Method For Monitoring Gravel Packed Wells

Abstract

The effectiveness and competency of a well gravel pack and changes therein are determined by monitoring the location of radioactive pellets within the gravel pack. The size and specific gravity of the radioactive pellets is substantially the same as that of the gravel particles which comprise the gravel pack.

Parent Case Text

This application is a continuation of Ser. No. 786,008, filed Dec. 23, 1968, now abandoned.

Description

BACKGROUND OF THE INVENTION

Gravel packs have been found necessary and have been used to great advantage in an attempt to effectively control movement in unconsolidated earth formations in which the well cavities are located. Examples of wells in which gravel packs have been incorporated include producing, injection, source, and storage wells. Control of sand or other unconsolidated earths in fluid or gas producing wells has been of major concern in order to avoid recovery equipment wear and failure as well as improving the quantity of materials produced. The control of earth formations surrounding injection wells which includes not only those used in secondary oil, water or gas recovery but storage wells, source wells, disposal wells and barrier wells is also important. Slumping, sagging or collapse of such formations usually necessitates redrilling the well or abandonment altogether. Although gravel packing has been used rather successfully in reducing or avoiding the aforementioned well failures, methods of improving packing effectiveness have been sought and attempted.

Gravel packing is recognized as a somewhat elaborate procedure, which, in order to be effective or satisfactory, must be carried out under carefully controlled conditions. A number of techniques for placing the gravel into wells have been developed, the variations of which, depending on the type of well being packed, are well known to those skilled in the art and will not be discussed in detail here. Generally, the methods most commonly used are gravitational, circulation and prepacked liner methods. However, notwithstanding the extent to which these gravel packing techniques have been developed and the care used in well packing, failures are common. As a result, such failures have necessitated redrilling and repacking of a well cavity before suitable production, injection source or storage processes can be continued. Although the following discussion is generally directed to problems associated with producing wells and the gravel packing used therein, it should be appreciated that the present invention and its advantages relates broadly to any gravel packed wells such as those set forth hereinabove.

The use of gravel packs in oil wells has been especially important not only because of the high costs of drilling but, in addition, to maintain maximum production capacities. The gravel packs which surround a perforated or slotted liner through which the fluid oil is directed and recovered are effective in screening oil sand from the recovered fluid when such packs are in proper position. Without the control of an effective gravel pack in wells surrounded by unconsolidated earth formations, the small particles of sand and the like which are entrained in the fluid to be recovered would otherwise be unrestricted and pass through a perforated liner along with the fluid thereby causing undesirable abrasion of metal parts and necessitate frequent clean outs. Obviously, the production costs of such a well would be significantly increased. The presence of an effective gravel pack between the perforated liner and the walls of the well thereby reduces equipment failures and increases production rates. In preparing a well for gravel packing, it is common to ream or cut out an area of the well formation somewhat larger than the size of the liner to be placed in the wall. Thereafter the liner is centered into the well cavity and the gravel is placed into the space around the liner by a suitable technique. Once production is initiated, the gravel particles present in the pack and which are significantly larger than the finer sand or earth particles present in the oil-containing earth formation surrounding the well act as a filter to allow the desirable fluids to pass therethrough into the perforated liner. At the same time, the sand grains initially contacting the gravel particles at the gravel pack-earth interface become trapped and as production continues, a gravel-sand screen or bridge is built up which further prevents significant amounts of said from extensively entering the gravel pack. Another advantage of efficient and compacted gravel packs is in preventing sagging, slumping, and possible collapse of the surrounding unconsolidated earth formation from which fluid is being taken and recovered. However, it is appreciated that as effective as the forementioned gravel packs may be in screening out most sand particles, a number of fine grains which are entrained in the fluid produced will be removed from the earth formation or be shifted in position and may pass through the pack. Voids in the formation may also be created by fracturing during drilling, injection or remedial operations. This sand removal or shifting, although not necessarily seriously impeding the fluid recovery, eventually may cause the sagging or slumping of the adjacent unconsolidated earth formation surrounding the gravel pack. If the slumping is extensive, the gravel pack formation itself may become disrupted and voids or bridges may result with concomitant exposure of some liner perforations to the unconsolidated sand formation. Obviously, when this occurs, the undesired particles may enter the liner and be directed to the well head with the fluid thereby causing the undesired abrasion of metal parts or sand filling of the well as previously noted. In addition, if large quantities of sand are eroded or removed from the resulting unconsolidated formation, a collapse of the overlying strata may also result and damage the well casing itself.

