to 0.63 psi/ft in areas of ND. The maximum horizontal-stress orientation has been measured at 340° in MT, but varies across the basin, turning to the NE direction. There is little stress anisotropy between the maximum and minimum horizontal stress, which results in some complexity to the hydraulic-fracture geometry. This complexity has been noted by Bessler (2007), where interference has been observed in wells located E-W from the treatment well.