Investigation Into Strengthening Methods for Stabilizing Wellbores in Fractured Formations (original) (raw)

IADC/SPE 128728 Wellbore Stability in Fractured Rock

Numerous wellbore instability problems related to drilling through potentially fractured formations have been reported. Often, these rocks are characterized by the abundance of macro and micro scale bedding planes and/or networks of natural fractures. The presence of fractures weakens the rock mechanically and produces potentially higher-permeability fluid-flow paths within the low-permeability rock matrix. Practically, it is difficult to identify fracture size and fracture density without a costly core sample. A number of wellbore stability case studies have therefore been published where the author relied on anecdotal evidence to postulate failure mechanisms involving fractured rock, and recommendations for how to mitigate the observed instability range from increasing to decreasing the wellbore pressure. This paper presents results from a geomechanical investigation of a wellbore instability incident experienced in a fractured shale formation. As part of this assessment, a preserved core was obtained from the fractured shale interval and the presence of fractures was identified both by CAT scan and visual inspection. A series of triaxial tests were conducted to characterize the mechanical properties and failure strength of this shale. This data, combined with wellbore stability modeling, suggests that the residual strength, rather than the peak failure strength, is a more representative measure of a fractured rock's in-situ strength. The Hoek and Brown (1982) failure criterion was found to be particularly suitable for modeling fractured rock. Multi-arm caliper logs from two boreholes through the same fractured shale suggests that wellbore instability is more complex in the fractured interval than in the over-and underlying intact rock. These caliper logs also clearly demonstrate that the borehole quality was significantly improved by increasing the drilling fluid density, which contradicts the conclusions drawn in a number of published wellbore stability case studies (Santarelli et al.

Design and Evaluation of Wellbore Strengthening Materials for Fractured Reservoirs

Volume 10: Petroleum Technology

Fractured formations are commonly encountered while drilling carbonate reservoirs. Drilling of fractured formations usually results in lost circulation. In some cases, target zones cannot be reached due severe to lost circulation incidents. Fractures encountered before reaching target zones can be sealed off by using wellbore strengthening materials (WSM). WSM are drilling fluid additives which are specially sized and designed particulates. They are generally used to strengthen the wellbore and to prevent the occurrence of losses by propping and sealing the fractures while drilling. In this study, experiments were carried out to determine optimum concentration and particle size distribution (PSD) of WSM to seal off fractures with certain sizes. A non-damaging water-based polymer drill-in fluid with acid soluble marble particles having different concentrations and PSD were used in this study. Plugging efficiency of different blends were tested using modified Permeability Plugging App...

Stabilizing Wellbores in Unconsolidated Formations for Fracture Stimulation

All Days, 1998

A wellbore stabilization system incorporating fracture stimulation has been developed and modeled in the laboratory for unconsolidated and dilatant formations. The profile of resin leakoff and penetration depth into the formation is relatively dependent on the pattern of perforations. Use of dissolvable particulates as a fluid-loss control agent proved to be effective in diverting resin, resulting in a uniform distribution of resin over the highly varying permeability interval. A significant drop in permeability of the near-wellbore area occurred after the resin treatment, but high productivity was re-established by connecting the wellbore with the untreated formation through highly conductive fractures. During companion testing of the resin-consolidation technique, the unconsolidated sand pack became a competent formation with compressive strength in excess of 2,000 psi. This competent material will be affected less by high drawdown pressures and stress cycling during production/sh...

Wellbore Strengthening -Where Field Application Meets Theory

SPE Latin American & Caribbean Petroleum Engineering Conference, 2010

In recent years there has been a growing awareness of the importance of Wellbore Strengthening Material (WSM) for sealing drilling-induced and natural fractures both in permeable and impermeable zones. This paper details the application of a software tool designed specifically for determining the optimum blend of WSM to drill the formations of the challenging Cashiriari field in Peru. Several theories have been proposed to explain techniques which are in vogue for mechanically strengthening wellbores so as to control fracture propagation: Hoop Stress Enhancement ("Stress Cage") theory involves increasing the near-wellbore stress, which is usually carried out by opening fractures to tighten the wellbore and sealing the fractures immediately in that open state; "Fracture Closure Stress" theory utilizes high-fluid-loss materials to plug and tighten existing fractures; and "Fracture Propagation Resistance" theory involves isolation of the fracture tip. The design and successful utilization of a blend of WSM based on the opening and sealing of fractures suggests that these three theories are, in many respects, complementary. Any theory explaining a particular problem is ultimately judged by its practical application. Cashiriari field, known for expected mud losses to both natural and drilling-induced fractures in primarily permeable formations, is where this wellbore strengthening application took place. The optimum blend of particulates, designed using wellbore-strengthening (WBS) software, was applied consistently on the wells drilled by the operator in the Cashiriari field in the 2009-2010 campaign. Fracture resistance was increased up to 0.8 lb/gal above the minimum horizontal stress for drilling and up to 1.68 lb/gal for running casing and cementing operations. Specialized logs showing the presence of WSM inside fractures were an important tool for optimization of the WSM blend. WSM design software takes in account a multitude of drilling parameters and rock properties to predict the fracture width and estimate the required size distribution and concentration of the WSM particulates. Consequently, the WBS solution reduced lost circulation as a primary contributor of costly and time-consuming lost circulation incidents in the Cashiriari field. Further, it is anticipated that as the WBS approach is applied more widely, the incidence of lost circulation events will be greatly reduced.