Similar failures may take place in injection or storage wells where fluids or gases are directed into the well cavities under pressure. The surrounding earth formation is thus continuously exposed to the injected material as it passes through the gravel pack with gradual erosion of the earth formation and the gravel pack with possible well failure resulting.

Again referring to wells containing a liner or pipe through which fluids or gases are recovered or injected, it is especially important that a gravel pack surrounding the liner or pipe be as uniform as possible in cross-section throughout. Considering that gravel packs often are only a few inches in thickness, there is little room for error in establishing an effective pack. Although serious enough in essentially vertical wells, the chances of poor pack formation are even greater in high-angle holes in which means for centralizing the liner must be used to prevent liner contact with earth formation. Generally, the initial depth and size of the well cavity can be reasonably estimated. However, even assuming that the liner is centered within the well, variations along the sides of the surrounding earth formation may take place as the gravel particles are directed into the well and circulated around the liner during preparation of the gravel pack. In addition, unexpected holes or cavities may be present, the entrances to which may be common to the well cavity. Yet, short of noting obvious differences in the amount of gravel calculated as necessary to form the pack and that actually used, there is presently no suitable means for efficiently determining the placement effectiveness of the gravel pack formation before production begins and extensive failure occurs. Further, once the well is in use, there is no suitable means for effectively monitoring its competency, that is, its continued effective condition short of realizing failure after it has taken place. Yet if the gravel pack formation could be periodically monitored for movements, voids, bridges, slumping and the like, failures could be anticipated and forecast and remedies such as repacking could be instituted. In addition, should gravel pack failures occur, points of failure could be determined and appropriate steps to obviate or reduce the effects could be taken.

BRIEF DESCRIPTION OF THE INVENTION

The present invention comprises a method of determining the effectiveness of formation of well gravel packs and their continued competency by distributing radioactive pellets within the gravel pack and monitoring the location of the pellets by radioactive detecting means. The radioactive pellets are essentially the same size and specific gravity of the gravel particles. The radioactive pellets are added to the gravel as it is directed into the well cavity at preselected intervals so that the individual pellets are preferably at least somewhat uniformly distributed throughout the gravel pack.

BRIEF DESCRIPTION OF THE DRAWING

The FIGURE shows a schematic cross-section of a well containing gravel pack surrounding a perforated liner and wherein radioactive pellets are distributed throughout the pack.

DETAILED DESCRIPTION OF THE INVENTION

In preparing gravel packed wells according to the invention, it is desirable to incorporate therein radioactive pellets which meet certain requirements. The pellets should be of a size and specific gravity closely approximating that of the gravel particles. Generally, in the wells containing slotted or perforated liners, the minimum designated size for gravel particles is usually slightly larger than the slots or openings of the perforated liner around which the gravel pack is located. Accordingly, preferred sizes for the radioactive pellets are those of average cross section between about 0.01 and about 0.25 inches corresponding to between about 60 and 3 mesh respectively, U.S. series. Pellet sizes within this range may be varied depending upon the critical limits of the gravel particle sizes. Specific sizes will depend not only on the width of the liner slot opening but also the type of earth formation into which the well cavity is formed. Should a radioactive pellet be significantly smaller than the gravel particles, it may be forced through the gravel pack along with the fluid. Such an occurrence would result in erroneous surveys in determining movements and changes in shape of the gravel pack formation. On the other hand, radioactive pellets significantly larger than those of the adjacent gravel particles may be unduly strained and worn.

The specific gravity of the radioactive pellets should also be approximately the same as the grain density of the gravel particles, in order to prevent erroneous monitoring data caused by excessive movement through the gravel pack with the fluids injected or recovered. Generally, suitable specific gravity ranges are between about 1.0 and about 5 grams per cubic centimeter and more preferably between about 1.5 and about 3.5 grams per cc.

