Borehole Stability (Geomechanics) Modeling and Drilling Optimization Practices Improve Drilling Curves in Naturally Fractured Shale—A South Argentina Experience

Abstract

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 planesUse of mud additives to seal-off natural fracturesDrilling 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. INTRODUCTION 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.

Wellbore Pressure Management in Unstable Shales: It's Not Just About Rocks

“…When stability problems occurred in the wellbore, cavings from the fractured shales fall into the wellbore, and that mechanical action at the wellbore wall (drill pipe rotation, BHA design, drilling practices, etc.) can further destabilize weak shale (Fontana 2007).…”

Well of the Future: A New Approach to Real-Time Wellbore Stability Monitoring in the Colombian Foothills

“…Drilling records suggested that large boreholes and drilling problems had been encountered while drilling and tripping indistinctively with high and low mud weights. On the other hand, a number of wellbore stability case studies were reviewed, where the authors relied on anecdotal evidence to postulate failure mechanisms involving fracture rocks, and recommendations on how to mitigate the observed instability increasing and decreasing the mud weight (Santarelli et al, 1992;Last et al, 1995;McLellan and Cormier, 1996;Økland and Cook, 1998;Labenski et al, 2003;Edwards et al, 2004;Wilson et al, 2007;Gallant et al, 2007;Fontana et al, 2007). Some experts advise that increasing the mud weight is not effective for the Colombian Foothills, or even detrimental in these fractured zones; others suggest the instability could be cured by simply adding blocking solids or fracture sealing agents to the mud in order to delay fluid invasion and pressure diffusion into the unknown fracture network.…”

Modelo Geomecânico Aplicado À Análise De Estabilidade De Poços Com Ênfase Em Folhelhos

“…Figura 2.14 -Acompanhamento do perfil caliper para a avaliação da integridade das paredes dos poços (Modificada de Fontana et al, 2007). Araujo et al, (2009) Na Figura 2.16 a validação de um peso de fluido de perfuração para um novo poço do campo foi feita com o fim de reproduzir os problemas experimentados nas mesmas profundidades e litologias nos poços perfurados (Araujo et al, 2009).…”