Bài giảng môn Khoan-Khai thác dầu khí - Bài 7: Well completion and stimulation

ormation is tested and evaluated.  To complete the production well, casing is installed and cemented, and the drilling rig is dismantled.  A service rig is brought in to perforate the production casing and run production tubing along with downhole equipments.  Production begins after surface safety equipment installation inished. INTRODUCTION 30’’ CASING 20’’ CASING 13 3/8’’ CASING 7’’ LINER RESERVOIR SEA BED PLATFORM Production casing (9 5/8) GEOPET Well Completion and Stimulation  5 WHAT IS COMPLETION? Well completion creates a dependable pathway to the surface for the hydrocarbons. The term ‘completion’ describes the assembly of downhole tubulars and other safety equipments that is required to enable the safe and efficient production of oil or gas from the well after it has been drilled. GEOPET Well Completion and Stimulation  6 BASIC WELL COMPLETION TECHNOLOGY  Each drilled wellbore awaiting completion is unique. Even nearby wells drilled to the same reservoir can have differencies in:  depths,  formation characteristics,  and hole sizes  A wide variety of equipment designs and procedures have been developed to provide safe, efficient conduits from subsurface reservoirs to the surface in different situations.  The ideal completion design  minimizes initial completion and operating costs,  providing for the most profitable operation of an oil or gas well over its entire life. GEOPET Well Completion and Stimulation  7 Natural Completions Natural completions are those in which little or no stimulation is required for production. Sandstone and carbonate systems with good permeability and mechanical stability are ideal for natural completions. Stimulated Completions These completions are generally applied to improve the natural drainage patterns of hard, low-permeability formations. It is used to remove barriers that prevent easy passage of fluids into the wellbore. Sand-Control Completions Sand-control completions support the formation while allowing the flow of fluids. They are performed in young, unconsolidated or less mechanically competent sandstones. TYPE OF COMPLETION GEOPET Well Completion and Stimulation  8 TYPE OF COMPLETION The design of a particular completion depends on: 1. The number and type of productive zones, 2. The expected pressures and flow rates, 3. The need to control sand production, 4. The need for artificial lift or stimulation the regulations governing operations in the area. GEOPET Well Completion and Stimulation  9 WELL COMPLETION ACTIVITIES Well completion activities include:  Conducting well test  Setting production casing  Running production tubing along with downhole equipments  Installing surface safety equipments  Starting production flow GEOPET Well Completion and Stimulation  10 BASIC COMPLETION METHODS  Once we drill to the target and evaluate our well by  Mud analysis: density & viscosity  Well logging (electrical, ascoustic, nuclear, etc)  Coring: at bottomhole or sidewall  Welltest: bottomhole pressure vs time -> reservoir properties Next decision is whether to complete or abandon it????  In the latter case:  set a cement plug or plugs in the hole,  possibly recover whatever casing can be removed,  and return the drill-site to its original condition.  The more fortunate is one in which our well not only is productive, but economically justifies a completion. GEOPET Well Completion and Stimulation  11 The next step usually involves the running of the final string of casing - the production string. The manner in which this is done determines the basic completion method and may follow one of several configurations: (interface between the wellbore & reservoir)  the openhole completion,  the liner completion,  the cased and perforated completion • Without liner • With liner BASIC COMPLETION METHODS GEOPET Well Completion and Stimulation  12  The openhole completion: the producing formation is not isolated by the casing, which extends only to the top of the producing interval.  