The composition of the radioactive particles comprises a base material, a radioactive component, and if necessary, additional substances which will add weight to the particle in order to achieve the desired specific gravity. Depending on the method of formation, adhesives, coloring agents, etc., may also be present. The body or matrix of the radioactive pellet may comprise one or more materials such as glass, ceramics, metals, wood, synthetic resins and the like. The latter materials include, for example, phenolic resins, epoxy resins, diene-vinyl aromatic copolymers, polyesters, polyolefins, vinyl and vinylidene polymers including acrylic polymers, polyvinyl chloride, fluoro-carbon polymers such as polytetrafluoroethylene (Teflon) and the like. The particular base material utilized is not especially critical as long as it possesses the necessary strength requirements in addition to its being essentially inert and unaffected by the materials to which it will be exposed in the gravel pack. Unsuitable materials may be altered physically such as by swelling or other degradation in the presence of the fluid hydrocarbons, water, etc., to which they will be subjected. In practice, polyesters and polytetrafluoroethylene have been found to be most suitable in view of their high strengths, chemical and water resistance.

The radioactive element may be of any suitable radioactive material such as, for example, iridium 192, europium 152-154, cobalt 60, scandium 46 and the like. The specific radioactive element used is not especially critical so long as it has a radioactive half life sufficient to ensure radioactive monitoring, for example, over a period of years. Within a single gravel pack it may be desirable to use individual pellets containing different radioactive materials whereby the individual pellets can be identified and their respective locations relative to one another can be determined by appropriate monitoring techniques. The radioactive material may be incorporated within the pellet by any convenient means at the time of casting or thereafter embedded into the base composition by insertion or injection, impregnation or coating methods. It may also be desirable to color or color code the pellets for visual identification. Where necessary, the radioactive pellets may also contain a material which increases its weight and desired specific gravity. Lead has been found suitable for this purpose. The use of suitable adhesives may also be desired, in order to ensure that the radioactive material is essentially permanently confined within the pellet. For general usage it has been found that by incorporating a small piece of cobalt 60 wire within a resin pellet, very suitable results have been obtained, with cobalt 60 having a half life of about 51/2 years. The radioactive pellets so prepared are superior to radioactive particles prepared, for example, by irridating or otherwise radioactively charging particles or gravel by exposure to radioactive sources. Such particles if placed in a gravel pack could become worn by repeated frictional engagement with other gravel particles or by surface erosion caused by flowing fluids with gradual degradation of radioactivity and loss of detection. Further, should such particles fracture, erroneous monitoring surveys would result.

The accompanying drawing represents a schematic cross section of a gravel packed well cavity which surrounds a perforated or slotted liner 11 having a number of perforations 19 throughout its length and which pack contains uniformly dispersed radioactive pellets 14. The gravel pack 10 meets the earth formation or sand body 12 from which the desired petroleum fluids are obtained at the interface 13. As also shown, the radioactive pellets 14 are essentially uniformly displaced at desired intervals throughout the gravel pack 10. Other parts of a conventionally completed oil producing well shown include cement sheath 15, well casing 16 and liner adapter 17. A surrounding shale body formation is shown as 18.

It will be appreciated that the well cavity is usually formed by first reaming or scraping the well throughout the space in which the gravel is to be placed. The walls of the well cavity are generally annular in shape, the average diameter of which will depend both on the diameter of the perforated liner to be inserted and the desired thickness of the gravel pack. The amount of gravel and the distance between the exterior of the perforated liner and the walls of the cavity will also be determined by the type of unconsolidated earth formation in which the well is to be placed. Gravel pack thickness, i.e., the distance between the exterior wall of the perforated liner and the wall of the earth formation, may be between about 2 and about 30 inches. In fluid recovery wells, gravel pack thickness of between about 3 and about 5 inches are common. As the gravel is placed in the well cavity, the radioactive pellets are added to the gravel at the desired intervals so that their locations throughout the gravel pack will be at least somewhat uniform. The specific intervals used for adding the radioactive particles will depend on the desired intervals between particles throughout the gravel pack and on the volume of gravel added to the well cavity. For example, where it is desired that the radioactive particles be approximately 50 linear feet apart within the gravel pack and the annular volume of gravel is about 1 cubic foot per linear foot of gravel pack, a radioactive pellet will be added to about every 50 cubic feet of gravel.

