Exploratory and appraisal drilling in extensive salt intervals poses a severe challenge in terms of nonproductive time (NPT) and associated costs. This paper introduces the concept of the mechanical specific energy (MSE) index for creep control in real time. This index is fundamental to leveraging an understanding of MSE behavior while drilling and managing an effective equivalent circulating density (ECD) plan.
An ECD management plan must be restricted to a salt creep operational window (SCOW) between a lower limit and a higher ECD limit. The higher limit is based on a leakoff test (LOT) or formation integrity test (FIT). The challenge in defining a SCOW is therefore assessing the lower limit because its variations with depth determine the minimum ECD. A retrospective analysis of a Brazilian presalt exploration campaign determined that, while drilling in salt layers, a direct relationship exists between creep effects (caused by insufficient ECD to stabilize the wellbore) and the behavior of applied MSE (MSEa) with depth. This relationship was analyzed to create an MSE index as a probability vs. depth matrix to help estimate the lower ECD limit during predrill planning and to make necessary adjustments to operational parameters while drilling.
This paper presents two salt creep control cases in the Brazilian pre-salt.
In the first case, drilling occurred in an extremely soluble salt environment with a high tendency to creep and cause stuck pipe. Geomechanical analysis in real time helped to control creep in the entire drilled salt interval. This analysis resulted in exceptional performance in a critically important section by eliminating five of the ten planned operational days and managing ECD within the predicted SCOW by a good safety margin.
Conversely, situations occur in both thin post-salt depositional environments and large shallow and evaporitic sections in which the SCOW does not permit creep control with a good safety margin. The second case presents specific solutions for this complex scenario, which is characterized by low LOTs/FITs, causing lost circulation and stuck pipe. In response, the authors propose that the industry adopt a new concept, the MSE index, to support the SCOW design in predrill planning and real-time operations.
The aim of this paper is to help reduce operational days, drilling risks, and costs in drilling environments with complex geopressure variations and narrow pore pressure windows by providing techniques to maintain pressure control, reduce NPT, decrease the potential for mud losses, and reduce stuck pipe events.