The slotted liner completion: which is not cemented and not "tied back" to the surface. BASIC COMPLETION METHODS GEOPET Well Completion and Stimulation  13  The cased and perforated completion  Without liner: cementing the production casing across the productive interval and then perforating the casing for production  With liner: a liner is cemented and perforated as a cased and perforated completion BASIC COMPLETION METHODS GEOPET Well Completion and Stimulation  14  One of these configurations will be the basis for the completion design, which may incorporate:  one or multiple strings of tubing: single, dual, or triple, etc  and a variety of tubing components to facilitate production (production method): pumping, flowing, etc.  from one or multiple zones: single or multiple zones  For our purposes, a cased and perforated well with a single tubing string will serve to illustrate the typical completion procedure. BASIC COMPLETION METHODS GEOPET Well Completion and Stimulation  15 Subsea production systems are wells located on the sea floor, as opposed to at the surface. The safety equipments are installed underwater on the seabed. They enable early production from deepwater, remote, and marginal fields. Subsea production system offer a means of producing field extremities not reachable by directional drilling from existing platforms, or where field economics do not justify the installation of one or more additional platforms. SUBSEA COMPLETION GEOPET Well Completion and Stimulation  16 2. COMPLETION PROCEDURE GEOPET Well Completion and Stimulation  17 COMPLETION PROCEDURE  After the contract casing crew runs the final casing, cementing follows the usual procedure, although stage cementing may be necessary to cement an extremely long string.  The production string has been hauled out to the location and the inside diameter checked to make sure that imperfections will not prevent the subsequent running in of tubing and packers after the string is set. GEOPET Well Completion and Stimulation  18  Special care: to prevent the possibility of future leaks.  If stage cementing is necessary:  the bottom section is first cemented in place and then  a series of plugs are pumped down the casing to open ports that allow the upper end of the annulus to receive cement.  After the cement has set, the inside of the casing must be  drilled out and  flushed clean of cement and other debris to a depth below that of the proposed completion.  It is important that the inside diameter of the production casing be clean and smooth. COMPLETION PROCEDURE GEOPET Well Completion and Stimulation  19 COMPLETION PROCEDURE It is also important that the cement form a competent seal between the casing and borehole over the entire openhole interval. To ensure this,  an acoustic cement bond log is sometimes run on wireline  to determine if voids exist between casing and hole because cement has bypassed the drilling fluid. GEOPET Well Completion and Stimulation  20 COMPLETION PROCEDURE  If the bond is poor in an area, particularly if the area is between productive formations, a cement squeeze will be required.  Often the cement bond log is run in conjunction with a gamma ray log and a casing collar log. The drilling engineers can correlate this gamma ray log with the logs run earlier during formation logging.  This correlation is important because as we zero in on the target - the productive formation - our need to locate tools precisely relative to that formation is critical.  The open hole logging sondes are subjected to a greater amount of "drag" when being pulled up the hole, the depths at which formations are recorded may differ somewhat from the formation depths on the gamma ray log run inside the casing. GEOPET Well Completion and Stimulation  21 COMPLETION PROCEDURE  If we were to perforate the casing according to the openhole log depths, we might miss the formation entirely. By using the correlation log and casing collar log to set packers and perforate, we are assured of precise placement.  At this point, many operators  move the drilling rig off location and  replace it with a less expensive, and often less powerful, completion rig.  This gives the operator time to design the rest of the completion, provide for a sales contract, and order equipment. GEOPET Well Completion and Stimulation  22 COMPLETION PROCEDURE Whichever rig is used, the next step in the completion is to measure the tubing while running it into the hole.  A careful count must be kept of the exact number of tubing joints run into the hole and their total length.  With the tubing in the hole, the BOP stack, which is now attached above the tubing head where the tubing will hang, may be tested.  The casing may also be pressure tested,  and a filtered completion fluid may be circulated into the well to displace the drilling mud prior to perforating.  This fluid is usually a heavy brine, because it: o provides the hydrostatic pressure needed to control the well, o does not contain solids that can plug the perforations and damage the formation. GEOPET Well Completion and Stimulation  23 COMPLETION PROCEDURE  If perforating is to be done at this point,  the tubing is removed and  the perforating gun is lowered and positioned according to the correlation log and casing collars.  It is critical that the gun be placed precisely;  once inaccurate perforations are made, they can only be plugged off with a costly cement "squeeze." GEOPET Well Completion and Stimulation  24 COMPLETION PROCEDURE  With the well perforated, it may now be time to stimulate the well by either  acidizing or  hydraulically fracturing the formation.  