First Openhole Sidetrack in Deep Horizontal Well Saves Time and Lowers Cost: A Case Study
Authors: Dang Lurui , Peng Ping , Zhou Guangxian , Chen Zhenyu , Phan Chinh Van , Walker John , Zhao Jingkai , Zhao Zhiguo , Liu Guoyu 
Published: May 2016

Abstract
Recently, the first successful openhole sidetrack without a cement plug in Chong Qing field (central China) was drilled in a 5 7/8-in. horizontal section. Since 2009, approximately 50 horizontal wells using an LWD tool for well placement have been drilled in the field. Most of the wells are deep and geologically challenging, with uncertainty of formation dip and potential faults along the long lateral section. As a result, with only the near-wellbore formation measurements to use during drilling, it has been common to unintentionally drill out of the target zone during the well placement.
There have been approximately 10 sidetrack wells in the field. Commonly used techniques for sidetracks in an open hole include time drilling without a cement plug, sidetracking from a cement plug, and sidetracking from a whipstock. In this field, the operator wanted to avoid using whipstocks. Openhole sidetracking without cement plugs using basic time drilling has had a low success rate. Therefore, sidetracking from a cement plug was the operator's first option. However, the time to set the cement plug in these deep horizontal wells averaged approximately 12 days, including a bottomhole assembly (BHA) trip. It was therefore necessary to re-evaluate openhole sidetracks without a cement plug to save overall time and cost.
Two previous failed openhole sidetracks without cement plugs were analyzed. Key lessons were learned and critical factors were identified, which could lead to future success through the application of a more consistent drilling technique.
Once the feasibility of the method was established, the technology was successfully implemented to drill a sidetrack without a cement plug. The sidetrack was completed in only 4 days—at least 7 days faster than the openhole sidetracks with a cement plug in this field.
This sidetrack represents a step change in efficiency and a reduction is risk during openhole sidetracking in these deep horizontal wells in Chong Qing field. Application of the right techniques will save drilling costs and time, thereby offsetting the geologic challenges in this drilling environment.

Introduction
The Sichuan basin is located in central China (Fig. 1) and holds the biggest gas reserve in the country. PetroChina Southwest Oil & Gas Company (PetroChina SWOGC) is one of subsidiaries of PetroChina Company Limited and is the operator in the Sichuan basin. The eastern Sichuan gas operator (Chong Qing Gas) is one of the PetroChina SWOGC branches operating in the eastern part of Sichuan basin, located in Chong Qing area.
The main zones of interest range in age from Paleozoic to Mesozoic. The major economic reservoir for Chong Qing Gas was discovered and then developed in a thick Carboniferous dolomite.
This Carboniferous carbonate reservoir (referred to as C2hl) is primarily dolomite with some limestone. Average porosity of this section is only 2.90%. The C2hl target reservoir has very low permeability, from 0.05 to 10 md, with an average of only 0.05446 md. The reservoir is classified as a fracture-porosity type. Fractures developed within the thin dolomite target are expected to strongly enhance the reservoir quality and its production rate potential.
In this particular target, we do not see consistent layering within the reservoir. Instead of widespread layered dolomite, the sweet spot with the best reservoir properties is found to change laterally along the geosteered section. Reverse/thrust faults and high structural dip uncertainty are present in this major compressional tectonic area. Lateral property changes in the dolomite target reservoir and a stratigraphic thickness of the reservoir that varies from 6.0 to 20.2 m along the offset wells (Fig. 2) add to the difficulty.