Geomechanical Factors in the Successful Implementation of Barefoot Horizontal Completions Totaling 100,000 ft Long, Alpine Reservoir, Alaska

Abstract

This is a case study of a successful, field-wide implementation of horizontal, barefoot completions in a moderately competent formation - the Alpine Reservoir in Western North Slope, Alaska, Figure 1. A barefoot completion is a borehole without tubular liners and no cemented support - the least costly but riskiest completion strategy. (In this paper, barefoot and unsupported mean the same and are used interchangeably.) The Alpine experience provides a successful example where the benefits of unsupported boreholes outweigh the risks of borehole failure. To date, this aggressive yet simple completion technique has an aggregate length of more than 160,000 ft, all unsupported. Combined with good drilling practices, the success of barefoot horizontal wells in Alpine is also due to the following petrophysical and geomechanical factors:Consistent reservoir quality within the layerAbsence of shale-breaks in the producing zoneModerate strength in normal fault geotectonic settingLow variability of strengthLinearly elastic behaviorNon-severe, slight weakening when water-saturatedGood permeability retained under post-elastic strains Introduction In general, barefoot horizontal completions are implemented only in very competent, hard formations that pose little risk for wellbore collapse and/or sand production, such as dolomites, hard limestones, hard sandstones, and shale-free siltsones. The advantages of barefoot completions are:Low completion costSimple and fast implementationPotentially lower completion skins if undamagedHigh productivity per unit length (producers)High injectivity (injection wells)Higher critical drawdown pressure for sanding However, the risks and disadvantages of unsupported, long, horizontal wells in Alpine are:Potential for collapse or sanding in weak zonesHigher wellbore stresses compared to vertical wellsCostly and limited options for zonal isolationHigher sensitivity to formation damageLimited options for stimulationLimited options for future remediationHigh friction factors for future coiled tubing workover There are very few reservoirs that are developed exclusively with barefoot horizontal wells, owing to the abovementioned risks. One recent example is described in Australia (Allard, 1998). The Alpine experience is the first reservoir in Alaska developed exclusively with barefoot horizontal wells.

Predicting Horizontal-Openhole-Completion Success on the North Slope of Alaska

“…This strength is low enough, at 7,000 ft true vertical depth (TVD), for potential wellbore collapse and loss of productivity. However, the measured ranges of UCS for the Alpine C facies support a high critical drawdown pressure when minimum-principal-stress support is included in the evaluation, justifying the choice of an openhole completion (Ramos et al 2002).…”

“…The static E of 2.69 ± 0.47 × 10 6 psi and average UCS of 7,794 ± 751 psi are indicative of moderate strength. Their low standard deviation of 9.6 to 17.6% suggests low strength variability within a well and from well to well (Ramos et al 2002).…”

“…In general, barefoot horizontal completions are implemented only in very competent, hard formations that pose little risk for wellbore collapse and/or sand production, such as dolomites, hard limestones, hard sandstones, and shale-free siltsones (Ramos et al 2002). In Alaska's North Slope Petroleum Province, a normal-faulting geotectonic regime is the norm.…”

“…Values for S hmax were inferred from borehole breakouts and tensile cracks. Although wells along the S hmax azimuth are less stable and are susceptible to drilling-induced tensile fracturing (and, consequently, fluid losses), they were desirable from the point of view of Alpine's reservoir management (Ramos et al 2002).…”

Predicting Openhole Horizontal Completion Success on the North Slope of Alaska

“…In general, barefoot horizontal completions are implemented only in very competent, hard formations that pose little risk for wellbore collapse and/or sand production, such as dolomites, hard limestones, hard sandstones, and shale-free siltsones 5 .…”