Ball sealers

[Β] (Π) EXPLANATORY STATEMENT «No. 152467

<td>NORWAY</td><td>(51) Int Cl⁴ E 21 B 33/13, 43/18, 43/25</td>

[NO]

<td>BOARD</td><td>(21) Patent Application No. 783267</td>

FOR THE INDUSTRIAL (22) Filing »27.09.78

<td>DEFENSE</td><td>(24) Running day 27.09.78</td>

(41) General information available from 15.05.79 (44) Application published, publication document published 24.06.05

<td>(30) Priority requested</td><td>14.11.77, United States, No. 850879</td>

<td>(54) Designation of the invention</td><td>PROCEDURE FOR TREATING A FORMATION AROUND A WELL.</td>

<td>(71) (73) Applicant / Patent Holder</td><td>EXXON PRODUCTION RESEARCH COMPANY, PO Box 2189, Houston, TX 77001, USA.</td>

<td>(72) Inventor</td><td>THOMAS WAYNE MUECKE, Houston, TX, CLAUDE EVERETT COOKE, JR., Houston, TX, CLAY GRUESBECK, JR., Houston, TX, USA.</td>

<td>(74) Agent</td><td>Bryns Patentkontor A / S, Oslo.</td>

<td>(56) Cited publications</td><td>US Patent No. 2754910, 3174546, 3011548.</td>

The present invention relates to the treatment of underground formations through which a borehole penetrates. More specifically, it is based on a method for selectively treating a variety of formation intervals using ball seals. It is common practice when oil and gas wells are completed, to put down a pipe string called casing in the well and use cement around the outside of the casing to isolate the various hydrocarbon producing formations the well penetrates. To provide fluid communication between the hydrocarbon-bearing formations and the interior of the casing, the casing and the cement casing are perforated.

At different times during the life of the well, it may be desirable to increase the rate of production of hydrocarbons by acid treatment or hydraulic fracturing. If only a short, simple hydrocarbon conducting zone in the well has been perforated, the treatment fluid will flow into this producing zone. As the length of the perforated zone or the number of perforated zones increases, treatment of the entire productive zone or zones becomes more difficult. For example, the layers that have the highest permeability are likely to consume the bulk of a particular stimulant treatment so that the less permeable layers are left virtually untreated. To overcome this problem, it has been proposed to divert treatment fluid from high permeability zones to low permeability zones.

Various techniques for the optional treatment of multiple zones have been proposed, and sealing devices, baffles and spheres, bridging plugs and ball seals are used.

Packing devices have been used extensively to separate zones to be treated. Although these devices are effective, they are expensive to use because of the associated work equipment one must have while handling the packing device for the pipes. Furthermore, mechanical reliability will tend to decrease as the depth of the well increases.

When using a baffle and a ball to separate zones, a conductor that fits between two joints in the casing is used and the ring has a somewhat smaller internal diameter than the casing so that a large ball or body released into the casing will settle on space on the baffle. As the ball is on the baffle, the ball prevents fluid from flowing down the casing. A disadvantage of this method is an additional cost of placing the baffle. In addition, if two or more baffles are used, the inner diameter of the lower baffle will be so small that a standard perforating device cannot be used for perforation below the lower baffle.

A bridge plug consisting mainly of shims, a plug mandrel and a rubber sealing element has also been inserted into and secured to the casing to insulate the lower zone while treating the upper zone. After fracturing or acid treatment of the well, the plug is generally lowered to the bottom of the wellbore with a percussion device. One difficulty with the bridge plug method is that the plug sometimes does not withstand the high pressure differences. Another problem with this diversion technique is that the placement and removal of the plug can be quite expensive.

One of the more popular and widely used diversion techniques is using ball seals. In a typical process of this kind, ball seals are pumped into the well along with treatment fluid for the formation. The beads are then brought into the well bore and to the perforations using the fluid flowing through them. The balls come into contact with the perforation and are held there by the pressure difference over the perforations.

