Impact of hole cleaning and drilling performance on the equivalent circulating density

Highlights

• New integrated model to predict the Equivalent Circulating Density was presented.

• The model presented allows to optimize drilling operation not only on the MSE only but also ECD and cuttings transportation.

• The wellbore geometry and mainly the drill pipe size has major effect on the hole cleaning and the ECD.

• Developed ECD model help engineers optimize drilling and injection rate to prevent wellbore instability and mud losses.

Abstract

Efficient hole cleaning in drilling operations is essential to ensure optimum penetration rates. This complex problem involves the simultaneous analysis of multiple parameters, including cuttings characteristics, fluid rheology, and annulus space geometry. The effect of the mud density increase due to the cuttings concentration, which itself is a function of the settling velocity and rate of penetration (ROP), must be considered for accurate calculations of the equivalent circulation density (ECD).

Mechanical Specific Energy (MSE) models have been widely used in bit selection, drilling efficiency quantification, drilling performance monitoring, drilling performance optimization, and ROP improvement. We attempted to employ MSE for optimized hole cleaning and controlled ECD. Cuttings concentration was integrated with the drilling MSE, which was calculated to determine the effect of different drilling parameters on hole cleaning and ECD. We proposed a new model for predicting the ECD in vertical and deviated wellbores that takes fluid and formation properties, as well as wellbore and drill string geometry and drilling operational parameters, into account. The model predicts the cuttings concentration and equivalent circulation density in vertical and deviated wells. The workflow implements the critical and settling velocity models, which aids in optimizing drilling performance and hole cleaning. The developed model was used to study the effect of different drilling parameters on ECD and help engineers optimize their operational parameters. Integrating the drilling operational parameters to provide controlling options to drillers as they monitor ECD values while maintaining safety and optimizing the drilling job is critically important.

Introduction

Weighting materials, such as barite, are used to increase the bottom hole pressure during drilling operations to prevent the influx of formation fluids into the wellbore. This density increase can cause mud circulation loss when drilling into unstable or naturally fractured formations unless the pressure is maintained below the formation fracture pressure, which introduces a range of appropriate mud densities used during the drilling operation, known as safe mud weight windows (Bourgoyne, 1991). The pressure required to circulate the mud during drilling operations and overcome pressure loss in the annulus is added into the hydrostatic pressure of the mud, leading to an increase in bottom hole pressure. The new circulating mud gradient is known as the equivalent circulating density (ECD). Most researchers only take the effect of pressure loss on the ECD into account; however, the impact of the cuttings concentration on the ECD, especially at low flow rates or high rates of penetration (ROP), needs to be studied.

Several peer-reviewed journal articles have reported on research involving models for cuttings transportation in the annular space, including settling velocity. These studies focused on experimental investigations (Agarwal and Chhabra, 2007; Ahmed, 2012; Badrouchi et al., 2020; Badrouchi and Rasouli, 2020; Baldino et al., 2015; Buscall et al., 1982; Chhabra et al., 1996; Faitli, 2017; Fidleris and Whitmore, 1961; Jacobs et al., 2015; Jayaweera and Mason, 1965; Johnsen, 2014; Kelessidis, 2003; Khatmullina and Isachenko, 2016; Nolan, 1970; Peden and Luo, 1987; Sharma and Chhabra, 1991; Wang et al., 2018; Xu et al., 2017) and numerical simulations (Badrouchi et al., 2020; Badrouchi and Rasouli, 2020; Blackery and Mitsoulis, 1997; Bush, 1994; Butcher and Jr, 1990; Dazhi and Tanner, 1985; Dhole and Chhabra, 2006; Gavrilov et al., 2017; Ghosh and Stockie, 2015; Gumulya et al., 2014; Missirlis et al., 2001; Prashant and Derksen, 2011; Trofa et al., 2015; Wachs and Frigaard, 2016; Zaidi et al., 2015).

Accurate ECD determinations require appropriate wellbore system modeling and understanding how the various sources of pressure increase in the annulus (Xiang et al., 2012). Attempts have been made to model ECD based on pressure loss; however, they neglect the cuttings effect (Al-Hameedi et al., 2019; Elzenary et al., 2018; Han et al., 2019; Kulkarni et al., 2014; Vajargah et al., 2014). Other researchers have attempted to include the effect of cuttings on ECD (Abdelgawad et al., 2019; Kerunwa, 2020; Xiang et al., 2012); however, most of these studies are limited by the range of particles and drilling fluids considered in their models. There has not been any work reported that relates the ECD to the fluid and formation properties as well as drilling operational parameters to the best of the author's knowledge.

Xiang et al. (2012) studied the effect of cuttings concentration on ECD while drilling. The authors performed a comparison between the ECD predicted with and without cuttings concentration effect and concluded that the cuttings concentration affects the ECD in the annulus. Controlling the cuttings concentration is vital to avoid losses, pipe sticking, and other problems that cause non-productive times (NPT); however, the model they used to predict the cuttings concentration was limited to a fluid model. Kerunwa (2020) reported that ECD would increase with increasing ROP and decrease with increasing mud flow rate. The authors mentioned that a high mud flow rate decreases the effect of the cuttings in the mud. Effective mud weight due to cuttings decreases with increased mud flow rate; however, high mud flow rates cause an increase in pressure loss, leading to higher ECD.

Maximizing the ROP to reduce drilling costs in oil and gas development is the primary objective for drilling researchers (Chen et al., 2014a, b, 2016a, 2016b, 2018; Chen and Gao, 2017). Many parameters increase ROP, especially rotary speed (RPM) (Chen et al., 2018); however, increased RPM may cause extra frictional pressure loss.

The process of optimizing drilling parameters, such as ROP, should be drilling system-specific and formation-specific (Chen et al., 2018). Mechanical Specific Energy (MSE) is defined as the mechanical work necessary to excavate a unit volume of rock, which could create an objective assessment of drilling efficiency. Teale (1965) presented the first MSE model for rotary drilling systems. His model, which was based on surface drilling parameter measurements, led to significant sources of error due to indirect bottom hole measurements at the surface. Numerous researchers have since attempted to develop more accurate models based on bottom hole data from logging while drilling (LWD) and more accurate measurements of WOB and torque (Armenta, 2008; Chen et al., 2018; Dupriest and Koederitz, 2005; Hammoutene and Bits, 2012; Mohan et al., 2014; 2009; Pessier and Fear, 1992).

MSE models have been widely used for bit selection, drilling efficiency quantification, drilling performance monitoring, drilling performance optimization, and ROP improvement. We have attempted to employ MSE for optimizing hole cleaning and controlling ECD. No comprehensive study is available in the literature concerning the effect of different drilling parameters on ECD, to the best of the author's knowledge; therefore, we present a new model for predicting the ECD in vertical and deviated wellbores that takes 1) fluid properties, 2) formation properties, 3) wellbore and drill string structure, and 4) drilling operational parameters into account. The developed model was used to study the effect of different drilling parameters on ECD and help engineers optimize operational parameters.

Integrating the drilling operational parameters to provide controlling options to drillers as they monitor ECD values while maintaining safety and optimizing the drilling job is critically important.

The objectives of this work are:

• Presenting an integrated workflow to predict hole cleaning efficiency in vertical and slightly deviated wellbores.

• Accounting for the effects of drilling parameters, such as WOB, RPM, ROP, T, and bit efficiency, on the hole cleaning process.

• Accurately predicting ECD to maintain the drilling mud weight window within a safe range to prevent breakouts and losses during the drilling operation.

• Providing a quantitative understanding of the effects of different parameters, such as drilling, fluid, and rock properties, for better field operational conditions and flexibility.