Borehole instability related problems due to massive formation caving in micro-fractured shale resulted in substantial non-productive times (NPT) and accounted for as much as 25% of the total programmed drilling time.
Geomechanics and image log analyses clearly showed the natural/micro fractures were responsible for large cavings. Drilling optimization strategies to mitigate formation caving were focused on:
Reducing mud weight to decrease fluid/pore pressure penetration in fracture planes
Use of mud additives to seal-off natural fractures
Drilling with downhole motor and stabilizers to minimize lateral vibrations.
Average drilling time prior to implementing these strategies was 14.75 days. The drilling time improved to 8.18 days after the implementation, an improvement of 44.5% with no occurrence of major instability events.
This paper will present a case study from an oilfield in Southern Argentina demonstrating how the assimilation of geomechanics modeling and drilling optimization practices is critical to improving the drilling curves.
The Austral Basin, located in the southern tip of Argentina, is an area of active hydrocarbon exploration and exploitation. Basin development comprises both onshore and offshore acreages with some of the reservoirs straddling into the Strait of Magellan. The main reservoir is the Springhill sandstone formation (Figure 1) which, despite being continuously present along the entire basin, has variable thicknesses. This formation overlay the Tobifera series which is a Jurassic basement composed mainly of volcanic and volcano-clastic rocks. The Springhill sandstone is covered by a thick sequence of marine shales of Cretaceous age. The lithology of this Palermo Aike shale formation is mostly clay with a high presence of smectite and illite components.
The fields of interest are the Cerro Norte and Campo Molino (Figure 1). Typical wells in these fields are mostly vertical, with their intermediate sections drilled with a 12 ¼" roller-cone bit to a depth of about 300 m. The final TD section is drilled using either a 8 ½" or 8 ¾" PDC or roller-cone bit to about 1,900 m, and then cased with a 5 ½" casing. During drilling, large cavings occurred in the Palermo Aike formation between 1200 and 1500 m resulting in hole enlargements equivalent to twice the bit size. These cavings caused numerous borehole instabililty-related problems such as pack-off, over-pull, hard reaming, etc., and consequently increased the non-productive times (NPT). An analysis of the drilling time data of the 19 wells drilled over a two-year period (2003 - 2005) indicated that the averaged drilling time was about 75% higher than the programmed drilling time (Figure 2). This less-than-optimal drilling performance has a substantial cost implication.
Because the Cerro Norte and Campo Molino fields are in the development stage, optimizing drilling practices and reducing NPTs are extremely critical to project success. Numerous attempts were made to solve cavings problems including changes in drilling fluid formulations (shale stabilizing additive, potassium ion concentration, API fluid loss, rheology, and mud weight) and operational controls (rate of penetration, weight on bit, mud flow rate, and rotational speed). None of these efforts was able to solve the problems of borehole instability, and drilling operations were continuously being plagued by high NPTs.