Experimental Study of Slimhole Annular Pressure Loss and Its Field Applications
Authors: Wang Haige , Su Yinao , Bai Yangmin , Gao Zhenguo , Zhang Fengmin 
Published: December 2007

Abstract
Large difference exists between slimhole annular hydraulics and conventional well's. It is one of the key factors which hinders a wider application of slim hole drilling. The paper has studied experimentally the slim hole annular pressure loss(APL) at the slim hole annulus flow loop test apparatus. Various parameters such as drill pipe rotation speed, the eccentricity between pipe and annulus, and mud properties are investigated. Rules of APL changing with pipe rotation speed and pipe eccentricity are obtained.
Test results show that when pipe rotation speed is lower, the APL decreases slightly with it. While as rotation speed greater than 70 rpm, APL increases fast. the APL decreases quickly with increasing eccentricity. With the annular gap decreases, APL changes dramatically, and becomes more sensitive to pipe rotation speed. Through the analysis of data collected in the test, the regressive models for pipe rotation and pipe eccentricity imposed on APL have been worked out.
Contrary to conventional drilling, in slim hole drilling, the annular mud flow regime is not only relied on Reynolds number, but also on Taylor number. The empirical equation is presented to identify the flow regime.
For the slim hole size of 6 inch, with pipe rotation speed being 140 rpm, and mud flow rate of 16.8 l/s, the ratio of pressure dropped in annulus to that in drill pipe lies between 25% to 30%, which is much higher than that in conventional wells.
On-site data of Miao 1–40 well in Jilin Oil Field are gathered and used to evaluate the valueness of the APL models developed in this paper. Moreever, these models have been used to Miao 5–40 well of Jilin Oil Field. The difference between the calculated pressure drop and measured pressure drop is less than 6%.

Introduction
Since the downturn in oil and gas industry in 1985–1986, the need for reduced exploration and production costs has been emphasized. Indentifing the advantages of drilling holes with reduced diameters in terms of costs and environmental impact. Many oil and gas companies, drilling contractors and service companies in the world have investigated slim hole technology for exploration drilling and development drilling. While slim hole drilling can be dated back to 1920s. In the late 1950s and 1960s, over 3000 slim holes were drilled in oil and gas field worldwide. Although significant cost savings were achieved, large holes became common in the 1970s and 1980s due to:short life of small diameter roller bits;reduced rate of penetration;high oil prices;the misconception of large holes heading everywhere.
However with the advance of new techniques and equipment, the positive results of slim hole drilling in terms of cost reduction, minimisation of the environmental impact of drilling activity, drilling efficiency and safety and quality of geological evaluation have been widely accepted. Slim Hole drilling has gained increasing interest in oil and gas industry since 1986.
Slim Hole drilling is different from conventional oil and gas well drilling because ofhigh rotation speed of drill pipe;the very slim annulus, the small annular gap.
These main differential characteristics make it absolutely essential the drilling hydraulics study, and make the slim hole hydraulics totally different from convential wells. In slim hole drilling, the "Couette effect" resulting from pipe higher rotation speed and "Crescent effect" due to pipe axis highly deviated from the axis of wellbore must be taken into account, and these two characterics which are of different points between slim hole and conventional wells governes the slim hole hydraulics calculation accurcy.
Early field pratice and labortary studies showed that in convential well drilling APL rarely exceeds 15% of the total pump pressure. However in a slim hole up to 30–50% pump pressure is due to pressure loss in the annulus, some people consider it is up to 90%. Therefore slim hole drilling hydraulics becomes one of the major concerns. This theoretical study and field application will allow a better understanding of phenomenor occuring in the annulus and thus an optimization of hydraulic parameters for a successful slim hole drilling.