Enhanced oil recovery (EOR) is fundamental to increasing the ultimate recovery of a reservoir. Among several parameters that govern the success of an EOR project, profile modification is a key factor when flooding heterogeneous reservoirs with multiple zones. Given the time and economic constraints, simultaneous flooding of all zones of interest is preferred although it can be difficult for reasons such as a varying pressure and fluid injectivity/productivity profile caused by permeability contrasts between zones. Injection fluids can bypass low-permeability oil-saturated zones, not achieving the full benefit of injection if the profile is not properly modified. Failure to modify production profiles can lead to early water and gas breakthrough. To mitigate these problems, various approaches such as mechanical downhole equipment or chemical treatments have been used.
This paper presents a comparison of the use of mechanical versus chemical conformance control systems in different scenarios. Conformance control is defined as the process that helps improve the drive mechanism for better sweep of reservoir. The basic premises of both mechanical and chemical systems are introduced. Their applicability, operational benefits, and disadvantages are also described. A 3D reservoir simulator is used to illustrate the varying sweep efficiencies, depending on the conformance control method used. The result from the best case with both injector and producer profile modification demonstrated nearly double the oil recovery compared to the base case with no treatment.
Water shutoff in mature reservoirs is traditionally achieved with cross-linked gels. By blocking the areas already swept by water, subsequently injected water can sweep an unswept area of the reservoir and thereby increase the oil recovery. However, it is a complicated process and the performance of polymer gel flood in complex reservoirs requires an accurate model that represents the reservoir features, chemical properties, and displacement mechanisms.
This paper presents a successful investigation of polymer gel behavior from laboratory to full field scales. First, a series of laboratory experiments were conducted to achieve a deep understanding of polymer gel behavior. The results show that the polymer viscosity, gelation time, and gel strength strongly depend on reservoir temperature, polymer type, polymer concentration, cross-linker concentration, pH and salt concentration, which are the successful keys of a polymer conformance control process. The optimal values of these parameters are proposed for applying in a pilot test. A series of numerical simulations are performed to history match with experiment data and generate parameters for field scale simulation. The adsorption phenomenon is fully integrated into the reservoir model for controlling and reducing this effect during the polymer flooding process.
According to the laboratory results, polymer gel flooding was applied for White Tiger which is the biggest oil field in Viet Nam. After a long time of waterflooding, water production becomes a serious problem in this field. Polymer gel treatment is simulated in full field scale and the results show that it is an excellent candidate for conformance control. Water production decreases from 4,800m3/d to slightly less than 2,000m3/d, while a significant increase in oil production has been achieved from unswept zones. That is a really successful evidence of the polymer conformance control technology in heterogeneous reservoirs.
Waterfloods increase both the oil rate and the volume of recoverable oil from a field, but they become very inefficient once the injected fluid channels through a reservoir directly to the production wells. High water production is a main concern in mature oil fields. A large amount of water production results in (a) the need for more complex water-oil separation techniques, (b) the rapid corrosion of well equipment, (c) the rapid decline in hydrocarbon recovery and (d) ultimately, the premature abandonment of a well. Also, the breakthrough of either the formation or injected water results in increased operational costs of pumping, treatment, and disposal of the produced water (Ogunberu, et al., 2006). As a result, the oil industry handles more water than oil.
One of the challenges for polymer application in offshore oilfield is the treatment of the produced fluids containing polymer. It would be extremely difficult to deal with when the injected polymer is produced in large volume on the platform. A practical example was illustrated in this paper. JZ west area, one offshore oilfield in Bohai Bay, has been implemented polymer flooding since 2007. In this field, incremental oil and reduced water cut were observed, as the concentration of the produced polymer rised to such a high level that nearly leading to a treatment failure on the platform.
In this paper, reasons of the polymer cross flow are analyzed, and methods to control the cross flow of the polymer solution in the pilot were described. The study shows that separate layer injection and in depth profile control are effective measures to improve the vertical sweep efficiency of the injected agent.
After the measures were applied in the field, the produce polymer concentration has been reduced and the treatment procedure resumed to normal.
It is showed that prevention of cross flow in advance is extremely important for offshore oilfields. This paper gave a practical experience for polymer application in offshore environment.