First Horizontal Water Injectors in Prudhoe Bay Field, Alaska
- D.A. Bodnar (BP Exploration (Alaska) Inc.) | P.J. Clifford (BP Exploration (Alaska) Inc.) | J.S. Isby (BP Exploration (Alaska) Inc.) | A.C. Lane (BP Exploration (Alaska) Inc.) | R. Loveland (BP Exploration (Alaska) Inc.) | L.I. Seymour (BP Exploration (Alaska) Inc.) | D.M. Urban (BP Exploration (Alaska) Inc.) | S.M. Shultz (Arco Alaska Inc.) | G.S. Walz (Arco Alaska Inc.)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Engineering
- Publication Date
- May 1997
- Document Type
- Journal Paper
- 104 - 109
- 1997. Society of Petroleum Engineers
- 2.5.1 Fracture design and containment, 1.6.3 Drilling Optimisation, 5.6.1 Open hole/cased hole log analysis, 6.5.2 Water use, produced water discharge and disposal, 2.2.2 Perforating, 3.3.1 Production Logging, 2.4.3 Sand/Solids Control, 1.14 Casing and Cementing, 1.6 Drilling Operations, 1.6.7 Geosteering / Reservoir Navigation, 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation, 5.4.1 Waterflooding, 5.1.2 Faults and Fracture Characterisation, 2 Well Completion, 1.1 Well Planning, 3 Production and Well Operations, 4.1.2 Separation and Treating, 4.1.5 Processing Equipment, 1.12.1 Measurement While Drilling, 1.8 Formation Damage, 1.6.6 Directional Drilling
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Water injection into horizontal wells is a new and distinct technology, where success will often depend on the management of thermal fractures. This paper describes the background, planning, drilling, completion and results of the first two high angle water injection wells in the Prudhoe Bay field. The procedures described resulted in re-establishing injection in the correct zone, achievement of target rates and good injection profiles.
The use of horizontal and high angle wells for oil and gas production has been an established and rapidly growing technology for several years. The use of the same well types for water injection, however, is just beginning. This paper describes the first examples on line in Prudhoe Bay field (Fig. 1). These wells (Y-05A and Y-11A sidetracks), are also the first injected by BP worldwide. Two more pre-produced injectors (X-20 and A-42) have been drilled in Prudhoe Bay, and will be put on injection in 1996.
Drilling of a horizontal injector requires a clear picture of what it can achieve, and in particular of why it can perform better than a vertical injector. Unlike a production well, this picture is complicated for most deep, hot reservoirs by the thermally induced fracturing of injection wells during cold water injection. In Prudhoe Bay, all conventional water injectors are thermally fractured. The fracture pressure for seawater injection is sufficiently low (0.53-0.54 psi/ft) that it was seen as inevitable that horizontal injectors would also fracture. A successful horizontal injector requires successful use of thermal fracturing.
The reason for attempting horizontal injection in the Y-Pad area of Prudhoe Bay is to achieve a higher injectivity than a vertical well, without risk of downward vertical fracture growth out of zone and into the aquifer. The 100 ft oil leg in this area is bounded above by the Shublik Formation (limestone & shale), which provides fluid and fracture containment. Below, it is bounded by a 30 ft layer of heavy oil / tar (HO/T), which acts as a barrier to fluid flow, but is no different in stress from the oil leg. Beneath the HO/T lies the Prudhoe Bay aquifer, which in this area is typically in higher permeability sand than the reservoir.
Vertical Injectors. The original vertical injectors in the area, Y-05 and Y-11, completed in 1985 and 1987, thermally fractured and initially achieved their injection targets on seawater injection. In 1991 the wells were changed to produced water reinjection (which characteristically reduces injectivity by 30-50% due to higher temperature and lower water quality). This, coupled with increased injection targets due to further development of the area, led to attempts in 1991 to inject more water at higher well-head pressure (WHP). This unfortunately resulted in vertical growth of the fractures downwards through the HO/T (Fig. 2). This process has been modeled in related cases. Although injection targets were nominally being achieved, much of the water was now being wasted into the aquifer.
Loss of containment is clearly shown for conventional injector Y-11 in its well performance plot (Fig. 3). During one month in 1991, the injection rate at given WHP increased by a factor of 4. Even when the well was taken to lower WHP, its performance remained permanently altered from before, and it is likely that a large proportion of all injection has since entered the aquifer. This is directly verified for Y-11 by comparing temperature logs before and after loss of containment (Fig. 4). The 1986 log shows only a temperature gradient into the HO/T layer due to heat conduction. By contrast, the late 1991 log shows cooling to bottom-hole temperature that extends 100 ft into the aquifer. The temperature log results are verified by repeat spinner surveys on Y-11 (Fig. 5). The performance of conventional injector Y-05 suffered a similar change.
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