As the hydrocarbon production in the Gulf of Thailand has matured, managing associated produced water has become a focus of attention. Produced water management in an offshore environment requires innovation, both from a surface facility and subsurface viewpoint. Produced water can be disposed in oil sands (waterfloods), aquifer or wet sands and in depleted hydrocarbon sands. This paper provides insights into the subsurface disposal alternatives of produced water management using examples from Chevron Thailand's greater B8/32 operating area.
The foundation of a robust produced water management strategy lies in the ability to accurately forecast future water production. Using historical water production data from existing platforms, future drilling activities and impact of artificial lift, we can generate forecasts of produced water. Currently, in B8/32 asset, we produce about 68,000 bbl/day of water and an additional 20,000 bbl/day of water is expected from new projects and artificial lift by end of 2007. Once produced water forecasts have been generated, we associate the produced water volumes with candidate reservoirs where produced water can be disposed. The B8/32 operator's strategy has been to focus first on reservoirs where produced water can be injected to drive oil (water flooding). Second, the focus is on injecting produced water in wet (aquifer) sands which may not be connected to a hydrocarbon system, but are very large and have small amounts of trapped gas. Lastly, but most importantly, depleted oil and gas sands can store significant volumes of produced water and in the future will play a significant role in storing large volumes of produced water in the greater B8/32 operating area.
A key element of the approach is to obtain good estimates of reservoir volume and connectivity. The reservoir sands in B8/32 area of the Gulf of Thailand are primarily found in tertiary sequences, which are predominantly of non-marine origin, and deposited in fluvial to lacustrine environments. The net-to-gross over the entire reservoir interval (over 3000 feet) is typically between 0.15–0.20. At this low net-to-gross, the vertical connectivity of sands is generally very poor. There is a high level of faulting, subdividing the channel systems, and thus reducing the connected sand area to around 20–50 acres. While larger compartment sizes (100–250 acres) do exist, and have been observed from pressure, production and seismic data, they are less frequent. The sand area/volume connected to a well, remains one of the largest uncertainties in estimating water disposal volumes.
The large volumes of produced water to be disposed and the typically small reservoir compartments to dispose produced water make produced water management a challenging proposition for the B8/32 asset. Artificial lift and secondary recovery to increase oil recovery have further added produced water to the system. While efforts are underway to reduce water production, a key component of the produced water management strategy is to dispose produced water in an environmentally friendly manner. We discuss the three main buckets for disposing produced water in the following sections.
Opportunities for water flooding are limited in the Gulf of Thailand, because of small disconnected sands and significant faulting. If large oil sands can be identified in time, water injection can be very effective in improving recovery efficiency. Injecting produced water can have a double benefit. Firstly, typical recovery factors with depletion drive are in the 10–15% range, however, with waterflood and good voidage replacement (see Figure 1) they could reach between 30–40%. Secondly, we can dispose produced water in waterflood sands, thereby reducing water overboard.