Page 292 - Introduction to Petroleum Engineering

280 RESERVOIR PERFORMANCE
Seawater injection commenced in 1984. The scope of the water flood was deter‑
mined by the character of the reservoir, aquifer strength in different parts of the res‑
ervoir, water sources, production performance, and the timing of gas sales. Water
flood infrastructure had to be built and included a seawater treatment plant, water
injection plants, and a network of seawater distribution pipelines.
The reinjection of hydrocarbon gas created an opportunity for implementing a
miscible gas injection project. Miscibility is achieved by injecting gas at a high
enough pressure that the interfacial tension between injected gas and oil is signifi‑
cantly reduced so that the gas and oil phases combine into a single phase. Miscibility
occurs when reservoir pressure is greater than minimum miscibility pressure (MMP)
of the system, which can be measured in the laboratory. Injected gas mixes with in
situ oil and swells the oil in a miscible process. A small‐scale pilot program was
conducted in 1982 to confirm the viability of miscible gas injection as an EOR
project. The large‐scale Prudhoe Bay Miscible Gas Project began in 1987 and was
subsequently expanded.

Example 14.4 Production Stages
A. Primary recovery from an oil reservoir was 100 MMSTBO. A water flood
was implemented following primary recovery. Incremental recovery from
the water flood was 25% OOIP. Total recovery (primary recovery plus
recovery from water flooding) was 50% OOIP. How much oil (in
MMSTBO) was recovered by the water flood?
B. What was the OOIP (in MMSTBO)?

Answer
A. Total recovery (primary + water flood) = 50% OOIP
Incremental water flood recovery = 25% OOIP
Therefore primary recovery = total recovery – water flood recovery = 25% OOIP
Water flood recovery = primary recovery = 100 MMSTBO
B. Primary recovery plus water flood recovery = 200 MMSTBO
Total recovery = 200 MMSTBO = 0.5 × OOIP
So OOIP = 200 MMSTBO/0.5 = 400 MMSTBO

14.4 PERFORMANCE OF AN UNCONVENTIONAL RESERVOIR

In this section we consider examples of unconventional reservoir performance.

14.4.1 Barnett Shale, Texas: Shale gas Production
Geologists discovered a shale outcrop near the Barnett Stream in San Saba County,
Texas, in the early twentieth century. The stream was named after John W. Barnett
who settled a large tract of land in central Texas. The thick, black, organic‐rich shale