"Smart polymers" limit water production without hindering oil, while reducing cuts and costs.

Chevron Texaco Exploration and Production Technology Company (EPTC) has successfully applied water control treatments using a novel relative permeability modifier (RPM) called AquaCon, to high water-cut wells in the Bekasap Field, Indonesia. The AquaCon system underwent core flow test evaluation and was subsequently field-tested successfully in oil-producing wells in Bekasap Field.
Exorbitant water production is a continuing and increasing burden on the oil industry, especially as lifting and facility costs rise and disposal of produced water becomes increasingly expensive and environmentally sensitive. Producing undesirable water increases operating expenditures, and high water rates significantly can reduce well productivity. Excessive water production problems emanate from a number of sources, including, but not limited to: channels behind casing casing leaks, coning, encroachment, injection water breakthrough and natural or induced fractures.

Several different methods aimed at reducing water inflow into producing wells have been proposed, developed and tested in the field over the years. These methods generally consist of placing a barrier across the water-producing zone (e.g., mechanical packers or bridge plugs, cements, resins or cross-linked polymers). Unfortunately, these methods usually block the oil or gas flow and water inflow. Also, mechanical solutions require well intervention and can be expensive. Polymer gels represent a valid and economic alternative to mechanical isolation, but their application requires that water zones are identified and isolated from hydrocarbon-producing zones during treatment. This usually involves direct well intervention by setting temporary plugs or isolation devices using wireline or coiled tubing.

Relative permeability modifiers

RPMs have been developed for use in chemical treatments to selectively reduce water production in a hydrocarbon zone without significantly reducing hydrocarbon production. RPMs are water-soluble, hydrophilic long-chain polymer systems that associate with the rock matrix. In the presence of hydrocarbons, these RPMs remain flaccid and do not significantly affect the relative permeability of hydrocarbons. When exposed to water, RPMs become hydrated and rigid, or expand, resulting in a loose occupation of pore spaces. Being strongly hydrophilic, they attract water and repel oil and, as a net result, exert a drag force on water flow in the pores with minimal (and sometimes positive) effect on oil flow.
Generally, RPMs provide a number of potential advantages. They are relatively low cost in that the chemicals are used in limited quantities and treatment does not require zonal isolation. They are relatively low risk, in that they reduce water permeability without adversely affecting oil or gas permeability. Finally, they are simple to apply in the field and can be applied where mechanical isolation is not a viable option.

AquaCon treatments unique

AquaCon treatments incorporate unique RPM products exclusive to BJ Services. The AquaCon molecule is a moderate molecular weight hydrophilic ter-polymer based on a polyacrylamide molecule. AquaCon is also composed of sulfonated moieties that enable the polymer to tolerate high-salinity brines and especially those containing divalent cations (Ca+2 and Mg+2). The inability to handle such cations is a shortcoming of some RPMs. This characteristic, coupled with a unique chemical structure that allows the polymer to bind to rock rather than simply associate through charge attraction, provides the basis for AquaCon performance.

The polymer, having its special structure, is designed to partition itself both onto reservoir rock and into reservoir brines. This behavior in pore throats results in a significant reduction of permeability in water-rich environments. The polymer is also able to deform in oil-rich environments so that pore throat restrictions associated with the polymer are minimized. This behavior is observed as minimal impairment of oil permeability.

Characteristics of unique chemistry

With its unique polymer chemistry, AquaCon treatments display a number of characteristics that make the application effective. For example, the low viscosity (3 to 4 centipoise) makes the treatment solution easy to pump; it can be bullheaded or placed through coiled tubing. The chemical formulation is effective in both sandstones and carbonates, and can be placed without the need for well intervention or mechanical isolation.

AquaCon imparts a very high disproportionate reduction in water permeability - with little or no effect on oil or gas permeability. It can be applied in a variety of applications, including stand-alone matrix treatments, as a water control treatment incorporated into acid or fracture stimulation, and as a fracturing fluid efficiency enhancement (leak-off control) additive. Its compatibility with scale and salt block inhibitors make it ideal for one-step, multi-purpose water management/treatment solutions.

AquaCon case studies

The first treatments were pumped in the Bekasap Field, Sumatra, Indonesia. Wells producing in this area suffer from high water cuts (> 90%) and accompanying restrictions in oil production. Located about 6 miles west of the Dhuri field, Bekasap began producing in 1965. A peripheral waterflood was implemented in 1997 to increase recovery and extend the life of the field. Average injection of 250,000 b/d of water from 16 injectors has been established into 10 distinct reservoir intervals. Current field production is about 27,500 B/d of oil, 2,400 b/d of condensate and 150,000 b/d of water from 75 producing wells. The geology of the field includes multiple reservoirs within three main shallow marine sandstones with a range of reservoir quality. A total field reservoir management project was initiated in 1999 to optimize the waterflood operation and maximize recovery.

The first ChevronTexaco AquaCon treatment was conducted in Well Titian #1. The well was treated with 6,000 gallons of 6% AquaCon solution - designed for 7-ft radial treatment penetration across the 20-ft perforated interval in this well. The treatment was bullheaded through tubing.

Prior to these procedures, this well produced 1,098 b/d of fluid with 94% water cut, from a fairly high permeability (~700 mD) sandstone interval that was 20 feet thick (3,940-3,960 ft). The produced oil had a 4 cp viscosity at 203 YF. The results for the first 4 months of post-treatment production were unprecedented in the field's water control workover history (Figure 1, Table 1).

The RPM effect, which is often delayed, increased as time progressed. Just 4 weeks following the AquaCon treatment, the water cut had been reduced by about 30% and the oil production increased by 330%. After 7 weeks, the total fluid production had stabilized (800 b/d), while oil production reached a peak of 436 b/d at a water cut reduced to 46%. Although the water cut began to rise thereafter, as water production seeks a more preferable path to the wellbore, the effective treatment life in this well was about 6 months and treatment payout was in 2-3 weeks.

The second AquaCon-treated well in the Bekasap area was well Penasa No. 2. In this well, a cement squeeze was followed by re-perforation 6 ft above the anticipated water-oil contact (from 3,105 ft to 3,118 ft), just prior to the AquaCon treatment. The well was treated with 5,500 gallons of 8% AquaCon solution - designed for a 7-ft radial treatment penetration across the 13-ft re-perforated interval. Results are similar to those from Titian No. 1, again unprecedented relative to field history. Again, the RPM effect was delayed, but resulted in a dramatic decrease in water cut and even more dramatic, six-fold increase in oil production 2 weeks after treatment (Table 2).

The success of water control measures such as these AquaCon treatments supports the use of RPM as an effective tool for helping to optimize recovery from waterfloods in mature reservoirs.

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