Slimhole Lateral Well Drilling Across Faults from 4-1/2" Cased Producers in the Denver-Julesburg Basin, Colorado
Authors: Boone Lynn E. , Clausen Frederick J. , Birmingham Thomas J. , Schappert Neil 
Published: December 2007

Abstract
The first slimhole lateral drilling from 4–1/2" cased wells in the Denver-Julesburg Basin was performed by HS Resources, Inc. in 1996. This project also included the first reported lateral drilling in Colorado using coiled tubing and the first reported lateral cementing operations of 2–3/8" and 2–7/8" liners in the Rocky Mountains. The objectives of the project were three-fold: develop reserves with minimum surface impact and operating costs by utilizing existing locations, production facilities and gathering systems and, exploit additional tight gas sand reserves in the Codell formation by drilling across faults into undrained areas. The following factors were key to the success of these slimhole lateral wells:an accurate interpretation of fault location,minimizing the amount of fractured interval encountered in the overlying Pierre Shale in which the casing window is located,maintaining dogleg control to allow for cased hole tool access,using slimhole mud motors with maximum durability and,using a stable hydraulic fracturing fluid.

Introduction
The Codell Sandstone (the primary objective of the slimhole lateral drilling project) in the Denver-Julesburg (D-J) Basin, Wattenberg Field currently produces from an estimated 5,401 wells (Fig. 1). The formation ranges in gross thickness from 10 to 35 feet and occurs between 6800 and 7700 feet in depth. The Codell sandstone was deposited as a shallow marine shelf and shoreline sand, under low sea level conditions. The Codell contains as much as 30% clay by volume which substantially decreases the in-situ permeability of the reservoir. The effective permeability measured by pressure transient analysis is typically less than 1 md. Porosity in the reservoir ranges from 8% to 22%. HS Resources, Inc. considers the Codell reservoir fluid to be a gas-condensate with lower GOR portions of the field located farthest from the basin axis. It is necessary to hydraulically fracture the Codell in order to produce at economic rates.
Several areas throughout the D-J Basin, and specifically in the Wattenberg Field, contain regions which are intensely faulted and fractured. Although some fracturing has been observed near regions of faulting within the Codell interval, the listric faulting style (normal; high angle progressing to low angle with increasing depth) that is present can both improve the chances of natural fracturing which enhances productivity or eliminates the reservoir by vertical block movements. Therefore, it is necessary to both avoid faulting to preserve as much of the reservoir as possible, but also to place the producing interval in close proximity to faulting to enhance the chances for a naturally fractured Codell section.
Spacing for development is one well per 80 acres with the option for a second well (effectively one well per 40 acres). The Niobrara formation is typically included in the completion of the Codell or is added as a recompletion zone. The Codell and Niobrara formations in the Wattenberg Field are approaching complete development under current spacing orders. Additional potential remains in areas isolated by faults. Two factors heavily influence the development of the reserves remaining in the Codell; 1) the relatively low margin of profitability and, 2) the surface impact of additional development in an active agricultural area. These factors can be addressed by utilizing existing wellbores thereby reducing operating costs and minimizing surface impact. However, the following four conditions must be considered in planning the use of existing wellbores:the size of the casing (4–1/2") and therefore, size, availability and durability of slimhole drilling and completion equipment,the requirement for high pressure stimulation (between 5000 and 6000 psi),the effect of the liner and hole sizes on cementing, completion and recompletion operations and,the fluid stability required for slow rate hydraulic stimulation.
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