A conventional method of gravel packing comprises centering the perforated liner into the well cavity and thereafter circulating gravel into the annular space about the liner with the aid of a circulating fluid. The particles of gravel to be used should be cleaned, washed and closely sized so as to prevent contamination of the produced fluids and to insure essentially uniform permeability of the pack surrounding the liner. However, in water recovery or storage wells, a wide range of gravel size may be used. The gravel is circulated into the well cavity by mixing it with a fluid and thereafter forcing the mixture into the well. Once a portion of the gravel has been added and fluid circulation is established, more gravel is then added and this cycle repeated until the well cavity has been filled. Although not especially critical, in establishing uniformity it may be desired to add a radioactive pellet to the initial portions of gravel being placed into the well. Further, any pellets which are returned to the surface with the circulating fluid may be readily retrieved. For this purpose, as well as for safety reasons, it will be evident that the radioactive pellets should be marked or colored so that they can be easily recognized and distinguished from the gravel particles. It has also been found that where the pellets are added to the gravel at the hopper which gravel is thereafter directed to the vessels in which it is mixed with the circulating fluid, control of uniform distribution is somewhat inferior. On the other hand, much greater control of the pellet placement within the pack may be accomplished by adding individual pellets to the fluid-gravel mixture as it is directed to the well from the mixing equipment.

According to the invention, during gravel pack formation, monitoring surveys may be carried out whereby the effectiveness of formation of the pack can be readily determined. Thus, for example, as the individual radioactive pellets are added to the gravel-fluid mixture being directed to the well, their entrance and positioning within the forming pack can be detected as well as further shifting during continued pack formation and development. In this manner, should the well contain unexpected cavities or irregularities into which the gravel is being directed, the relative location and even the approximate size of the cavity may be determined by noting significant losses of radioactive intensities of individual pellets which are monitored during packing. The size of the cavity may be estimated by the volume of additional and otherwise unaccounted for gravel required to complete the gravel pack. It is evident that by such a method, an extremely useful determination of the competency of the gravel pack can be made with further forecasts of the necessity of repacking in order to achieve the desired gravel pack before fluid injection or recovery is initiated.

Immediately upon completion of the well, or shortly thereafter, a radioactive survey should be made to determine the initial position of the radioactive pellets within the completed gravel pack. This may be ascertained by a gamma ray log, scintillometer survey or other suitable and effective monitoring means. A probe 20 may be lowered into the well on a cable 21. Periodically thereafter, the gravel pack may be resurveyed to determine any shifting in the relative positions of the radioactive pellets. Accordingly, the initial log will evaluate the gravel pack in terms of fill and hole condition with later surveys indicating pack settling, slumping, subsidence, voids, bridges and the location thereof. Surveying of the gravel pack is generally accomplished by lowering the survey apparatus into the pipe extending into the well by a wire or other appropriate means extending from the well head. In order to more accurately determine the vertical positions of the individual radioactive pellets, it may also be desirable to conduct a gamma-ray collar log which will indicate the depth of the collars used to join the series of pipe lengths within the well. The relative positions of radioactive pellets to these collars will more accurately reflect their depth within the well because of the known distances between the collars and the well head as compared to attempts to measure the length of a flexible wire or line used to lower the survey apparatus. The surveys are monitored by appropriate recording equipment. The well can be designed so that when it is determined that the gravel pack is settling from the top down, as may be the case in an injection well, it can then be repacked either through port collars or over the top of the liner. If it is found that settling is occurring in the middle of the pack, the settling area can also be ascertained and the point or points of sand entrance may thereafter be isolated and shut off by a suitable manner such as with a bonding or consolidating agent or filling the liner with a material up to the area of failure. It will be evident to those skilled in the art that a gravel packed well prepared according to the present invention will provide a method of determining its packing efficiency and its continued competency whereby changes and variations can also be easily monitored in order to anticipate and forecast problems and failures. Accordingly, costly damages, loss of production, extensive redrilling procedures and the like may be avoided or eliminated as a result of the invention described herein. These as well as other advantages will be apparent to those skilled in the art.

Claims

1. A method of detecting any change in the characteristics of a gravel pack in a well cavity, including the steps of:

a. sensing the individual locations of discrete, spaced radioactive pellets in the gravel pack;

b. again sensing the individual locations of the discrete, spaced radioactive pellets at a later time; and

c. determining whether any changes in the individual locations of the discrete, spaced radioactive pellets occurred between the two sensing

2. A method as set forth in claim 1 wherein the radioactive pellets have substantially the same size and specific gravity as the gravel particles comprising the gravel pack.