Inverted-ESP Completion Boosts Oil Rate While Disposing of Produced Water
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This paper aims to provide an introduction to the early management of downhole produced water in strong waterdrive reservoirs using inverted electrical-submersible-pump (ESP) technology. This technology facilitates a method of downhole water disposal within the same wellbore and, therefore, reduces the costs associated with bringing water to the surface or subsurface for treatment. The technique also saves the costs needed for drilling disposal wells.
Problems Associated With Produced Water
Water produced with petroleum is growing in importance from an industrial and environmental standpoint. In the past, this water was considered to be waste and required disposal. Early on, little attention was paid to the fate of the produced water in the environment. Later, it became clear that possible contamination from produced-water disposal, especially on the surface, needed to be considered.
This unwanted water is also a limiting factor in the productive life of the well. Many factors influence the drive for improved water control—loss of hydrocarbon production, environmental effect of disposal, government regulations, and public opinion. The environmental issues and costs related to produced water and its disposal are becoming major considerations for producers. The economic factors of reducing water production far outweigh the cost of typical water-control treatments. Historically, water-control treatments have often failed because of one or more of the following problems: the source of the problem was not properly identified, the wrong treatment was carried out, or the correct treatment was performed improperly.
For disposal or injection into reservoir rocks, this water must be processed and treated through a series of time-intensive and costly operations. Suspended solids and oil must be removed to an appropriate degree to reduce oil losses and prevent plugging of the disposal formation. Also, increasingly stringent regulations on the entrained and dissolved oil and other chemicals in the produced water must be met.
Economics of Produced Water
Except in the case of gas production from coal seams, water-production rates usually start slowly from the initial development of a property. Facility designers may forestall construction and installation of water-handling equipment deliberately at the beginning of a project to reduce initial capital costs. The eventual appearance of water production requires the addition of capital investment and operational expense to handle the growing water rates, which do not generate revenue to offset the cost. The natural tendency for companies is to minimize the immediate expense; as a result, companies often underdesign the equipment or fail to budget properly for operational expenses.
The general objectives for operators treating produced water are deoiling (removal of dispersed oil and grease), desalination, removal of suspended particles and sand, removal of soluble organics, removal of dissolved gases, removal of naturally occurring radioactive materials, disinfection, and softening (to remove excess water hardness). To meet these objectives, operators have applied many standalone and combined physical, biological, and chemical treatment processes to manage produced water.
Challenges and Opportunities
Water coning has long been a problem in the development of hydrocarbon waterdrive reservoirs and remains a major challenge in achieving maximum ultimate recovery. Several technologies are used currently in the oil industry to mitigate excessive production of formation water from strong waterdrive reservoirs while simultaneously maximizing hydrocarbon rates and recovery. These technologies include
- Partial perforation of the oil column
- Creating a low-permeability or nonpermeable barrier between the water cone and the upper oil column by injecting resins, polymers, gels, or cement across the water-flushed perforations
- Mechanical isolation of water-flushed perforations within the wellbore
- Horizontal completions with inflow control devices (ICDs)
To date, horizontal ICD completions have shown the highest success rate in delaying water-cone progression to the wellbore but exhibit little control when the water cone eventually reaches the wellbore.
Cone Development and Control
Cone-control technology has the fundamental principle of creating a counteractive pressure drawdown (ΔP) immediately below the oil/water contact (OWC) within the water leg equal to or marginally more than the ΔP across the perforated interval within the oil leg. This drawdown retards the progression of the water cone into the oil column and maintains the highest relative-permeability-to-oil (Kro) condition throughout the life cycle of the producing interval.
For high-permeability reservoirs (Kro greater than 1 Darcy), a more-effective technology, though not new, is being developed to maximize oil-production rates while reducing produced water significantly and improving ultimate hydrocarbon recoveries concurrently. This technology uses a cone-control completion in a vertical or deviated well to create a counteractive pressure drawdown immediately below the OWC in an effort to control the progression of the water cone.