Acid can be used to dissolve formation-damaging particles left by the drilling mud or, in carbonate formations, to create flow passages by dissolving portions of the rock itself.  Hydraulic fracturing involves the high-pressure pumping of fluid into the formation to split the rock apart and increase its flow capacity of tight formations. GEOPET Well Completion and Stimulation  25 COMPLETION PROCEDURE Normally, a completion packer is run and set next, either incorporated into the tubing string or set independently on electric wireline.  The packer is pressure tested to ensure its sealing ability. (Many shallow, low pressure wells, however, do not require a packer to isolate the casing from produced fluids.)  The tubing must then be "spaced out."  This requires that a length of tubing be removed from the upper end so that it can be "landed" in the tubing head, which is some distance bellow the rotary table.  Once the tubing has been landed in the tubing head, a temporary plug can be set inside the tubing while the BOP stack is removed and the surface flow control equipment ("Christmas tree") installed.  This plug is then removed through the Christmas tree, and the well is completed. GEOPET Well Completion and Stimulation  26 COMPLETION PROCEDURE  Of course, this procedure will vary according to  the specific brands of equipment being installed,  the characteristics of the well,  and the policies of different companies,  but the essential sequence of operations will be followed.  One variation is the procedure for perforating, which may be done after the tubing has been run.  This approach allows the formation to be perforated and immediately "cleaned up" by allowing it to flow as soon as the perforations are created. GEOPET Well Completion and Stimulation  27 COMPLETION PROCEDURE The rig will often be moved off location at this point, allowing the well to brought on production. On an offshore platform, the rig may be skidded to the next well slot.  If a rod pump is required on the well, it may be installed at this time and the necessary rods and downhole pumping mechanism run into the tubing.  If gas lift valves have been incorporated into the tubing string, gas may be used to blow the completion fluid out of the tubing and permit the well to flow on its own. In some cases, the well will be "swabbed in" at this point, by running a close-fitting plunger into the tubing on wireline and pulling it back up, thereby displacing the completion fluid in the tubing and allowing the formation to flow. GEOPET Well Completion and Stimulation  28 COMPLETION PROCEDURE After an initial well test, which may be conducted with temporary test facilities, the flow line needed to produce the well on a continuous basis will be connected. GEOPET Well Completion and Stimulation  29 3. PERFORATING GEOPET Well Completion and Stimulation  30 PERFORATING  The use of cemented steel casing to line the wellbore and isolate producing zones is only practical when a method for easily reopening those zones for production exists.  Jet perforating is the procedure whereby an explosive charge is used to selectively open passages to the formation through the casing and cement sheath. This method:  the most widely used today, because of its versatility and power.  Having evolved from the military bazooka, the jet perforator relies on a conical-shaped charge of explosives to produce a high pressure stream of particles.  Bullet perforators fire metal projectiles at the inside of the casing to penetrate casing, cement, and rock.  This technique has pressure, temperature, and penetration limitations that have made jet perforating the more popular choice for completions. GEOPET Well Completion and Stimulation  31 PERFORATING  Jet perforating guns consist of  a carrier with a series of explosive charges linked together by a detonating cord.  A variety of gun designs exist; they vary according to: - the gun is to be run on an electric conductor line or attached to the bottom of the tubing; - the gun is to be run through the casing on electric line or tubing, or is to be lowered through the tubing on electric line; - the gun is retrievable following detonation or is expendable (meaning it is destroyed when the gun is fired); - the diameter and length of the perforation desired. GEOPET Well Completion and Stimulation  32 PERFORATING  Wider, longer perforations require  larger, stronger jet charges,  and, larger guns to hold them.  The charge itself is held in a metal case that is linked to similarly shaped charges by a detonating cord ending in an electric detonator.  