Optimum particle size distribution design for lost circulation control and wellbore strengthening
Introduction
WBS is an effective technique to help negotiate challenging wells with narrow drilling margin (van Oort and Razavi, 2014). It can be defined as the extension of the drilling margin through enhancement of the fracture pressure. Fracture pressure enhancement is usually achieved by plugging the fractures (whether these are drilling-induced or natural) existing in the proximity of the borehole. The plugging solids used for WBS are generally known as LCM in the drilling industry. Since the introduction of the LCM to the industry, numerous experimental investigations have been conducted to understand the true underlying mechanics of fracture bridging which occurs due to the presence of LCMs in drilling fluids (e.g., Drilling Engineering Association (DEA) 13 (1985 and DEA 13 (Phase II) Final Report, 1988, Dudley et al., 2000, Guo et al., 2009, Guo et al., 2014). Building on the results of these experimental investigation, several theoretical studies were conducted to determine the fracture pressure of boreholes with various formation types, inclination angle, and in-situ stress values (e.g., Morita et al., 1990, Chen et al., 2015, Dokhani et al., 2014, Dokhani et al., 2015, Mehrabian et al., 2015).
Several design guidelines were introduced to determine the optimum PSD, concentration, type, and shape of the LCMs required for WBS applications. Abrams (1977) pioneered the work on the design of bridging solids. His work led to the introduction of the well-known “one-third rule”, aka the Abrams' rule. The one-third rule recommends the following guidelines for the size and concentration of bridging materials:
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The median particle size of the bridging additive should be equal to - or slightly greater than - one-third the median pore size of the formation.
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The concentration of the bridging size solids must be at least 5 percent by volume of the solids in the final mud mix.
Abrams' work was primarily aimed at reduction of formation damage due to reservoir impairment. However, the one-third rule can be applied to determine the size of bridging solids used for various particle plugging applications, including WBS. Building on Abrams' work, Vickers et al. (2006) employed the Pore Plugging Apparatus (PPA) and return permeability testing to minimize fluid loss. This work resulted in the introduction of the “Vickers criteria”, which prescribes the following standards for the PSD of the bridging LCM blends:
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D90 = largest pore throat
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D75 < 2/3 pore throat
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D50 = 1/3 of the mean pore throat
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D25 = 1/7 of the mean pore throat
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D10 > smallest pore throat
In addition, the authors recommended that the concentration of bridging material needs to be greater than 30 pounds per barrel (ppb) for water based mud (WBM) (this may be reduced for oil-based mud). This concentration, however, is lower than the 5 percent solid volume recommended by the Abrams' rule.
Fuh et al. (1993) patented a method for inhibiting the initiation and propagation of fractures by using LCM of a specific size. Their method was the result of the experiments conducted at the DEA 13 investigations (1985 and 1988) and several field applications, which employed the LCMs for WBS purposes. The patent prescribes adding 30–50 ppb of LCM with a critical size ranging from 250 to 600 μm to the drilling fluid. The preferred LCM types are nut shells or calcined petroleum coke.
Dick et al. (2000) conducted another major effort for the selection of bridging particles by adopting the “ideal packing theory” from the paint industry to practical oilfield use. Originally, the ideal packing theory was introduced by Andreasen and Andersen (1930) who proposed a power law relationship between the Cumulative Volume, CV, and the particle size, d, (CV∝dx) for effective bridging. The exponent value (x) typically ranges between 0.5 and 1. Kaeuffer (1973) states that ideal packing occurs when the CV varies linearly with the square root of the particle size ( ). More recently, Chellappah and Aston (2012) improved upon this power law model by employing particle plugging apparatus (PPA) testing and suggesting that the optimum value of exponent (x) is closer to 1 than to 0.5.
Although LCMs have become a standard part of fluid design for drilling formations with a narrow drilling margin, the industry still lacks a comprehensive framework to optimally select LCMs for WBS applications. Confusions persist on the underlying mechanics of fracture sealing and the true location of the seal formation along the fracture length. Very few in-depth experimental studies have been carried out to evaluate the validity of the proposed mechanisms. Furthermore, the above-mentioned guidelines for LCM concentration and PSD have not been examined independently in realistic fracturing experiments.
In this paper, we apply an experimental approach to study the fracture plugging during the WBS phenomenon. In section 2, we briefly describe the experimental set up, the tested fluid system and rock samples, and the testing procedure. In section 3, we present the results of parametric studies using synthetic-based fluids loaded with graphite- and Gilsonite-based LCMs. Post-fracturing analyses such as thin-section and CAT scanning imaging were conducted to study the geometry and structure of formed plugs on the fracture surface. In addition, the existing models to design the bridging blends are evaluated based on the conducted WBS experiments and post fracturing analyses. Finally, we propose a novel method to determine the optimal LCM PSD which maximizes the strengthening benefits. In section 4, we list a summary of our finding and conclusions.
Section snippets
Experimental set up: the UT MudFrac system
A state-of-the-art experimental set up was designed and manufactured for in-depth WBS investigations. The UT MudFrac hydraulic fracturing system (Fig. 1a–b) is a dual flow-loop and pressure-intensifying system which tests 4 inches diameter x 6 inches length cylindrical rock samples. A 9/16 inch borehole is drilled and flow lines are inserted 2.5 inches into each end of the sample, leaving 1 inch of the rock surface for fracture initiation and propagation. The flow lines are epoxied to the rock
Effect of LCM PSD
PSD is one of the major parameters affecting the sealing capabilities of an LCM blend. To investigate the effect of PSD on WBS, three different grades of graphite-based LCMs were tested. Unimodal fine and coarse grades were tested individually. They were also mixed (with a ratio of 10 to 7 for coarse and fine grades, respectively) to form a bimodal “medium” grade, which was also tested. Fig. 4 presents the PSD and cumulative PSD curves of the tested LCM grades. Particle size parameters such d10
Conclusions
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Optimum PSD is critical to maximize the strengthening benefits. Further, the optimum PSD appears to be largely independent of the type of the used LCM.
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Fracture sealing occurs along the fracture length and typically near the fracture tip, not at the borehole face. This observation supports the fracture propagation resistance model, and contradicts near-borehole WBS models, such as stress caging.
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A bimodal PSD has a clear strengthening advantage over a unimodal PSD. The reason seems to be related
Acknowledgements
The UT MudFrac equipment and the experiments done with it would not have been possible without an enabling donation by Schlumberger and the active involvement of Susan Rosenbaum and Jim Friedheim, to whom we owe a great deal of gratitude. The authors would like to sincerely thank ConocoPhillips and Schlumberger as the lead sponsors of this R&D project. Our very special thanks for their guidance and support go out to Dave Beardmore, Greg Mullen, Bob Pantermuehl, Gary Collins, Bret Borland, Ernie
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2023, PetroleumCitation Excerpt :The fracture width is a crucial parameter for optimal selection of LCM particle size distribution. Rapid loss control cannot be achieved without optimal LCM particle size selection [12–15]. Although the fracture width can be approximated analytically or numerically on the basis of the mud loss rate data but reliable prediction of this parameter is challenging as it is affected by a versatile set of different interconnected variables [3].