Analysis of Drilling Fluid Circulating Pressure Loss in Hole Annulus for Microhole Drilling
The microhole drilling (MHD) technology is one of cutting-edge drilling technologies with wellbore diameter less than 88.9 mm and taking coiled tubing (CT) as drilling string to deliver the bottomhole assembly to drill ahead. The normal circulation of drilling fluid is affected because of huge annulus drilling fluid circulating pressure loss (ADFCPL) caused by small diameter wellbore, narrow hole annulus and deep well. For three flow regimes of power-law fluid in MHD, the computing models of ADFCPL are built by analyzing the relationships among annulus drilling fluid parameters according to power law fluid flow state equation. The ADFCPL with different flow rate, CT diameter, well depth, eccen- tricity and so on is calculated for the hole annulus of MHD. The calculation results show that: in 89mm diameter MHD, the ADFCPL is huge. It increases with the increase of outer diameter of CT and average flow rate of drilling fluid, and in- creases linearly with the increase of well depth, and decreases with the increase of eccentricity. The bigger the hole annu- lus is, and the lower the flow rate is, the less the impact of eccentricity acts on the ADFCPL is. Thus, the ADFCPL can be reduced by decreasing the outer diameter of CT and the average flow rate of drilling fluid in hole annulus. Only low flow rate can be used to drill deep well. The research results may be useful for MHD parameter selections related to the ADFCPL.
Analysis of the Critical Buckling Loads and Contact Loads on Coiled Tubing String in a Vertical Microhole
Microhole technology is a novel technology for wellbore diameters less than 88.9 mm and uses coiled tubing (CT) as the tubing string for downhole operations. The CT cannot rotate in the downhole due to the small diameter of the microhole, so that a sliding force is needed to trip the CT into the microhole. As a result, high friction is generated between the CT and the wellbore. Problems such as buckling and even lock-up occur under compressive loads, which severely impairs downhole operations of the CT in the microhole. In this work, a detailed analysis is made of CT operating conditions in a small-diameter vertical wellbore. The critical buckling loads and associated contact loads on CT strings of different diameters are calculated using well-developed models, and a study is made of the dependence of the critical buckling loads and corresponding contact loads on various parameters. The results show that the critical buckling load increases with CT diameter and that coil pitch width increases with CT diameter and a decrease in the annular clearance between the CT and the wellbore wall. The contact load between the CT and the wellbore wall increases with an increase in the axial compressive load and decreases with an increase in CT diameter. To reduce friction and contact loads and to trip the CT smoothly into vertical microholes, a larger-diameter CT and a smaller compressive load should be used while maintaining adequate annular clearance.
Correlations predict friction pressures of fracturing gels
Friction pressure calculations are very important in the design of any hydraulic fracturing treatment. They are useful in determining horsepower requirements, bottomhole treating pressure, and maximum wellhead pressure. This article presents a classical approach in which flow data of various gels in multiple pipes are presented as two dimensionless quantities, Fanning friction factor and generalized Reynolds number. These dimensionless quantities are then correlated as a function of power-law flow behavior index n'. Fluids with a wide range of generalized Reynolds number (N' /SUB Re/ = 2 X 10/sup 3/ to 5 X 10/sup 5/) and flow behavior index n' = 0.272 to 1.0) have been investigated. The correlations can be used to estimate friction pressures of both uncrosslinked and crosslinked gels. A method predicting friction pressures based on these correlations has already been implemented on a field transportable rheology system and good success has been reported. The friction pressures estimated from these correlations have been compared favorably with field data from several fracturing treatments. Effect of pipe roughness pressure of gelled fluid has also been examined. This testing indicates that pipe roughness does not affect pressure at high gel concentration. This contradicts previous thinking.