A unique wellbore strengthening initiative is credited with dramatically slashing drilling fluid losses in the 121¼4-in. interval of an offshore well drilled through intermittent sections of high-pressure shale and depleted sand.

In order to stabilize the well bore while drilling through the shale, the operator was forced to


An MI survey showed the magnitude of wellbore stability problems encountered in the field. The graphs show the percentage of intervals drilled in which a problem was reported, starting in January 2003. (Graphic courtesy of MI LLC)
use a higher mud weight than could be tolerated by the depleted sands. Data from offset wells suggested mud loss in excess of 6,000 bbl should be expected. Consequently, by strengthening the well bore with specially designed loss prevention material (LPM) as drilling progressed through the interval, the operator reduced anticipated loss of drilling fluid more than 15 fold when compared to offset wells.

In a recent survey, E&P companies estimated that US $400 million a year is being spent globally solving wellbore stability problems, many of which are related to unstable or chemically reactive shales, unconsolidated formations and reservoir depletion. The cost of wellbore stability arises from lost drilling time (so called non-productive time, or NPT), rig time and lost fluid and equipment. Accordingly, the tolerance for wellbore stability-related problems can vary significantly depending on geographical location, well design sophistication of the well design and the downhole equipment used. For example, relatively inexpensive onshore operations can absorb several days of lost drilling time without too much financial pain. Conversely, in deepwater operations where rig day rates can exceed $500,000, the cost of delays mounts quickly and considerably.

The problems related to wellbore stability are experienced in many different ways even though the primary causes may be the same or similar. Hole collapse for example, resulting from too low a mud weight can be observed and identified as stuck pipe, active gumbo, poor hole cleaning or even under the catch-all “wellbore instability.” More indirectly, lost circulation can lead to hole collapse, which occurs when losses to the formation result in a sudden loss of hydrostatic pressure in the well, thereby removing support to the wellbore walls and causing hole failure; this is not an uncommon experience.

Wellbore stabilization program
Recognizing the economic impact of hole instability, M-I SWACO established a multi-disciplinary team of specialists, whose primary role is providing wellbore stability analysis for high-cost and high-risk wells and providing ongoing training for field operations. One of the key initiatives was the development of the proprietary and patented Opti-Stress wellbore stability module that helps design LPM for wellbore strengthening applications. The software calculates the width of induced shallow fractures and recommends LPM formulations and particle size distributions (PSD) to prop and seal the fractures. An innovative feature allows the module to use probabilistic techniques based on Monte Carlo simulation to incorporate expected uncertainties in rock properties and drilling parameters. This generates distributions of fracture width and the required LPM formulation to provide optimum bridging and sealing.

Field results
In the targeted 121¼4-in. section of the pilot well, varying permeability and formation strength generated different pressure over-balance for the various sections of the interval. This resulted in different induced fracture apertures, requiring assorted particle sizes for plugging and sealing the fractures. Calculations using geomechanical data and wellbore pressures gave estimates of required fracture apertures ranging from 180 to 650 microns. Two candidate LPM blends were developed. One was for the widest fracture (650 microns), and the second for a mean fracture width of 415 microns. The combination of the specifically sized graphitic and carbonate LPM blend was used in an invert emulsion fluid with density 10 to 11.3 lb/gal.

The 7,217-ft (2,201-m) interval was drilled successfully without major hole problems. Mud losses during drilling were reduced to about 400 bbl. During drilling, the bit was pulled out of the hole several times to check the bit and bottomhole assembly for signs of wear. Each time, the trip out of the hole and return in the hole were trouble-free. Mud properties and hole cleaning remained good during the entire drilling of the interval.