Wellbore separation is becoming increasingly viable as the process becomes more accepted and completion designs become more standardized. Successful wellbore separation has been achieved with same-well clean water disposal using a combination of downhole pump and hydrocyclones. Gravity wellbore separation, often referred to as "suppressed coning," also has been demonstrated where the oil and water components have been produced separately and where gas/water or oil/water separation has been adequately achieved using the retention time available in the well annulus.
Downhole separation and injection also is becoming more attractive as its benefits generate improved Net Present Value through earlier production, and increased recoverability by extending the life of the well and affecting water flood patterns and pressure maintenance.
This paper addresses some of the problems related to downhole separation with respect to the completion design, and suggests design concepts developed to overcome some of these problems.
It is reasonably well understood how the production of water can undermine the relative economics of producing oil and gas wells. Conventional concerns relate to management of the increasing water volumes, and the effect of saltwater hydrostatic on the production formation. A less obvious concern is the effect of the water in the reservoir, where it increases the relative saturation of the gas and/or oil in the near wellbore and, in fact, impedes the flow of hydrocarbon into the wellbore.
This phenomenon can be alleviated in gas wells using a GASPROTM system or, in the case of oil wells, using a HydroSepTM (Figure 6) system (which comprises ESP, separator and packer, tools etc).
In the gas well, gas/water separates by gravity in the wellbore and the water in pumped into a disposal zone. In the oil well the necessary degree of separation is achieved through use of a hydrocyclone separator.
A number of new completion techniques and equipment designs are now available which expand the application of wellbore separation technology. The different completion designs can vary widely depending on the location of the injection zone, the casing size and lift method and, in some cases, can be quite complex. Combinations of pump type and gas lift or power lift are also envisaged but are not pursued here. Rather, it is felt that these combinations will tend to be very customized and site specific.
In order to design the most efficient completion for the application, it is necessary to start by clearly defining what the completion must do. Electrical Submersible Pumps (ESP), Vortoil SubSep downhole separators, packers, tools, instrumentation, and chemical injection are all available and add to the complexity.
A newly released ESP Packer (Figure 9) now makes it possible to complete existing wells with 5–1/2" casing, low BHFP (Bottom Hole Flowing Pressure) and an upper disposal zone. In addition, a method developed by Baker Hughes Process Systems for the same application, now allows the power cable to pass through the SubSep downhole separator.
There are several types of downhole HydroSep systems available for different casing sizes. With each system, the completion design must accomplish the following:
separate the oil from the water
provide energy to dispose of the water and ensure the oil can flow to surface
provide zone isolation for water injection
The following completion requirements are not strictly necessary but are recommended based on experience: