Artificial lift

Artificial lift systems are mostly required later in a field s life, when reservoir pressures decline and therefore well productivities drop. If a situation is anticipated where artificial lift will be required or will be cost effective later in a field s life, it may be advantageous to install the artificial lift equipment up front and use it to accelerate production throughout the field s life, provided the increased revenues from the accelerated production offset 

The following types of artificial lift are commonly available today  

The progressive cavity pump consists of a rotating cork-screw like sub-surface assembly which is driven by a surface mounted motor. Beam pump rods are used to connect the two. The flowrate achieved is mainly a function of the rotational speed of the subsurface assembly. There Is in principle very little that can go wrong with progressive cavity pumps. Progressive cavity pumps excel in low productivity shallow wells with viscous crude oils and can also handle significant quantities of produced solids. 

The principle of operation of the hydraulic reciprocating pump is similar to the beam pump, with a piston-like sub-surface pump action. The energy to drive the pump, however, is delivered through a hydraulic medium, the power fluid, commonly oil or water. The power fluid drives a downhole hydraulic motor which in turn drives the pump. A separate surface pump delivers the hydraulic power. The power fluid system can be of the closed loop or of the open type. In the latter case, the power fluids are mixed with the produced fluid stream. The performance of the hydraulic pump is primarily monitored by measuring the discharge pressures of both surface and sub-surface pumps. 

The jet pump relies on the same hydraulic power being delivered sub-surface as to the hydraulic reciprocating pump, but there the similarity ends. The high-pressure power fluid is accelerated through a nozzle, after whioh it is mixed with the well stream. The velocity of the well stream is thereby increased and this acquired kinetic energy is converted to pressure in an expander. The pressure is then sufficient to deliver the fluids to surface. The jet pump has no moving parts and can be made very compact. 

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There will be some uncertainty as to the well initials, since the exploration and appraisal wells may not have been completed optimally, and their locations may not be representative of the whole of the field. A range of well initials should therefore be used to generate a range of number of wells required. The individual well performance will depend upon the fluid flow near the wellbore, the type of well (vertical, deviated or horizontal), the completion type and any artificial lift techniques used. These factors will be considered in this section. 

The end of field life is often determined by the lowest reservoir pressure which can still overcome all the pressure drops described and provide production to the stock tank. As the reservoir pressure approaches this level, the abandonment conditions may be postponed by reducing some of the pressure drops, either by changing the choke and separator pressure drops as mentioned, or by introducing some form of artificial lift mechanism, as discussed in Section 9.7. 

The objective of any artificial lift system is to add energy to the produced fluids, either to accelerate or to enable production. 

Figure 9.18 provides an overview of the application envelope and the respective advantages and disadvantages of the various artificial lift techniques. As can be seen, only a few methods are suited for high rate environments gas lift, ESP s, and hydraulic systems. Beam pumps are generally unsuited to offshore applications because of the bulk of the required surface equipment. Whereas the vast majority of the world s artificially lifted strings are beam pumped, the majority of these are stripper wells producing less than 10 bpd. 

Artificial lift techniques are discussed in Section 9.6. During production, the operating conditions of any artificial lift technique will be optimised with the objective of maximising production. For example, the optimum gas-liquid ratio will be applied for gas lifting, possibly using computer assisted operations (CAO) as discussed in Section 11.2. Artificial lift may not be installed from the beginning of a development, but at the point where the natural drive energy of the reservoir has reduced. The implementation of artificial lift will be justified, like any other incremental project, on the basis of a positive net present value (see Section 13.4). 

Tften, other forms of artificial lift are used to produce oil wells such as ( ivnhole submersible pumps and rod pumps that require that most of 1 formation gas be separated downhole and flowed up the annulus... 

In addition to pressure maintenance in a resei voir, there are other ways to maintain or improve well productivity as the formation pressure falls. A pump can be used to raise the oil, a process called artificial lift. One type is the familiar beam pump with its surface power unit driving up and down one end of a center-mounted beam while the other end executes the opposite down-up motion. The second end is attached to a string of sucker rods that extend down to the bottom of the well and operate a pump arrangement, consisting of a cylinder, plunger, and one-way A alves. Other types of pumps in use eliminate the sucker rod and may have the power unit at... 

Brown, Kermit E., and Beggs, H. Dale, The Technology of Artificial Lift Methods, Vol. 1, PennWell Books, Tulsa, 1977. 

