This report will basically centre on how the followings - Subsea Completions and Workover, Subsea Trees and Subsea Processing are applied to maximize oil production in the Gulf of Mexico.
Well completion involves the installation of a production conduit into which has been incorporated various components to allow efficient production, pressure integrity testing etc [2].
Workover -The recompletion of the well to restore production or change the well function [2] or the process of replacement and maintenance operations on the tools in an oil or gas well.
A subsea tree, also called a “wet tree, is an assembly of control valves, gauges and chokes that control oil and gas flow in a completed well” [2]. The tree also enables methanol and chemical injection, pressure and temperature monitoring and allows vertical access for intervention [2].
The removal of unwanted constituents and recovery wanted constituents is called process of hydrocarbon under a condition of pressure and temperature. Subsea processing is the processing of hydrocarbon fluids on the seabed. Processes of these subsea processing involved include water re-injection, multiphase boosting, phase separation gas compression. Not all processes are done offshore; some are still designed for onshore processing.
The Gulf of Mexico region is the arm of the Atlantic Ocean and is bound on the northeast, north and northwest by the Gulf coast of the United States, on the southwest and south by Mexico, and on the southeast by Cuba [3]. In this region, completions and workovers, subsea trees and subsea processing have been designed to do a particular task. The Gulf of Mexico is richly able with hydrocarbon deposits in deepwater. Below is a picture showing the Gulf of Mexico and the countries around the region.
The Gulf of Mexico [4]
Subsea Well Completion involves all the work done on the well prior to production and the installation of subsurface equipment e.g. tubing hanger, blow out preventer (BOP), etc in order to successfully produce from the well. Completion consists of the lower and upper completion processes. The upper completion involves installation of all the various components from the base of the production tubing right to the top, while lower completion takes place around the production area. Some categories of lower completions are:
2.1 Barefoot Completion: This type of completion is suitable for hard rock, multilerals and under balance drilling. Not suitable for weaker formations requiring sand control and wells that require selective isolation of oil, gas and water intervals [5].
Barefoot completion [8]
2.2 Cased Hole Completion: The portion of the wellbore that has had metal casing placed and cemented to protect the open hole from fluids, pressures, wellbore stability problems or a combination of these. This is also the process whereby a casing is run down through the production zone and cemented in place. This type of completion encourages good control of fluid flow [5].
Cased hole completion [7]
2.3 Open Hole Completion: This type of completion is more advantageous in horizontal wells because the technical hitches and the high cost of cemented liners is associated with horizontal wells [5].
The simplest types of oil well or gas well completion, open hole completions have several limitations and disadvantages. Consequently, they are typically limited to special completions in formations capable of withstanding production conditions [6].
Open hole completion [6]
Perforating Guns: This type of component is used to create predefined pattern of perforation in the sides into the reservoir by means of explosive charges, to allow the flow of oil into the well [9]. An example is shown below.
Perforating gun [9]
2 Wellhead: This is the main component that houses the valves that controls fluid from the well to the manifold. It also acts as an interface between the production facility and the reservoir.
Wellhead [10]
Tubing Hanger: This component is located on the top of the manifold provides support for the production tubing. See picture below.
Tubing Hanger [11]
Production Packer: “This is a standard component of the completion hardware of oil and gas well and it is a seal between the tubing and the casing. It is used to isolate one part of the annulus from another for various reasons”. This is done to separate different sections like the gas lifts section from the production section. It is also used in injection wells to isolate the zones. [12].
Production paker [2]
Production tubing: This is the basic channel through which hydrocarbon flows from the reservoir to the surface. The diagram is seen below.
Production Tubing [13]
Downhole Safety Valve: This is used to protect the surface from the uncontrolled release of hydrocarbons. It is a cylindrical valve with either a ball or flapper closing mechanism; it is installed in the production tubing and is held in the open position by hydraulic pressure from surface [5]. See the diagram below.
Downhole Safety Valve [14]
6 Annular Safety Valve: This is needed to isolate the production tubing in order to
prevent the inventory of natural gas downhole from becoming a hazard. See the diagram below.
Annular Safety Valve [15]
Landing Nipples: This is a receptacle to receive wireline tools. It is also a useful marker for depths in the well, which can be difficult to accurately determine as you can see in the diagram below [4].
Landing Nipples [16]
Downhole Guages: This is an electronic or fibre optic sensor to provide continuous monitoring of downhole pressure and temperature. Gauges use a 1/4" control line clamped onto the outside of the tubing string to provide an electrical or fibre optic communication to surface as shown in the diagram below.
