Development and Field Application of PDM with Double Bent Housing for Sidetracking Medium-Short Radius Horizontal Wells
Authors: Su Yinao , Cheng Zuxi , Tang Xueping , Wang Xinmin 
Published: December 2007

Abstract
Sidetracking through casing window is an effective measure for developing old oil fields. Medium-short radius, as known as intermediate, horizontal well is required to produce the residual oil in the original well network for short landing point displacement.
5LZ95×7.0TW double bent housing PDM with build-up rate between 30°/30m and 45°/30m has been developed to meet the concrete requirement of sidetracking through window milled in 7" casing in Liaohe oilfield. The design physical parameters of the PDM are determined, and double bent housing structure is adopted through analyzing its passing capability through casing, the performance of BHA and predicting the build-up rate. 5LZ95×7.0TW PDM is easy to manufacture, repair and maintain, and the drilling string is easy to run through casing, and its build-up rate can be predicted accurately.
The field application of the PDM in two wells of Liaohe oilfield demonstrated that it fully reached its design purpose. It is superior to single bent housing PDM jointed with a bent sub, and is welcome to users.

Introduction
Horizontal wells can be divided into four basic types: long radius(K&lt; 8°/30m), medium radius (K=8°~20°/30m), medium-short radius(K=0.7°~3°/m), short radius (K=3°~10°/m) in terms of the curvature of their wellbore trajectories. Medium-short radius horizontal well has advantages of short control section, small TVD error, ease of hitting thin oil layer, and is often used to sidetrack old wells. Medium-short radius horizontal well sidetracking technology is an effective approach to reducing cost and raising benefit by sidetracking horizontal wells to exploit the residual oil and improve the ultimate recovery.
PDM for medium-short radius well is different from that for long, medium radius well in its high bent angle or double bent housing causing high build-up rate(0.7°~3°/m), and its small size(F120.65~88.9mm)i.e. low rigidity while used in drilling old wells. One of the crucial parts to develop medium-short radius (curvature radius=82~20m) horizontal well sidetracking technology is to design and select small size bent housing PDM to meet the requirement of build-up rate. It is very important to design such PDM that can meet the features of sidetracking medium-short radius horizontal well and requirement of its build-up rate, and is easy to run into casing, use, repair and maintain.
Based on the equivalent relation of PDM structure and theoretical foundation of mechanical analysis, prediction and sensitivity analysis of build-up rate, and the passing capability analysis of PDM through casing [1], a 5LZ95×7.0 double bent housing PDM which meets the requirements of field application has been designed and manufactured.
Equivalent Relation of Structure of Double Bent Housing PDM and Single Bent Housing PDM
Single bent housing PDM and double bent housing PDM are often used in horizontal well drilling. Through analyzing the equivalent relation between them, it's found that a geometrical equivalent relation exists between them (Fig. 1).