Abstract

Since 1992, two 300–400 m long horizontal wells have been drilled in the Brazeau River Nisku D pool at a true vertical depth of 3200 m. The pool has been under vertical hydrocarbon miscible flood for over fifteen years. The purpose of these horizontal wells was to increase ultimate pool recovery by reducing well drawdown leading to improved GOR behavior.

The initial discovery and vertical well development were based on 2-D seismic data. As the GOR increased, alternative seismic and drilling technologies were used to optimize pool recovery.

Three component vertical seismic profiling was conducted to locate the porous zone and to aid in the design of the length, depth and azimuth of the first horizontal well drilled in 1992. In late 1993, a 3-D over the pool seismic program was conducted which was used to map the Nisku base structure and porosity to determine the deepest corridor along which the second horizontal well would traverse. This led to the second horizontal well, drilled in 1994. In all, three forms of seismic technologies, including 2-D seismic, have been used in the development of the Nisku D Pool.

Both horizontal wells were drilled as reentries from existing wellbores and open hole completed, to minimize cost. The initial oil rate of the first horizontal well was 250 m3/d with an expected incremental reserves of approximately 190 000 m3, or seven percent of the OOIP. The second horizontal well had an initial rate of 200 m3/d, and is expected to contribute another 81 000 m3 of incremental reserves. In both instances, producing GOR was reduced to the initial solution GOR of approximately 140 m3/m3, from a pre-reentry high of over 2500 m3/m3. The low GOR was maintained for over 6 months. The reentry costs were paid out within the first year of production.

3-D seismic with reservoir characterization over the pool has proven to be superior in providing a more complete understanding of the reservoir and, with a properly planned acquisition program, can be effectively used for the design of horizontal wells to maximize pool recovery.

Introduction

The Brazeau River Nisku D pool, located 150 km southwest of Edmonton, Alberta, is a pinnacle reef containing sweet light volatile oil having a density of approximately 800 kg/m3 and a solution GOR of 140 m3/m3. Figure 1 shows the location of the D pool along with other producing Nisku reefs in the area. The Nisku D pool was discovered in June 1978 with the drilling of well 7-33-48-12 WSM. Average porosity ranges from 7 to 10 percent and the average net pay is 40 m. Average pool permeability is approximately 50 mD. The original oil-in-place of 2700 103m3 was established by material balance. Table 1 summarizes the reservoir and fluid properties of the pool.

History

The pool was initially under primary production using wells 5–34 and 7–33 until October 1980 when it was shut in. By this time, the reservoir pressure had declined from an initial 34 500 kPa to 20 000 kPa, 480 kPa below the bubble point pressure and created an estimated three percent critical gas saturation. A vertical lean hydrocarbon miscible flood with a minimum operating pressure of 36 000 kPa was implemented in August 1981 with gas injection at 10–33. The pressure history is shown in Figure 2. A third producer, 15–28, was added in 1983.

Gas breakthrough occurred at 7–33 six months after injection started, followed by the 5–34 a year later. Gas broke through at the 15–28 well in 1985. As shown in Figure 3, the pool produced at a relatively stable oil rate of 400 m3/d until 1987 when decline began due to limited capacity to handle the increasing gas production as GOR increases, as shown in Figure 4. Due to high GOR's, the 7–33 well was suspended in 1988, and the 5–34 in 1991.

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