Laboratory Evaluation of Sealing Wide Fractures Using Conventional Lost Circulation Materials
Authors: Alsaba Mortadha , Nygaard Runar , Saasen Arild , Nes Olav-Magnar 
Published: October 2014

Abstract
Preventing or mitigating fluid losses caused by the presence of natural fractures is a major challenge faced by operators when drilling exploratory wells in the Barents Sea. Natural fractures widths vary significantly which makes it difficult to design treatments using conventional lost circulation materials (LCM's). Different solutions have been used to overcome this problem, such as chemical sealants, hydratable LCM's pills, rigid-plugs and cement. This study intends to expand the usage of conventional LCM's in sealing wide fractures.
Experimental evaluation of LCM's is a crucial step prior to field application. Currently, particle plugging apparatus is often used to evaluate LCM's performance using straight slots or tapered slots by applying a constant pressure and measuring the fluid loss. However, fluid loss values are not a good measure of the formed seal integrity. Therefore, another means of evaluating LCM's performances is required. In this work LCM sealing efficiency was defined as the seal/bridge maximum breakdown pressure. The tests were conducted on a fit-for-purpose apparatus designed to evaluate the performance of LCM's by measuring the sealing efficiency under high pressures (10,000 psi) and temperatures.
This paper presents an extensive laboratory evaluation to investigate the feasibility of sealing wide fractures using conventional LCM's. The ability of different LCM's in sealing fractures was evaluated using a set of tapered slot sizes up to 2000 microns fracture width. Four different types of conventional LCM with 13 different particle sizes (D50) and one new foam wedge based LCM were investigated. A total of 200 tests were run to investigate the effect of different parameters on the overall performance of both conventional and unconventional LCM. These parameters include LCM's type, concentration, tapered slot size, particle size distribution (PSD), temperature, and injection rate. Ground walnut shell showed a superior performance in sealing large fractures (sealing pressure up to 2200 psi) when used as a concentrated pill. Microscopic studies revealed that this performance is due to the ability of these materials to deform and plug fractures under elevated pressure. However, selecting the right LCM concentration is critical to avoid screen out at the fracture openings. Unconventional LCM used in this study was able to seal fractures up to 5 mm where conventional LCM failed to seal. Laboratory procedures to evaluate high fluid loss squeezes by means of hesitation squeeze were developed and recommended injection rates were determined.
This research highlights the different factors affecting LCM sealing efficiency and seal integrity. A full understanding of the factors contributing to successful treatments for sealing fractures is very important. Appropriate laboratory evaluation of LCM treatments is an important step prior to field application. The authors believe that a better understanding of these factors will improve the way LCM treatments are designed.