Although ball sealing diversion techniques have been widely used, the bullets often do not work effectively because only a fraction of the bullets introduced in reality

152467 comes in contact with perforations. Ball seals with a greater specific weight than the treatment fluid will often provide a low and indeterminate sealing efficiency, a degree highly dependent on the difference in specific weight between the ball seals and the liquid, the fluid flow rate through the perforations and the number, the distance between and the orientation of the perforations. . The result is that closure of the desired number of perforations at the right time during treatment becomes very random. It is also difficult to determine which perforated interval of the perforated casing is closed with spheres and in many cases stimulation is obtained in undesirable parts of the formation.

Ball seals with a specific weight smaller than the treatment fluid have been proposed to improve the efficiency of the seal. In this method, liquid containing light ball seals is introduced into the wellbore at a rate such that the velocity of the liquid downwardly is sufficient to exert a downward driving force on the ball seals greater than the buoyancy force of the ball seals. Once the ball seals have reached the perforations, they will come into contact with them and seal the perforations so that the treatment fluid is diverted to the remaining open perforations. A problem with using light ball seals is that if the fluid flow rate down the tube is slow as is the case with matrix treatments with acid, the treating fluid will not be able to overcome the upward buoyancy of the ball seals, and thus the ball seals may not be transported to the perforations. Another problem is that it is sometimes difficult to determine which interval of the formation will be treated. Lightweight balls brought into the casing of the heavier treatment fluid will often close the upper perforations before closing the lower perforations.

The present invention provides an improved method for temporarily limiting the flow of a treating fluid through the lower perforations of the casing into a wellbore by introducing treating fluid through the upper perforations into the well. In general, the invention is that in the casing of a well perforated at a variety of levels, ball seals intended to close at least one of the perforations in the casing are introduced, a first fluid of a specific weight greater than the ball seals. specific weight and a further fluid having a specific weight less than the specific weight of the ball seals. The ball seals, the first fluid or the second fludium can be introduced into the well simultaneously or in any order. The first fluid is introduced into the well in an amount sufficient to fill the lower portion of the well to a level between the perforations to remain open and the perforations to be temporarily restricted in fluid flow. The difference in specific weight between the ball seals and the fluid in the well causes the beads to move to the interface or transition zone between the first fluid and the second fluid. As soon as the ball seals reach the level to be treated, a treatment fluid is introduced into the well. Fluid flow through the perforations below the ball seals will bring the ball seals against the perforations where the seals will abut and divert further introduction of treatment fluid through the upper perforations. This process can be repeated for processing any number of zones in the formation.

In a preferred embodiment, the first fluid is an aqueous saline solution having a specific weight greater than 1.1 g / cm, while the second fluid is diesel oil having a specific weight of less than 0.95 g / cm and the ball seals are made of a syntactic foam core and a polyurethane cover

3 with a specific weight between about 1.0 g / cm and 1.05 g / cm.

This invention makes it possible to use light ball seals to limit the flow of treatment fluid through the perforations in a lower part of the borehole with 100% efficiency without affecting the introduction of treatment fluid through perforations in an upper part of a well. This method therefore offers significant advantages over prior art methods used for fluid diversion.

The invention is characterized by the features set forth in the claims and will be described in more detail below with reference to the drawing which represents a section through a well, where the invention is used.

The invention can be used in wellbores with a casing extending through a plurality of hydrocarbon emitting layers, formations, zones or strata. The oil-producing layers are often superimposed and may be separated by non-productive intermediate regions. When treatment fluids are introduced into a well where it is connected to the multiple layers or intervals, the layers that have least resistance to the treatment will have their permeability or productivity increased, while the layers less susceptible to treatment will not increase their permeability or productivity. One zone is therefore treated at the expense of another. The present invention is particularly suitable for increasing the permeability or productivity of a higher producing layer by stimulation methods, e.g. by hydraulic breaking or acid treatment, while limiting fluid flow to a lower productive layer.

