Development and diversification of directional drilling tools continues, as do the applications in which they operate. As a result, there is continual demand for development and refinement of drill bits and string tools to meet the latest challenges and issues faced. One such challenge is the ability to reliably deliver consistent directional performance in very soft formation applications. Issues such as hole washout, inappropriate drilling parameters, stabilization, and hole quality can all contribute to poor directional drilling performance.
Several commercial projects are reviewed where technical merit has justified use of Rotary Steerable Systems (RSS), but due to the very soft lithologies, these systems have been unable to deliver the required directional control. In each example, a separate engineered solution is introduced. These include:An innovative concentric string reamer, featuring a mid-reamer section that enables effective stabilization of the reamer even if the pilot borehole is of poor quality or is over gaugeA soft formation fixed cutter drill bit design with an engineered hydraulic configuration to avoid hole washout and extended circumferential gauge geometryA near-bit stabilizer incorporating a full ring gauge that delivers 360 degree circumferential coverage, thus providing greater contact with the wellbore and higher potential for deviationA specific Bi-center design that utilizes a secondary component configuration on the face of the pilot to enable appropriate drilling parameters to be utilized for efficient directional control. The gauge geometry and profile is also tailored to suit soft formation drilling.
Global case studies document where these solutions, in combination with both Push and Point RSS, have proven extremely successful. These have provided greater flexibility with regard to tool selection, well planning options, and delivering lower cost per foot in RS projects.
Introduction
Approximately 60 to 80%1 of the formations drilled in the oil and gas industry are shale or shaly type formations which are mechanically weak and easily penetrated. These present multiple drilling challenges.
From a drill bit perspective, designs are developed in order to overcome the potential of bit balling, a scenario where formation cuttings pack together and stick to the bit face, resulting in reduced drilling efficiency. Solutions may include optimizing the open face volume, though potentially at the cost of reduced durability of the designs. Generally, steel bodied bits are considered optimal in this environment as they typically have higher blade standoff and thinner blades when compared to that of a matrix bodied design. This affords them an increased open face volume and larger junk slot area. Matrix bodied bits are composed of an infiltrated matrix of tungsten carbide powder, offering excellent surface hardness properties with an exceptional ability to resist fluid erosion, significantly higher than their steel bodied counterparts. This is beneficial as flow rates utilized in soft formation tend to be high in order to maximize hole cleaning, potentially leading to body erosion and reduced bit life. However, tungsten carbide matrix is less strong than steel and this means that matrix bodied bits generally have wider blades and reduced standoff, limiting the open face volume that can be achieved.
