Correlations of Mud Rheological Properties With Cuttings-Transport Performance in Directional Drilling

Abstract

Summary An experimental study comparing the effects of mud rheological parameters on annular drill-cuttings buildup was conducted in a large-scale flow section. Rheological parameters examined were mud yield point (YP); plastic viscosity (PV); YP/PV ratio; power-law exponent; consistency index; Fann™ V-G Meter dial readings at 600, 300, 200, 100, 6, and 3 rev/min; effective viscosity (calculated by the power-law derivation); and initial and 10-minute gel strengths. Fifteen bentonite/polymer water-based muds were used, ranging from water to a mud with YP and PV values of 20 lbf/100 ft2 and 40 cp[19.6 Pa and 0.04 Pa·s], respectively. For each mud, flow tests were run at annular velocities from 120 to 240 ft/min [37 to 73 m/min] and hole angles from 30 to 90° from vertical. The effective drilling rate for all tests was 50 ft/hr [15/h]. Altogether, 180 tests are analyzed. When data were correlated with annular cuttings volume in steady-state flow, the best fit was obtained with low-shear-rate parameters. These parameters include the 6- and 3-rev/min dial readings and initial gel strength. Correlation coefficients for these were often from 0.9 to 0.95. For hole angles 45° (from vertical), increasing values of these parameters improved cuttings removal (i.e., reduced cuttings volume). The same was observed at angles near horizontal, but only in the laminar flow regime. Cuttings volumes were always smaller in the turbulent regime during near-horizontal flow. Introduction Adequate cuttings removal from a well during rotary drilling is critical for cost-effective drilling. High annular cuttings buildup is often responsible for stuck drillpipe and other impediments to standard drilling and completion procedures. Increasing hole inclination from vertical aggravates the tendency for annular cuttings accumulation, intensifying the need for mud hydraulics aimed at acceptable hole cleaning. Many factors affect cuttings transport,1 but the options available for effective transport are few. Such influential parameters as mud density and hole angle are predetermined for reasons ranging from rock and fluid in-situ characteristics to overall well-planning strategy. Other variables, such as drill-cutting physical properties, are uncontrollable. The windows of opportunity for ensuring adequate hole cleaning during drilling lie, to a considerable extent, with mud flow rate and mud rheology. While mud rheology does not (during high-angle drilling) affect annular cuttings flow as strongly as such factors as mud flow rate, mud density, and hole angle,1–6 its effect is significant. Furthermore, its effect depends appreciably on interactions with mud annular velocity and hole angle.2 The complex viscous nature of drilling muds has led to many proposed means of varying sophistication to characterize mud rheology. In turn, questions are (or should be) raised concerning the appropriate rheological parameters best suited for particular hydrodynamic applications. This study entertains the applicability of various parameters to annular solid/liquid flow behavior, focusing specifically on cuttings removal. Our study had two main objectives: to evaluate in greater depth the effects of commonly measured mud rheological parameters for various combinations of mud velocity and hole angle and to identify those parameters that correlate most closely with cuttings-transport performance. Background and Experimental Approach This work is a continuation of cuttings-transport studies focusing on high-angle drilling.1–6 Tomren et al.,1 Tomren,3 and Iyoho4 showed the likelihood of large cuttings concentrations in inclined hole sections. The inclination angles were not always extreme; for unweighted muds at annular velocities near 120 ft/min [37 m/min], cuttings volumes at 30° were often three to four times greater than those at vertical. At angles near 45°, cuttings displayed potentially hazardous behavior, tending to form beds that slip downward during cuttings injection (i.e., drilling) and to avalanche toward bottomhole immediately after circulation stops. Attention later focused on individual variables. Becker's5 experiments showed that cuttings concentrations with muds weighted to 14 lbm/gal [1678 kg/m3] often were only about one-third the amount of accumulations with unweighted muds. For an annular mud velocity of 150 ft/min [46 m/min], concentrations were favorable (2 to 6 vol%) for holes from vertical to 45° when mud density exceeded 12 lbm/gal [1438 kg/m3]. Okrajni and Azar2 focused on mud YP, PV, and YP/PV ratio. One significant finding was the degree to which increasing YP/PV ratio augments the influence of yield value; another was how hole angle influences determination of the optimal flow regime. Testing confirmed that laminar flow is preferable for holes from vertical to 45° but that turbulent flow is recommended for inclination angles from 55° to horizontal. Because of the need for optimal rheology where mud density is limited, degree-of-linearity comparisons are presented to facilitate hole cleaning. Specifically, annular cuttings concentration (during simulation of drilling with unweighted muds) is correlated with YP, PV, YP/PV ratio, apparent viscosity, effective viscosity, power-law model consistency index, Fann viscometer dial readings (including 100, 6, and 3 rev/min), shear stress at the borehole wall, shear stress at average annular shear rate, and initial and 10-minute gel strengths. Fifteen bentonite/polymer water-based muds were tested, ranging from water to mud with a YP of 20 lbf/100 ft2 [9.6 Pa] and a PV of 40 cp [0.04 Pa·s]. Table 1 lists the test parameter values. Tests were conducted with a 40-ft [12-m] -long cuttings-transport test apparatus2 at the U. of Tulsa. The annular section had a 5-in. [12.7-cm] -ID outer pipe and a 2-in. [5.08-cm] -OD inner pipe rotating at 50 rev/min. The inner pipe was located halfway between the center and the outer-pipe wall. This test column can be oriented to accommodate mud and cuttings upward flow at any angle from vertical to horizontal. Angles selected for this study were 30, 45, and 70° from vertical. (Tests were also run at horizontal. Data trends were very similar to those for 70°; annular volumetric cuttings concentrations at horizontal were typically 1 to 2% higher than those at 70°.) Just before a test, the predetermined flow rate was established, after which cuttings (simulated by 1/4-in. [0.635-cm] ASTM limestone gravel) were injected at a rate simulating drilling at 50 ft/hr [15 m/h]. When the cuttings discharge rate from the test section became constant, cuttings injection was stopped, the cuttings were flushed from the test section and screened from the mud, and their volume was measured.