An embodiment of the invention will be described with reference to the drawing which shows a well 10 having a casing 12 extending to the bottom of the wellbore. The well extends through an upper hydrocarbon producing layer 14 and a lower hydrocarbon producing layer 15. It is believed in this example that the lower layer 15 has a greater permeability than the upper layer 14. The casing is shown bonded to the sides of the borehole with cement on the outside to hold the casing in place and to insulate the layers 14 and 15 through which the well extends. The cement layer 13 extends upward from the bottom of the borehole to the earth's surface. The layer 14 is in fluid communication with the interior of the casing 12 through perforations 17, and the layer 15 has corresponding fluid communication with the interior of the casing through perforations 16.

Hydrocarbons from the producing layers 14 and 15 flow through the perforations 16 and 17 to the interior of the casing 13 and are transported to the surface through the production tube 19. A packer is placed near the lower end of the production tube 19 and over the layer 14 to form a pressure seal between the production tube 19 and the casing 12. Production pipes are not always used and in this case the entire inner volume of the casing is used for transporting hydrocarbons to the soil surface. Because the lower layer 15 has a higher permeability than the upper layer 14 in order to stimulate the upper layer 15 by breaking or acid treatment, it is necessary to limit the flow of treatment fluids into the lower layer 15.

The first step in the isolation of the lower layer 15 from the upper layer 14 according to the invention is that a fluid of greater specific gravity is introduced into the wellbore than the specific weight of the ball seals. This heavier fluid, designated 20 in the drawing, is pumped into the well in an amount sufficient to fill the lower portion of the wellbore up to a level between the perforations 16 for the lower layer 15 and the perforations 17 for the upper layer 14.

The heavy liquid 20 used to fill the lower portion of the well must have a specific weight greater than the specific weight of the ball seals introduced into the well. This is desirable for the ball seals to flow on the heavy fluid 20 over the perforations 16. The specific weight of the fluid 20 will, of course, depend on the specific weight of the ball seals used in the well, but the fluid will normally have a minimum specific weight of more than 1 g / cm and preferably a specific weight of more than 1.10 g / cm . Any liquid having the specific weight required and inert to the ball seals can be used in the practice of the invention. Suitable heavy fluids may include aqueous fluids in the form of a saline or calcium bromide solutions and non-aqueous fluids such as e.g. ortho-nitrotoluene, carbon disulfide, dimethyl phthalate, nitrobenzene and isoquinoline.

After the heavy fluid is introduced into the casing, a fluid having a specific weight less than the specific weight of the ball seals is introduced into the casing. This light fluid designated 21 in the drawings will lie in the well above the heavy fluid and preferably fill the well to a level close to the perforations 17 of the layer 14. Any liquid having a specific weight less than the specific weight of the ball seals can be used at practice of the invention. Suitable light fluids include hydrocarbons such as diesel oil and light hydrocarbon capacitors. The light fluid 21 may also be the same fluid used to treat the layer 14, provided that the specific weight of the treatment fluid is less than the specific weight of the ball seals.

After the heavy fluid 20 and the light fluid 21 are introduced into the well, ball seals 22 are introduced into the well, the ball seals having a specific weight which lies between the specific weight of the heavy fluid 21 and the specific weight of the light fluid 20. The ball seals are designed to have an outer coating which is sufficiently malleable to seal a perforation formed by rays and also has a solid, rigid core which prevents the ball seals from being pressed into or through the perforations. The spheres are roughly round, but other geometric shapes can be used. Due to the difference in specific weight between the ball seals and the light fluid 21, the ball seals will sink to the bottom of the light fluid 21 and flow on top of the heavy fluid 20.