When the gun is fired, an electric current from the surface sets off the blasting cap detonator, which secondarily ignites the detonating cord leading to the main explosive charges. GEOPET Well Completion and Stimulation  33 PERFORATING When a charge is fired  The metallic liner collapses to form a stream of high pressure, high velocity jet particles.  Traveling at 30,000 ft/sec (9100 m/sec), the jet stream strikes the casing at some 15x106psi (100x 106kPa) a fraction of a second after detonation, to form a perforation. a) before detonation b) after detonation, showing collapsing liner & swelling casing. c) volatilizing metal liner and formation of particle d) jet lengthens as process continues GEOPET Well Completion and Stimulation  34 PERFORATING  Retrievable hollow carrier guns have cylindrical steel bodies with closed ports opposite each jet charge.  Fully expendable guns enclose the charges in a frangible aluminum or ceramic case that disintegrates on firing.  while semiexpendable guns consist of wire or metal strip carriers that are retrieved after firing.  Through-casing and through-tubing guns of these types differ primarily  in the diameter of the gun • 3 to 5 inches [7.6 to 12.7 cm] for casing guns, • 1 to 2 inches [2.5 to 5.1 cm] for tubing guns  and in the size of the jet charges. GEOPET Well Completion and Stimulation  35 PERFORATING Type of perforating guns • Retrievable hollow carrier guns • Fully expendable guns • Semi expendable guns GEOPET Well Completion and Stimulation  36 PERFORATING GUNS GEOPET Well Completion and Stimulation  37 PERFORATING As mentioned earlier, perforating can be carried out in several different ways:  Conventional overbalanced perforating is done through casing with an electrical conductor line and heavy fluid in the hole. This completion fluid is usually a low-solids solution of sodium or potassium chloride, or sodium or potassium bromide.  Conventional underbalanced perforating is usually carried out after tubing has been run and equipment is installed to control the sudden pressure surge when the higher pressure formation is opened to the lower pressure wellbore. GEOPET Well Completion and Stimulation  38 PERFORATING  For a typical formation the difference between wellbore and formation pressure may be 300-500 psi (2000-3500 kPa).  For a low permeability formation, the typical difference between wellbore pressure and formation pressure may be 2000 psi (13,800 kPa) or higher. -> the immediate surge of formation fluids to prevent the clogging of the perforation tunnels with debris.  When a maximum pressure differential is desired, a tubing-conveyed perforating gun may be used.  it is possible to have the tubing run empty with a ported vent, which opens when the packer is set.  After firing, the gun component of the tubing is released with a wireline shifting tool to allow full flow into the tubing. GEOPET Well Completion and Stimulation  39 PERFORATING  In addition to perforation diameter and length, two important considerations in all types of perforating are  the shot density  and phasing of the perforations.  The shot density, or shots per foot, is usually 2, 4, 8,12, or 16 holes in each foot of perforated interval. GEOPET Well Completion and Stimulation  40 PERFORATING  Phasing pertains to the direction of each successive shot relative to its neighbors;  if each charge is pointed 90 away from the next, we have 90 phasing.  In the case of 180 phasing, each shot points directly opposite from the next one in the carrier.  Gun phasing can be particularly important when perforating a fractured well, a highly deviated well, or a multiple completion, where the gun must be oriented to avoid perforating an adjacent tubing string. GEOPET Well Completion and Stimulation  41 PERFORATING  The decision about the interval to be perforated is often made by the geologist or by the engineer and geologist responsible for the area in which the well is drilled.  Consideration will be given to maximizing flow rate and minimizing production problems such as produced sand, water coning, or excessive gas production in an oil well.  The decision is often made after careful review of the log and core data back at the company office.  The geologist's input concerning net pay, sidewall core descriptions, and the areal extent of sand intervals can be crucial in determining the best interval to be perforated.  One of the advantages of the cased and perforated completion: ability to selectively stimulate specific formations. GEOPET Well Completion and Stimulation  42 MULTI-LAYER PERFORATION GEOPET Well Completion and Stimulation  43 4. Stimulation • Acidizing • Fracturing GEOPET Well Completion and Stimulation  44 STIMULATION  In many cases,  acidizing  or fracturing is a routine part of the completion program.  