Downhole Guage [17]
Wireline Entry Guide: This component is often installed at the end of the tubing (the shoe). It is intended to make pulling out wireline tools easier by offering a guiding surface for the tool string to re-enter the tubing without getting caught on the side of the shoe.The diagram is shown below [5]
Wireline entry guide [18]
Centralizer: In highly deviated wells, this component may be included towards the foot of the completion. It consists of a large collar, which keeps the completion string centralised within the hole [5].
Centralizer [19]
Mensa field is an example of completions in the Gulf of Mexico. It consists of three wells and gathers gas into a manifold and transports it to West Delta 143 platform 68 miles. See the diagrams below [20].
Subsea development [20] Subsea Production manifold [20]
Well Performance Sensitivities [2]
“Reduced production, scale, tubing and components leaks, artificial lift failures e.g. ESP failure, water shut off and re-perforation, change of well function e.g. producer to injector are some events needed for workover operation on a well” [2]. “A brief summary of the completed workovers in the Gulf of Mexico are:
A-10: Cleared debris and zone was re-perforated. Initial production 140bopd with 10/64 chokes. Well continues to produce at a rate of 140 bopd.
A-2: Cleared debris and oil flowed to the surface followed by emulsions. Currently, the well is being analyzed to determine the appropriate solution needed to liquefy the emulsions so that the well can flow without interruption.
A-16: Cleared debris and re-perforated. Well did not produce from existing zone. Currently under analysis to determine if other zones can be considered as candidates for perforation” [21].
This can be classified into three types based on tree Configuration, tree functionality and
tree Installation.
Schematic of the subsea tree [22]
Horizontal Trees
The following below are the features of a horizontal tree
– “The valves are set off to the side.
– Well intervention can be done through them.
– No valves in the vertical bore
– Tree run before the Tubing Hanger
– Tubing Hanger orients from Tree (Passive)
– Internal Tree Cap installed
– Tubing Hanger seals are exposed to well fluids” [23]
Horizontal Tree [24]
Conventional Dual Bore (Vertical) Trees
Below are the features of a dual bore tree:
– “Master & Swab valves in vertical bore
– Tree run after Tubing Hanger
– Tubing Hanger orients from Wellhead or BOP pin (Active)
– External Tree Cap installed
– Tubing hanger seals isolated from well fluids” [23]
Conventional Dual Bore Tree [24]
A third type is the Mudline tree. These are usually used for shallow water applications and typically installed from jack-up rigs. They have minimal hydraulic functions [24].
Trees generally can either be used on production wells or on injection wells. Thus we have
Production Trees
Injection Trees
Trees can be installed either with Guidelines or Without Guidelines.
Examples of installed subsea trees in the Gulf of Mexico are:
This was used at the Shell-operated Silver tip field, part of the Perdido Development located to set a current subsea deepwater completions record of 9,356ft [25].
Enhanced Deepwater Subsea Tree [26]
This was the world’s first 15,000psig subsea tree. The tree was adapted by Cameron from an existing mono-bore mudline tree, with modified components from its 10,000psig tree design [27].
Gyrfalcon Subsea Tree [27] During Installation [27]
This was to be supplied by FMC in the Blind Faith Development which is located in approximately 7,000ft of water [28].
15k Enhanced Horizontal Tree [28]
Troika oil field, located 150 miles offshore Louisiana in Green Canyon 244 unit and lies in water depth of 2700ft made use of conventional, non-TFL, 10,000psi dual bore 4in×2in configuration, installed using guidelines [29]
“Deployment of subsea processing systems has seen a marked acceleration in the past couple of years, with various separation and boosting systems being ordered for deployment in the North Sea , the Gulf of Mexico, West Africa, South America and Australia” [30]. A driving factor for this is cost. Cost reduction is obvious when large and expensive topside facilities are eliminated for the subsea ones. Other drivers include “flow management and flow assurance, accelerated and or increased recovery, development of challenging subsea fields” [31]. Deployments in the Gulf of Mexico include:
Submerged Production System [32]
“The start-up of Aker’s MultiBooster pump technology at a water depth of 5,500ft below surface is expected to boost BP’s production at the King Field by an average of 20%. The MultiBooster system is a subsea multiphase pump module, combining field-proven twin screw technology with Aker’s suite of processing and subsea technology” [33].
Aker Kvaerner’s MultiBooster [33]
Also in the Perdido development, FMC’s scope of work included the supply of subsea caisson separation and boosting system [34]. The gas/liquid caisson separators with ESPs where used because of the fields low reservoir pressure and heavy oil [31].
Gas/Liquid Caisson separator at 2500m/8200ft water depth
for the Perdido Project [31]
Subsea technology and development in the Gulf of Mexico has improved for ages, this is a result of bringing new innovation to move the industry forward and optimize the abundant natural resources beneath the deep water and different technologies involved. This also makes production activities and exploration and in this region to be more fruitful for both operators as well as the marketers.
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