After the ball seals 22 are placed in the well between the layers 14 and 15 and preferably after all the balls flow on the top of the heavy fluid 20, as shown in the drawings, a treatment liquid is introduced into the well to treat the formation or the layer 14. The treatment liquid can be a acid, a water solution, or a hydrocarbon solution so that the formation permeability or productivity increases by physical crushing or breaking up or by reaction with a chemical agent, e.g. an acid that acts on the material in the layer. When treatment fluid is introduced, any fluid flow to layer 15 causes the level of heavy fluid 20 to decrease. As soon as the beads 22 arrive at the perforations 16, the flow of fluid 21 through the perforations 16 will bring ball seals across and abut the perforations. The ball seals are here kept by the pressure difference and thereby they will effectively close the perforations 16. As the perforations 16 in the layer 15 are closed, the pressure builds up in the casing and treatment fluid passes through the perforations 17 and into the layer 14.

The specific weight of the treatment fluid may be equal to or greater than or less than the specific weight of the ball seals. If the treatment fluid has a specific weight greater than the specific weight of the ball seals, the light fluid 21 may not be the same as the treatment fluid, because the light fluid must have a specific weight less than the specific weight of the ball seals to ensure that the balls are kept lower. than the perforations through which the treatment fluid is to flow.

After the layer 14 is appropriately treated, the pressure on the wellhead is relieved and the pressure difference from the formation towards the wellbore causes the ball seals to be released from the perforations 16. Other layers (not shown) can then be optionally treated according to the invention by introducing additional heavy liquid 20 into the well so that the ball seals flow to a position above the perforations in the next higher layer to be temporarily closed and below the perforations for the next higher layers to be treated while introducing additional light liquid to replenish the light fluid lost during the preceding treatment step, with subsequent introduction of additional treatment fluid to treat the next higher layer or optionally several layers higher than the ball seals.

Although the ball seals, heavy fluid 20 and light fluid 21 in the embodiment described herein were introduced into the casing in a particular order, it should be noted that the ball seals 22 and fluids 20 and 21 can be inserted into the casing in any order, and they can be introduced at the same time. In another embodiment, heavy fluid 20 and light fluid 21 can be pumped in at the same time, while the ball seals are then inserted into the casing at the wellhead by means of a feeder or other suitable introducer.

Ball seals introduced into the well of the invention do not interfere with the introduction of treatment fluids during multi-stage processing of a formation. Ball seals located in the well between layers 14 and 15 will abut perforations 16 when fluid flows through them and the sealing effect will be 100%. This means that each ball seal will engage and seal a perforation 16 so that there is a perforation 16 through which fluid flows. If the fluid 21 of low specific gravity flows through the lower perforations 16, ball seals are brought into contact. One will get a diversion that can be determined in advance because the number of perforations sealed by ball seals will be equal to the number of ball seals introduced into the casing. The number of ball seals used in the practice of the present invention therefore depends on the number of perforations one wishes to close. Due to the high efficiency of the seal, an excess of ball seals will normally be unnecessary.

In order to practice the present invention in the field, it is necessary to have ball seals with a specific weight less than the specific weight of the heavy fluid 20 and a specific weight greater than the specific weight of the light fluid 21, and at the same time have the strength to withstand the pressures they are exposed to in the wellbore. It is not uncommon for the pressure at the bottom of the well to exceed 700 kg / cm and even 1000 kg / cm during treatment of the well. If a ball seal does not withstand these pressures, they will collapse, thereby increasing the specific weight of the ball seals to a value that easily exceeds the specific weight of the fluid 21.

The heavy fluid 20 will generally have a specific weight of at least 1.0 g / cc and the light fluid 21 will generally have a specific weight less than about 0.8 g / cc. Therefore, the specific weight of the ball seals must be in the range of 0.8 to 1.1 g / cm.

It will be seen that the present invention has a number of advantages compared to methods heretofore used for the treatment and stimulation of multiple zones. With the method of the invention, any zone can be treated with any desired amount of treatment fluid with virtually no loss due to fluid being lost in perforations below the zone to be treated. The advantages of the present invention compared to methods previously used to prevent particular layers from absorbing introduced fluids include the simplicity since no expensive equipment is needed to perform the method, and the flexibility since changes in the level at which Treatment fluid to be made can be done quickly and cheaply.

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