Either type of stimulation may also be applied soon after a well  has been completed and  has tested at lower production rates than expected.  Stimulation may also be part of a remedial or "workover" program designed to improve productivity following a decline in production. GEOPET Well Completion and Stimulation  45 STIMULATION  Stimulation will often follow a formation pressure buildup test that was run to determine if the cause of low productivity was  permeability reduction near the wellbore,  low permeability throughout the reservoir,  or low reservoir pressure.  Acid stimulation can improve the first condition,  while fracturing is necessary to significantly improve the second condition.  Of course, the third condition can only be helped by pressure maintenance. GEOPET Well Completion and Stimulation  46 STIMULATION  Both acidizing and fracturing procedures involve the pumping of fluids down the tubing or drillpipe and into the formation.  In fracturing, the objective is to apply enough pressure to actually split the formation apart, thereby enhancing its flow capacity.  In acidizing  sandstone formations, the objective is to squeeze acid into the existing pore spaces of the rock matrix; this improves productivity by removing formation damage and dissolving clay particles.  In carbonate formations, acid treatments are designed to enhance permeability by actually dissolving part of the rock matrix. Acid-fracturing treatments are designed to create fractures that are simultaneously widened by acid dissolution. GEOPET Well Completion and Stimulation  47 Acidizing GEOPET Well Completion and Stimulation  48 ACIDIZING  Successful acidizing involves more than simply  pumping acid down the well  and allowing it to dissolve part of the formation.  The type of acid used, the chemicals added to improve its efficiency, the volumes pumped, and the pumping pressures maintained are dependent on  the characteristics of the reservoir rock and fluids  and the configuration of the well. Acidizing GEOPET Well Completion and Stimulation  49 ACIDIZING  Hydrochloric acid (HCl) is the most common chemical used in acidizing.  A solution of 15% HCl by weight is most often used in limestone or dolomite formations,  while a mixture of 12% HCl and 3% hydroflouric (HF) acid is often used on sandstone formations with interstitial clays, particularly in areas such as the Texas and Louisiana Gulf Coast of the United States.  Organic acids, such as acetic acid, or formic acid, are also sometimes used. GEOPET Well Completion and Stimulation  50 ACIDIZING A variety of additives help the acid work more efficiently.  Inhibitors prevent the acid from attacking the steel tubing and casing at high bottom hole temperatures.  In some applications, retarders can prevent the acid from spending quickly on the first formation rock it encounters, allowing the acid to be pumped further into the formation.  Surfactants added to the acid help prevent acid/oil emulsions from forming and reducing the ability of the fluids to flow.  Because the reaction of acid and iron compounds can create precipitates within the formation, iron sequestering agents are added to control these deposits. GEOPET Well Completion and Stimulation  51 ACIDIZING Some acid treatments are even designed to generate acid within the formation, again allowing deeper penetration of active acid.  In acid fracturing, it is important to keep the acid from leaking away as a fracture spreads out from the wellbore. Fluid loss agents can be added to keep the acid inside the fracture and allow it to penetrate farther into the formation.  Temporary plugging agents are also added, during matrix acidizing jobs, to divert the acid into different layers of the formation and improve overall permeability. GEOPET Well Completion and Stimulation  52 ACIDIZING  Preflush fluids designed to prepare the formation for  the acid,  the acid plus its additives,  and the displacing fluid that follows the acid, are all pumped at rates ranging from less than one barrel per minute to perhaps more than ten barrels per minute. GEOPET Well Completion and Stimulation  53 ACIDIZING  The actual rates will depend on the calculated fracture pressure required to split the formations, and whether a matrix or fracture treatment is preferred.  Volumes of 50 to 200 gallons of acid per vertical foot of formation are typical for most reservoirs, depending, of course, on the porosity and rock type.  The acid solutions are delivered to the wellsite in specially lined tanks brought by truck to land locations and delivered by boat to offshore wells. GEOPET Well Completion and Stimulation  54 ACIDIZING  High-pressure piping is connected to the well and the acid is pumped down the hole ( Figure ). GEOPET Well Completion and Stimulation  55 ACIDIZING  The size and configuration of the tubing in the well is important in calculating the volume of fluid required to completely displace the acid into the formation.  