Hole Cleaning in Full-Scale Inclined Wellbores

Abstract

Summary A 4-year multifactor experimental study was completed recently on hole cleaning in inclined wellbores. Ten variables-annular mud velocity, mud density, mud rheology, mud type (oil- or water-based), cuttings size, rate of penetration (ROP), drillpipe rotary speed, drillpipe eccentricity, drillpipe diameter, and hole angle-were studied over a range of values. The full-scale annular test section was 60 ft [18.3 m] long, with 3- and 4.5-in. [76- and 114-mm] drillpipe in an 8-in. [203-mm] ID (wellbore diameter). Hole angle varied from 45 to 90' from vertical. Results frequently showed a large buildup of cuttings in the annulus, and cuttings were difficult to remove once a bed had built up on the low side of the wellbore. Cuttings-bed sliding also was more intense than anticipated. Mud velocity and mud density have the greatest effect on hole cleaning. Several two- and three-factor variable interactions also emerged from a subsequent statistical analysis. Introduction Economic benefits of drilling nonvertical boreholes are firmly established in field practice. Such drilling problems as reductions in cuttings transport performance, however, are aggravated as hole inclination increases. For good hole cleaning, vertical flow is advantageous because cuttings fall in a direction opposite that of the drilling-mud flow. For an inclined well, the direction of cuttings settling is still vertical, but the fluid velocity has a reduced vertical component. This decreases the mud's capability to suspend drilled cuttings and results in faster particle willing velocities at greater hole inclinations. Particle trajectory (influenced by axial fluid movement and downward particle movement) is such that particles that slip through the fluid have little distance to particles that slip through the fluid have little distance to travel before striking the borehole wall. Local fluid velocities near the well are small, which reduces further particle movement. Particle dwell time is increased in the annular space, hence Particle dwell time is increased in the annular space, hence increasing the net volume of cuttings in the wellbore. For near-horizontal wells, particle dwell times usually are long enough to allow formation of contiguous cuttings beds. Fig. 1 illustrates cuttings behavior in inclined holes. Cuttings beds impede drillpipe movement into or out of a wellbore, and often the drillpipe gets stuck. In any event, cuttings beds increase nondribing rig time and costs. This hole-cleaning research identified how drilling parameters affect cuttings accumulation and bed formation so that controllable parameters may be adjusted for minimal cuttings buildup. Previous Investigations Previous Investigations Much of the early work on hole cleaning for vertical wells was summarized previously. I Batch (or unsteady-state) cases, where a set amount of cuttings is recovered per unit time, predominated during initial testing from the early 1940's to the early 1970's. Tests during 1972-797-12 used a constant (steady-state) cuttings feed rate. Hole-cleaning testing in inclined wellbores started about 1979. Much of the experimental testing was done at the U. of Tulsa. Data were still somewhat limited when Mobil's study began in 1984. Theoretical studies involved the calculation of slip (settling) velocities, 17-19 modeling flow in an annulus, 20-23 and modeling cuttings transport in an annulus. 24-26 Four recent papers 27-30 discussed hole cleaning in high-angle wellbores. Experimental Test Design The 10 factors in Table 1 were thought to affect hole cleaning and were investigated in three phases during this study. Results from 184 tests are discussed. Phase I examined effects of mud annular velocity, hole angle, mud yield point, and mud type. Phase 2 studied these and drillpipe rotary speed, annular eccentricity, and mud density. Phase 3 testing included drilling size, cuttings-feed concentration (simulating ROP), and drillpipe diameter. Experiments in Phases 1 through 3 followed a factorial design plan intended to identify interactions among test variables as well as their individual effects on cuttings-bed size during drilling. Table 2 shows the rheological character of the muds used. Although attention was given to mud yield point in these studies, the muds were designed with a strong correlation between yield point and low shear-rate values. point and low shear-rate values.