In 1991, a numerical model and field data were used to evaluate well performance for coning control using a dual-completion downhole water sink (DWS). The conclusion was that the pressure sink would control water coning and produce more oil with less water than conventional wells. Several short-term field trials were conducted with completion designs consisting of the ESP equipment currently available. These projects were aimed at testing the principle and feasibility of DWS. Typically, operators would install DWS in old wells that had been producing with high water cut for a long time. In such cases, the advanced water-flushing of the oil perforations usually shifted the Kro/relative-permeability-to-water relationship to an irreversible level at which even the best cone-control efforts could not improve ultimate recovery significantly.
In Kuwait, single-well models were simulated using this cone-control concept and yielded very favorable results. A water cone retarded by the counteractive drawdown applied immediately below the OWC yielded improved hydrocarbon-sweep efficiency and ultimate recovery.
Cumulative oil recovery was shown to increase by approximately 100% over a 3-year production period. This period also matches the industry average of a properly designed ESP system. An added advantage of the counteractive drawdown at the OWC is the capacity to exceed the coning-oil rate significantly and not produce copious amounts of formation water.
Superior Completion Design Using Inverted-ESP System
The inverted-ESP design not only reduced completion complexity but also provided increased counteractive drawdown capability below the OWC. The water-flow path alongside the ESP motor has increased exponentially now that a shroud is no longer necessary. This increased fluid-flow capacity provides increased drawdown capability at the OWC, thereby improving water-cone control and consequently increasing oil production.
Candidate Selection for Cone Control
One of the criteria for the candidate selected for the initial pilot using the reverse-flow Y-tool design was a well with an extensive production history and an advanced water cone. In addition, the candidate had a perforated interval greater than 50% of the original oil column above the OWC. Both of these conditions made for a challenging cone-control application. Despite these challenges, and even with the rate limitation of the reverse-flow Y-tool configuration, the water cut was reduced from 80 to 74% and remained stable at the reduced level until the system developed a tubing leak. The initial pilot was aimed primarily at proving the concept of cone control using the DWS technology, and it did so conclusively.
The fundamental principle of this technology is to establish the water cone first and then control it subsequently, not prevent it. For this reason, before the inverted ESP was energized, the well was kicked off by displacing the tubing to diesel before it was opened up to production. Choke sizes were increased gradually until a water cut was initiated and were allowed to increase further to approximately 36%. Surprisingly, the cone was established within hours, not days or weeks as previously anticipated. Fig. 1 shows a detailed response of the natural flow from the oil perforations before and after the ESP was energized.
Immediately after energizing the ESP, the water cut decreased almost instantaneously and significantly, from 38 to 14%, clearly indicating an immediate arrest of further cone development and actually effecting cone reversal. The reason for keeping the water cut present at a level higher than zero was to guarantee that no oil was pulled oil into the aquifer. In other words, the intent was to control the cone with this configuration and not reverse it.
To eliminate the chance of losing the water cut altogether in the oil-production interval and risk reversing the cone completely, the choke was increased immediately to offset the strong counteractive effect of the ESP drawdown.
The immediate water cut increase after generator trips and, similarly, the immediate water cut decrease after ESP startups and frequency increases confirmed that the drawdown at the OWC effected by the inverted ESP had remarkable control on water-cone development.
- The inverted-ESP cone-control completion created a superior and effective countersink drawdown that controlled water-cone progression, causing a significant reduction in produced water.
- Significantly higher oil-production rates above the critical coning rate now can be achieved.
- Cone control using inverted ESPs also can increase ultimate recovery from strong waterdrive reservoirs significantly without generating massive volumes of produced water.
- Disposal of countersink water to the underlying aquifer reduces the production of formation water and the associated processing, handling, and disposal costs markedly.
- Disposal of countersink water can be used for pressure support where applicable, thereby saving on associated capital and operating costs of injectors.
For a limited time, the complete paper SPE 187624 is free to SPE members.
Inverted-ESP Completion Boosts Oil Rate While Disposing of Produced Water
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