Once the well is drilled, the oil is either released under natural pressure or pumped out. Normally crude oil is under pressure (were it not trapped by impermeable rock it would have continued to migrate upward), because of the pressure differential caused by its buoyancy. When a well bore is drilled into a pressured accumulation of oil, the oil expands into the low-pressure sink created by the well bore in communication with the earth s surface. As the well fills up with fluid, a back pressure is exerted on the reservoir, and the flow of additional fluid into the well bore would soon stop, were no other conditions involved. Most crude oils, however, contain a significant amount of natural gas in solution, and this gas is kept in solution by the high pressure in the reservoir. The gas comes out of solution when the low pressure in the well bore is encountered and the gas, once liberated, immediately begins to expand. This expansion, together with the dilution of the column of oil by the less dense gas, results in the propulsion of oil up to the earth s surface As fluid withdrawal continues from the reservoir, the pressure within the reservoir gradually decreases, and the amount of gas in solution decreases. As a result, the flow rate of fluid into the well bore decreases, and less gas is liberated. The fluid may not reach the surface, so that a pump (artificial lift) must... 

The produced oil or gas is connected to surface flowlines from the wellhead pumping unit or surface regulating valve assembly typically referred to as a Christmas tree but to its arrangement. The flowlines collect the oil or gas to local tank batteries or central production facilities for primary oil, water, and gas separation. The reliability of electrical submersible pumps (ESPs) has increased to the point where the submersible electrical pump is commonly used for the production of liquid hydrocarbons where artificial lift is required for production. 

Reservoir support. An economic optimum method of developing the reservoir depends on the phasing of water injection, water-injection pressure, reservoir fluid properties, pressure at which the reservoir may fracture and be damaged, and when artificial lift may be economical. 

A detailed comparison of available fluid lift capacity with targeted rates indicated that artificial-lift capacity was not adequate in a number of areas. 

With the need to monitor reservoir performance closely and the ongoing requirement to change production targets, a computerized data system is beneficial for reservoir surveillance. Automation provides more frequent well test data to assist in developing well work programs and artificial lift improvements. 

An important feature of the distributed control system chosen is that the PCMs can communicate over a microwave link, thus allowing the well pad manifold modules to be included in the GC control system the hardwire data highway is limited to a distance of 5000 ft. (1524m) from the Host. The well chokes and manifold controls will remain on the SCAOA system, but the well pad will require continuous process control of the test separator, water injection flow control, and artificial lift gas flow control. 

Most wells on our lease in this field were equipped with high-volumc artificial lift in 1975-76 to take advantage of the high productivities of these wells The lease-wide oil production increase from 290 to 525 B D (46 to 83 m /d) was accompanied by an 18.000 BD (2862-m,/d) increase in water. Total water production,... 

When pressures in an oil reservoir have fallen to the point where a well will not produce by natural energy, some method of artificial lift must he used. Oil-well pumps are of three general types (I) pumps located a( the bottom of the hole run by a string of rods, (2) pumps at the bottom of the hole run by high-pressure liquids, and (3) bottom-hole centrifugal pumps. Another method involves the use of high-pressure gas to lift the oil from tile reservoir. 

The filtration properties of super-viscous crudes and the mechanism of their drainage from the reservoir rocks are now known as a result of the hydrodynamic studies. The latter were carried out, for example, at the Abino-Ukrainsk field in the course of oil production by artificial lift using bottom-hole pumps. 

The above value of 510 Darcies was obtained for well 41 at the time when the water cut of the existing wells in the Abino-Ukrainsk field was already 80%. At the same time, the water cut of well 41 was still only 30%, a value corresponding to the transition stage of production by free flow to production by artificial lift using downhole pump. These data indicate that at the ratio of 80/30 = 2.7, the productivity and consequently the permeability for oil should be higher, as per figures below ... 

Oil wells generally produce gas in addition to crude oil. Oil wells often require artificial lift as pressures decline. Artificial lift wells use rod pumps, submersible pumps, or gas lift. Injection wells are used in enhanced recoveiy operations or for disposal of produced water. In enhanced recovery, the injected fluids include steam, water, carbon dioxide, or polymer solutions. 

Gaviria, F., Santos, R., Rivas, O., and Luy, Y. (2007) Pushing the boimdaries of artificial lift applications SAGD ESP installations in Canada. Proceedings - SPE Annual... 

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