Wellbore-Strengthening Technique for Drilling Operations

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 105809, "Further Development, Field Testing, and Application of the Wellbore- Strengthening Technique for Drilling Operations," by Giin-Fa Fuh, SPE, and Dave Beardmore, SPE, ConocoPhillips Co., and Nobuo Morita, SPE, Waseda U., prepared for the 2007 SPE/IADC Drilling Conference, Amsterdam, 20-22 February. A significant increase in formation fracture resistance can be achieved as a result of a fracture sealing or plugging mechanism induced by a particle "screenout" effect resulting from the drilling fluid being loaded with an adequate amount of narrowly sized granular materials. Such an increase in formation fracture resistance is particularly valuable in helping to drill through depleted zones without losing fluid, strengthening the weaker formations that usually require additional casing strings for protection, avoiding lost circulation during cementing operations, and drilling high-angle well sections with high mud weights that normally would not be possible because of low formation fracture gradients. Introduction Lost circulation is perhaps the most costly mud-related drilling problem. Not only is rig time lost, but large volumes of expensive drilling fluids are lost to the formation. Lost circulation also can lead to severe well-control incidents. In response, several technologies referred to as "wellbore strengthening" (WS) are being developed to prevent or correct lost circulation. WS means that the breakdown pressure of the wellbore is greater after the treatment than before. These methods include use of special granular material, heating the wellbore, and use of rigid-plug-forming pills. The full-length paper focuses on a WS method that uses mud with special bridging and fluid-loss formulations. In the mid-1980s, a joint-industry project (JIP) was conducted to determine why oil-based mud apparently caused a lower fracture gradient than water-based mud (WBM). Data from this JIP were analyzed, and it was determined that they did not fit ordinary fracturing models of the time. These researchers noted that certain lost-circulation materials (LCMs) actually seemed to increase the pressure required for fracture propagation to more than what was seen with untreated drilling mud. They conducted laboratory tests outside the JIP to verify their observations. The result was the development of a loss-prevention material (LPM) that was found to inhibit the extension of a fracture tip. LPM was tested in the field several times and was quite successful. It was able to increase the wellbore break-down pressure in several cases, and subsequent application of the technology has allowed a casing string to be skipped in several cases. The technique affects only the near-wellbore break-down pressure, not the far-field fracture gradient.