Oilfield production

Introduction

 

In oil and gas production, large gravity based separators are commonly used to separate the different phases: water, oil, and gas, into clean phases. The produced water is cleaned in a number of treatment stages to reduce the oil content down to an acceptable level. The water is either reinjected into the reservoir as an increased oil recovery method, injected into a disposal well, or discharged. A typical North Sea offshore oil production system consists of two to four stages of bulk separation and two or three stages of produced water treatment, as illustrated in Fig. 1.

 

Figure 1. – Typical North Sea crude oil separation and produced water treatment system.

Figure 1. – Typical North Sea crude oil separation and produced water treatment system.

 

Upstream and in-between the different separators and produced water treatment stages, valves and pumps are installed to control the flow and the pressure. These valves and pumps are considered as a necessary evil with regards to the separation efficiency. Shear forces acting on the fluids passing through the valves and pumps cause droplet break-up and emulsification, which have a detrimental impact on the downstream separator efficiency.
A number of measures can be employed to counteract the negative effects from these valves and pumps. The residence time of a separator can be increased, more treatment stages can be utilized, heat can be applied, separation-enhancing chemicals can be used, etc. However these measures can significantly increase the CAPEX and OPEX by requiring larger footprint and weight, higher power and chemical consumption. A more constructive way is to deal with the cause of the issue/problem, which is to reduce the negative effect of the valves and pumps on the separation efficiency.
The philosophy of low shear production is that low shear solutions should be used where needed to prevent separation problems. Shear in itself is not necessarily a problem and a certain level of shear can even be desirable. Some degree of shear or turbulence can promote coalescence or enhance gas-liquid separation in the event of foaming. Too much shear would, however, lead to strong emulsification and result in reduced separation efficiency.

 

 

Choke valve

 

The most obvious application for low shear technology in the oilfield is a choke valve. Even a moderate or low differential pressure over the choke valve can develop significant shear forces in the flow and cause emulsification of the fluids. Tests performed with simulated and live field conditions have shown that an improved design of the valve internals enables low shear operation. Low shear valve, like the Typhoon® System, can have a tremendous impact on the separation efficiency of the downstream separators. Tests results of the Typhoon® System show a 50 to 90% reduction in the oil-in-water (OiW) concentration downstream of the separator compared to a conventional choke valve. Fig. 2 presents results of the test where the water quality is shown for both the Typhoon® System and a conventional valve, as a function of the water cut. The water-in-oil (WiO) concentration can be reduced with a low shear choke valve as well.

 

Figure 2. – Water quality as a function of the water cut.

Figure 2. – Water quality as a function of the water cut.

 

 

Control valve

 

Shear in level control valves, similarly to choke valves, can have significant negative effect on the downstream separation. Tests have shown that even with just 0.5 bar differential pressure over a valve, a low shear valve alternative can increase downstream separation efficiency considerably. Special interest presents the application of level control valve between a high pressure separator for two-phase separation and a low pressure separator for three phase separation. Lower emulsification of the water and oil phases over this level control valve would lead to enhanced efficiency of the process plant.

 

 

Pump

 

Pumps often increase the water pressure from the low pressure separators, and to transfer this water for further handling. Water can be recirculated to higher pressure separators, or it can be routed to the water treatment system. Traditionally a number of different pump types, ranging from simple and robust single stage centrifugal pumps to more complex twin screw or progressive cavity pumps, are chosen for these purposes. The use of low shear pumps in these applications is important in order to keep the dispersed oil droplets as large as possible. The larger dispersed droplets would tremendously accelerate the separation process by downstream treatment equipment. In many fields with high water cut, the capacity of water treatment facilities can be a bottleneck to the overall field production. Read more about produced water treatment.

 

 

Additional reading

 

Husveg, T., Bilstad, T., Guinee, P.G.A. et al. 2009 A Cyclone based Low Shear Valve for Enhanced Oil-Water Separation. Presented at the Offshore Technology Conference, Houston, Texas, USA, 4-7 May. OTC2029. http://dx.doi.org/10.4043/20029-MS

 

http://petrowiki.org/PEH%3AOil_and_Gas_Processing

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