Can Lost Circulation Materials (LCM) Cure Losses Across Fault Damage Zone?
Authors: Chan A. W. , Murray D. , De Gennaro S. , O’Reilly G. 
Published: September 2023

ABSTRACT
Severe lost circulation events have been observed from various assets across our global portfolio while drilling and completing through fault damage zones in recent years. Common characteristics of these events include: (1) pressures at which losses occurred in the proximity of fault zones appear to be significantly lower than the estimated lost circulation threshold (or fracture gradient, FG) for tensile failure of the intact formation; (2) most of these lost circulation events are dynamic in nature; (3) the more severe loss events happened when crossing fault damage zones in the more permeable reservoir rocks; and (4) Loss Circulation Material (LCM) have been deployed in attempts to cure these losses with minimal success when drilling resumed. In this paper, we will present a few examples of our more severe fault-related lost circulation events from assets in the North Sea and the South China Sea where LCM had been used with various degrees of success in curing these losses. At first glance through the drilling reports, it appears that LCM deployments had only temporarily cured the losses and the improvements disappeared as soon as drilling resumed. However, closer examinations of real-time and memory pressure-time data reveal that several phenomena somewhat similar to our extensive wellbore strengthening experience from depleted drilling can be made. Our detailed investigations also illustrated some of the potential root causes behind the apparent failure in various LCM deployment techniques in curing losses associated with fault damage zones. Based on these new insights, a pragmatic LCM deployment strategy has been developed in combating future lost circulation events while drilling (and completing) through fault damage zones.

INTRODUCTION
Lost circulation events due to reactivation of fault damage zones have been observed and reported more frequently in recent years. Prior to these sudden loss events when the well intersected the fault damage zones, drilling through the intact formation is typically uneventful with steady drilling parameters and Equivalent Circulation Density (or ECD) well within the predicted lost circulation threshold of the intact formation (or Fracture Gradient, FG). As noted by Abd Rahim et al. (2019) and Brem et al. (2019), some of these sudden drops in ECD can be quite significant and the stabilizing pressure can be up to 20 to 30% below the FG of intact formation. While the losses can be quite severe (above 200 to 300 barrels per hour, bbl/hr), they are typically dynamic in nature (i.e., the well is static when pumps are off). Based on additional observations from our global portfolio, Chan et al. (2022) proposed an integrated framework to de-risk fluid loss potential for well planning and fluid design when penetrating faults is unavoidable (Figure 1). While the framework provides a key tool for planning and mitigates the potential impacts of faults presented to drilling and completion operation, the large subsurface uncertainties (e.g., presence of faults, geometry of faults, stress characterizations) can still result in a relatively large range in FG estimates related to faults. An efficient and robust recovery measure should still be in place. Lost circulation material (LCM) pills are typically used to treat losses and most pills are designed against slot tests in attempts to plug up fractures with a given aperture (typically 2000μm to 6000μm's). The design of these LCM pills focus primarily on their de-fluidizing nature, particle sizes and/or inclusion of bridging materials such as fibers. However, our global experience suggested that the results from application of various LCM pills against fault-related losses are mixed at best. In other words, losses appear to be cured but resumed as soon as drilling started. In this paper, we will present some of our key observations from a detailed review on the effectiveness of LCM treatments against fault-induced losses based on a few more severe lost circulation events in the North Sea and the South China Sea. These new insights provided the basis to update our LCM deployment strategy and also feedback to our de-risking framework via the dynamic considerations (figure 1) in planning and combating future lost events while drilling through fault damage zones.