well completion and its types. PDF

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Well completion What are Completions? Completions are the interface between the reservoir and surface production. The role of the completion designer is to take a well that has been drilled and convert it into a safe and efficient production or injection conduit. Completion planning involves choosing and organizing the equipment to be used, selecting materials, establishing production line tubing dimensions, stipulating production intervals, and finally defining the mode of formation fluid production. Completion design is a mix of physics, chemistry, mathematics, engineering, geology, hydraulics, material science and practical hands-on wellsite experience. The best completion engineers will be able to balance the theoretical with the practical. Completion designs are based on raw data (e.g. measured reservoir pressure) or predictions (e.g. production profiles).

Fig: Data sources for completion design. MAIN FACTORS INFLUENCING COMPLETION DESIGN: There are many factors that influence completion design:  The well’s purpose  The environment  Drilling  The reservoir  Production  Completion techniques

WELL PURPOSE (TYPES OF WELLS) Based on purpose there are different types of wells: 1. Wildcat wells: Wildcats are drilled where little or no known geological information is available. The site may have been selected because of wells drilled at some distance have same geological features or based on analogy. This higher degree of uncertainty necessitates that the drilling crews be appropriately skilled, experienced and aware of the different types of parameters that are obtained from drilling. 2. Exploration wells: An exploratory well is drilled with the intent to discover a new petroleum reservoir. For such wells, site selection is done based on seismic data, satellite survey etc and no drilling data available about the prospective horizon. The prime objective of this well is to define  the nature of the fluids (water, gas or oil) in the reservoir rock  The characteristics of the pay zone and mainly the initial pressure, temperature and permeability. To obtain these data, it requires a number of measurements by means of tools lowered into the well on an electric cable (wireline logging) and running a temporary test string in order to carry out production testing(Drill stem testing). Well testing data of the well will determine whether to suspend the well for some period or to prepare abandonment program. The information obtained from this exploration well will help to complete the data already available from geology and geophysics. The combined data will help to decide whether to develop the reservoir or to drill one or more further wells to obtain additional information. 3. Confirmation / appraisal / delineation l/ step out well: wells drilled after a field is discovered by exploratory well, to determine filed extent (size of the reservoir). The purpose of these wells is to complete the data obtained from exploratory well. These wells help to determine:  Off wellbore permeability ( permeability of the formation away from wellbore)  Existence of the heterogeneity, discontinuity or faults  Reservoir boundaries In order to obtain these data, well testing is carried out for a longer period than for an exploratory well. All the data obtained from these wells will help to make decision whether to develop the field or not. If the answer is yes, next step is to prepare development plan with corresponding production forecasts. 4. Development wells: A development well is drilled in a proven area for the production of oil and gas. The main aim of these wells is not to make more measurements but to extract the reservoir fluids with maximum flow capacity. However, it is important to test this type of wells to assess the condition of the well and to check how effective the completion has been. There are different types of development wells:  Production well: they are numerous in number, their main aim is to optimize the productivity to price ratio.  Injection wells: they are less in number; they are used to inject water or gas to maintain reservoir pressure.

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 Observation wells: they are very few in number or sometimes none in a field. They are used to monitor variations in reservoir parameters (pressure). Infill wells: Drilling or adding new wells in between existing wells (production or injection wells). Site selection is done based on existing well patterns. Re-entry wells: Existing wells reentered to deepen sidetrack, multilateral drilling and rework. Various amount of planning required based on types on works.

Well completion Well Completion, in petroleum production, is the process of making a well ready for production (or injection). This principally involves preparing the bottom of the hole to the required specifications, running in the production tubing and its associated down hole tools as well as perforating and stimulating as required. Purpose of well completions: The purposes of a well completion are to 

Connect the reservoir to the surface so that fluids can be produced from or injected into the reservoir

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Provide a conduit for well stimulation treatments

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Isolate the producing reservoir from other zones

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Protect the integrity of the reservoir, especially in unconsolidated formations.

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Provide a conduit to measure the changes in flow rate and pressure needed to run a well test

Completion methods: 1) Open hole completion: In the open-hole type of completion, casing is set only to the top of or slightly into the completion interval Because the open-hole completion relies on the strength of the rock itself to support the wall of the hole, it has greater application in carbonate and basement formations.

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Figure: Wellbore diagram of an open hole completion. Advantages of open hole completion: Casing set at the top of the pay zone which minimizes formation damage. Full-hole diameter available to flow. No perforation is required. High productivity maintained when gravel packed for sand control. Disadvantages of the open-hole completion are: No way to regulate fluid flow from or into wellbore. Cannot control gas or water production effectively. No selective gas or water shut off job is possible. Wellbore may require periodic cleanout. Formation collapse is more common.