If gas lift valves or other points of communication exist between tubing and casing, precautions must be taken to ensure that acid is not  pumped into the casing/tubing annulus  and allowed to corrode the casing. GEOPET Well Completion and Stimulation  56 Fracturing GEOPET Well Completion and Stimulation  57 FRACTURING  As early as 1900, oil producers used explosives to "shoot" wells.  By detonating nitroglycerin opposite the producing formation,  the wellbore was enlarged and  the surrounding rock shattered.  As would be expected, this technique was  dangerous and  often damaging to the casing. Hydraulic fracturing GEOPET Well Completion and Stimulation  58 FRACTURING  In the 1940s: the inadvertent splitting of the strata during drilling or cementing might be purposefully carried out in a potentially productive formation to increase permeability.  The idea was  to pump fluid into a cased and perforated wellbore until the hydraulic pressure caused the formation to part;  continued pumping would force the fluid into the fracture,  propagating the fracture farther and farther from the wellbore. GEOPET Well Completion and Stimulation  59 FRACTURING  Early researchers realized that the fracture would close once the hydraulic pressure was relieved,  a solid material is added to the fracturing fluid to "prop" open the fracture.  Initial jobs consisted of perhaps 500 to 1000 gallons of gelled kerosene (napalm) as a fracturing fluid, with perhaps 1/2 lb of sand per gallon (Neely 1977).  These early fractures were assumed to be horizontal, following the bedding planes of the rock. GEOPET Well Completion and Stimulation  60 FRACTURING  Since that time, an enormous amount of research and field application of fracturing techniques has been carried out.  Theoretical mathematical models have been developed that permit engineers to predict the type of fracture and productivity increase that will result from a certain magnitude fracture treatment.  These calculations prevent the unnecessary use of enormous amounts of  costly fracture fluid,  proppant material,  and equipment horse power by tailoring the treatment to the particular well. GEOPET Well Completion and Stimulation  61 FRACTURING  Although there is still some disagreement among theorists concerning the behavior of rock under stress, we now know that:  Fracture orientation is dependent upon geologic conditions,  And that most fractures are vertical rather than horizontal. GEOPET Well Completion and Stimulation  62 FRACTURING  In order to significantly improve a well's productivity, a fracture must conduct fluid at a rate that is several orders of magnitude greater than the conductivity of the rock itself.  Creating a high-conductivity fracture involves  selecting the appropriate fluid, additives, and proppant,  determining the optimum volume of material to be pumped,  pumping the material at the appropriate rate and pressure. GEOPET Well Completion and Stimulation  63 FRACTURING  Desirable features for a fracturing fluid include  the ability to remain in the fracture and not leak off into the formation,  the viscosity necessary to transport the proppant into the fracture,  the ability to flow back into the well easily after depositing the proppant,  and low cost.  Water-based polymer solutions are popular, as are gel led hydrocarbons for water-sensitive formations.  A wide variety of additives are available to reduce fluid friction in piping, prevent fluid loss from the fracture, control contamination, and insure compatibility with the formation. GEOPET Well Completion and Stimulation  64 FRACTURING  The standard proppant used to hold open the fracture is silica sand.  Sand can be crushed, however, in deep formations where fracture- closure stresses are high.  In such cases sintered bauxite, zirconium oxide, or other high- strength materials are substituted for sand.  The goal is to create at least a partial monolayer of proppant within the fracture, holding the fracture open, but not plugging it completely. GEOPET Well Completion and Stimulation  65 FRACTURING  High-strength proppant forms a single layer of particle that holds open the fracture and permits flow GEOPET Well Completion and Stimulation  66 FRACTURING  At the wellsite, the equipment required for a large fracturing job is somewhat more sophisticated than that required for an acid stimulation.  The fracturing fluid is held in tanks, where any necessary additives are mixed.  Proppant is sorted in similar containers, from which it is conveyed to high-rate blenders.  Blend