Under-balance Drilling Technology of Slim Directional Well in Region Xiaoguai of Karamay Oilfield
Authors: Yang Long , Li Aijun , Cheng Xiaonian , Meng Yinfeng 
Published: December 2007

Abstract
The paper firstly describes reservoir geological and characteristics in the Xiaoguai of karamay oilfield. Through low-permeability, high angle fractured con-glomerate reservoir was analyzed, the necessity and availability of UBD of slim directional well must be discussed. Secondly, based on under-balanced drilling application in Xinjiang oilfield. The feasibility and howto operation Under-balanced drilling and completion techniques of slim directional well are summarizedand taken three slim(double-lateral, sidetracking) directional wells, especially well. GD1176 is the first double-lateral slim directional well as an example. They proved that using slim (multi-lateral) directional well to develop high angle fractured reservoirs could obtain remarkable results. The new technology would have widely applied.

Introduction
Region Xiaoguai has developed one of the karamay oil-field since the mid—-1990s. The early explored results show that pay- zone belongs to low permeable, higher angle fracturing sandstone formation. The pay-zone pressure coefficient is low, similar to fracturing formation loss pressure coefficient. The safety density window of drilling fluid is small. Lost circulation problems affected conventional vertical/directional drilling operations, but also damage formation reduced production in the vertical well drilled by over-balanced mud system. Lately, To improve well production rates, slim multi-lateral or sidetracking directional well have designed / developed. As they could drill across many higher angle fractures. The major purpose was to increase the con-tact area with reservoir. And under-Balanced Drilling (UBD) is gaining popularity as a significant and reliable technology of drilling loss pay-zone. since 1996, based on horizontal/ vertical UBD well successful experience in foreign oil -field, there have been 20's slim directional well that had drilled by UBD for low-pressure fractured reservoir in region Xiaoguai oilfield, including for G10–63, GD1179, GD1154, G2068, GD1176 etc. In these wells, It is mainly analyzed to how to operation U-BD program by taking GD1176——first double-lateral slim directional well as an important example. Through under-balanced drilling technology succeeded in region Xiaoguai, their result indicates that the follow UBD features:It allows for immediate discover, continuous evaluation and protect effectively oil / gas resources;Better control in cavernous and fracture areas, effective in drilling through loss-circulation zone ;Increased penetration rate;Minimum damage to liquid sensitive, solid plugging pay zone;Saving drilling cost and time.
Description of Geological & Characteristics
Region Xiaoguai lies to the south of karamy city, Xinjiang, P.R.C.. Its Xia Zijie (P2X1-4) reservoir is located between Hong rift zone and Xing rift zone. Belong to one-limbed structure. The formation angle 12·, Pay-zone's formation is made of sand and gravel. See Fig. 1, through recently exploration work done to it, it' s main characteristics:Permian reservoir has internal to low permeability, fractured conglomerate in region Xiaoguai, (Table 1).Its reservoir formation (P2X1-4) Pressure coefficient is low, and is similar to the fractured formation loss pressure coefficient. The safety density window of drilling fluid is less than the loss pressure (Table 2).Fig. 2 displays that well.G037 (3017---3536m) Scheunberger log FMI image. Early statistics analysis result: Xiaoguai P2X1-4 reservoir fractures are rich, the natural maximum fracture length is 12 meters, maximum width is 10 cm. The reservoir fracture belongs to vertical/ hig-her angle. They are one of main oil flow paths in producing well, also are one of main mud loss way.