2) Cased and perforated completion: In the cased and perforated completion, casing is set into or through the producing formation and cemented. The casing is then perforated to provide communication between the wellbore and formation.

Advantages of the cased and perforated completion include:  Ease of selective completion and workover operation in the producing intervals.  Can effectively control gas and water production by selectively perforating and isolating.  Can effectively control and monitor zonal fluid production.  Permits multiple completions  Can stimulate selectively.  Can be adopted to sand control, both as pre-pack gravel pack or conventional gravel pack. Disadvantages of the cased and perforated completion are:  Long interval perforation can be expensive  Effective wellbore diameter and productivity may be reduced.  Good cement job through production intervals is required.  More expensive than open hole.

3) Liner completion: In a liner completion, casing run is upto the top of the pay zone and a liner is set across the producing formation. Features of this type of completion are: The liner may be or may not be cemented. The cemented liner is perforated whereas uncemented liner is slotted. This type of completion is primarily applied to sand control, but could be used to control a sloughing formation. Completions using cemented liners to reduce casing costs are essentially considered cased and perforated completions. The advantages and disadvantages of the un-cemented liner completion are the same as the open-hole completion.

Figure: Wellbore diagram of (a) a slotted liner completion and (b) a cement liner completion. 4) Tubingless completion: The tubingless completion method does not use tubing. This is the simplest tubular arrangement and may be used in high-rate oil or gas wells where pressures and corrosive fluids are not detrimental to the casing. This application is used for developing fields with several reservoirs and is not limited to developing marginal producing properties.

Figure: Tubingless completion Advantages of tubingless completions are:

 Simplest completion operation  Individual zone control and completion are separately done. Tubingless completions have the following disadvantages:  Casing is subjected to pressure & temperature fluctuations resulting in frequent stress changes  Excellent cement job required  Difficult & expensive perforating procedures (gun orientation).  Possibility of losing entire production from one zone in light of repeated workover operations.  Paraffin and corrosive fluids are serious problems. 5) Casing with suspended tubing completion: Conventionally, wells are often produced with the tubing freely suspended in the well-bore. The completion with tubing is needed to:  Permit workover kill fluid circulation.  Prevent casing being subjected to paraffin and corrosive fluids.  Provide controlled flow path for producing /injection fluid.  Application of artificial lift becomes feasible. 6) Tubing and packer completion: Wells completed with a tubing and packer permit a number of tubing and casing configurations. Use of a packer increases completion costs, adds complexity to the system and decreases reliability. Packers are usually run for the following reasons:  To isolate the casing from corrosive fluids and/or high pressure (extra safety in the completion).  To stabilize and control flow from pay zones.  To selectively produce multiple zones (isolation).  Most flexible for well control using packer and subsurface safety valve.  Selective stimulation becomes feasible.  Wireline and downhole operations become feasible.  A variety of single zone completions are possible to meet many production requirements like putting the well on pump, gas lift, chemical injection etc. 7) Single String-Single Packer: One pay zone is flowed through tubing and the other pay zone through annulus. Its disadvantages are:  Casing is subjected to pressure, paraffin and corrosion.  Only one zone can be artificially lifted.  Upper zone sand production may stick tubing/cause packer unsetting problem.  Upper zone can't be produced through tubing unless lower zone is blanked off or commingled. 8) Commingled completion: In this multiple completion method, all zones produce through single string (one by one). Proration of flows amongst different layers is made possible by installation of down hole choke devices. This type of completion has the following advantages:  More than one zone is produced or injection is done through a single well.

 Wax deposition problem of one zone's production may be mitigated\ reduced by mixing it with other wax-free zone(s).  In some cases, one zone could help in the lifting of the other zone. The disadvantages are:  Layer wise production monitoring/accounting difficulty and surface monitoring yields only commingled stream data.  Selective killing, selective stimulation is very difficult.  For routine testing of one zone, other zone(s) has to be closed. 9) Multiple string completions: Dual completion (two packers and two tubing strings), and triple completion (two or three packers and three tubing strings).

These have the following advantages:  Possible to produce from/inject into more than one production / injection zone through a single well, thereby reducing overall development costs.  Selective zone well control is possible.  Use of natural energy (gas) from one zone to artificially produce another zone is possible. Disadvantages:  The large number of down hole equipment creates problems.  Expensive and more complicated completion and workover techniques.  Possible loss of production in zone due to mechanical problems and formation damage during workover....