A Novel Technology Of Formation Stimulation Involves Bi-Wellhead Horizontal Wells

Abstract

Abstract The Republic of Tatarstan possesses significant reserves of heavy oil and bitumen (more than 1.4 bln. tons), major portion of which are found in the Ufimian and Kazanian stages of Permian age. About 450 deposits have been discovered at rather shallow depths, 50 - 250 m subsurface. The first pilot project on heavy oil production through vertical wells dates back to 1970-ies. A number of thermal methods were tested including in-situ combustion, cyclic steam stimulation and gas-steam mixture injection. However, all these methods demonstrated their insufficiency. In 2006, a new pilot project was launched. This is a modified technology of steam stimulation through a pair of U-shaped horizontal wells. To date, three pairs of U-shaped horizontal wells have been drilled with the lengths of horizontal wellbores from 200 to 400 m. The wells were spudded vertically with a vertical drilling rig, hitting the surface at an angle, having thus two wellheads, vertical and slanted. The wells were completed from the surface: the casing with the sand screen was pulled from the slanted wellhead for the entire length of the wellbore. Two parallel horizontal wells were drilled through the producing formation, one above the other, at a vertical distance of 5 m. The bi-wellhead design contributed significantly to production increase. Based on modeling with STARS package, various operating practices have been proposed. The flow management was achieved through change of the produced fluid and injected steam volumes, as well as change of steam injection and pump setting points. All these made it possible to form steam chamber uniformly, avoid steam breakthrough, and increase oil production with minimum steam-bitumen ratio. The average production rate of the experimental horizontal wells is ten times higher than that of the offset vertical wells. Current steam-bitumen ratio makes 3.5 t/t. Introduction There are many applications of Steam Assisted Gravity Drainage method (SAGD) [1] in Alberta, Canada. The efficiency of SAGD method greatly depends on water-saturated and shale interlayers presence in the reservoir section. In 2006, a new pilot project was launched on Ashalchinskoye field (Fig.1) to test a modified low pressure SAGD technology using a unique pair of U-shaped horizontal wells (bi-wellhead wells) (Fig.2) [2]. This technique allows us to overcome some disadvantages of classic SAGD technology. This deposit occurs at a shallow depth (78 m) and has a rather complex structure. Water-saturated interlayers are found in the top and in highly oil-saturated zones of the formation, while shale stringers are found in the middle of the sequence. Both aquifers and tight, low-permeability, calcareous sandstones or a low-saturated reservoir can serve as the bottom of the formation. Due to deteriorated reservoir properties down in the formation, water/oil interface is irregular (Table 1).